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I'm trying to understanding base pairing. So every linear chromosome is a double stranded double helix. Is this double stranded property the same as saying an organism is diploid? Or do the chromosomes of a haploid eukaryotic organism, like a member of Fungi, each have 2 strands (with the purines always pairing w/ pyrimidines), which would imply that diploids organisms actually have 4 strands?
EDIT: Part of my confusion comes from the fact that in diploid organisms, each parent gives 1 chromosome to the child. If the double stranded property is the same as saying an organism is diploid, does this mean that say if a nucleotide on one chromosome is A, then the nucleotide on the other chromosome in the same loci must be T? Doesn't chromosome mean a paired up strand, not a single strand?
Is this double stranded property the same as saying an organism is diploid?
No. You are a diploid organism because you have two copies of every chromosome.* You got one from your mother, and one from your father. All your cells are also diploid, except for your gametes, each of which contains only one copy of every chromosome (due to crossing over, each chromosome is partially from your mother, partially from your father).
Bacteria just have one copy of their genome. They duplicate it prior to cell division, and each resulting cell has one copy of each chromosome.
*Unless your sex chromosomes are XY, or are anuploidic.
What Are the Base Pairing Rules for DNA?
The base pairing rules for DNA are governed by the complementary base pairs: adenine (A) with thymine (T) in an A-T pairing and cytosine (C) with guanine (G) in a C-G pairing. Conversely, thymine only binds with adenine in a T-A pairing and guanine only binds with cytosine in a G-C pairing.
Deoxyribonucleic acid, or DNA, contains the entire set of information essential for the survival of an organism. This set of instructions are encoded in a double-helix stranded structure composed of nucleotide monomers. Each nucleotide carries a phosphate group, a five-carbon sugar called a deoxyribose and one of four nucleobases. The four nitrogen-containing bases found in DNA are A, T, C and G. A and G are classified as "purines," while C and T are considered as "pyrimidines." Purines are bigger in size compared to pyrimidines.
An important discovery regarding the structure of DNA was made by Edwin Chargaff in 1949. In one of his experiments, Chargaff illustrated that the quantity of A is equal to that of T, while the quantity of C is equal to that of G. He then concluded that the complementary base of A must be T and the complementary base of C must be G. Chargaff's findings formed the basis for the base pairing principle of DNA.
Purines vs. Pyrimidines
When it comes identifying the main differences between purines and pyrimidines, what you’ll want to remember is the ‘three S’s’: Structure, Size, and Source. The very basics of what you need to know are in the table below, but you can find more details about each one further down.
The most important difference that you will need to know between purines and pyrimidines is how they differ in their structures.
The purines (adenine and guanine) have a two-ringed structure consisting of a nine-membered molecule with four nitrogen atoms, as you can see in the two figures below.
Chemical Structure of Adenine in vector format. Image Source: Wikimedia Commons Structure of guanine. Image Source: Wikimedia Commons
The pyrimidines (cytosine, uracil, and thymine) only have one single ring, which has just six members and two nitrogen atoms.
Cytosine chemical structure. Image Source: Wikimedia Commons Structure of uracil. Image Source: Wikimedia Commons Skeletal chemical structure of Thymine. Image Source: Wikimedia Commons
Because purines are essentially pyrimidines fused with a second ring, they are obviously bigger than pyrimidines. This size difference is part of the reason that complementary pairing occurs. If the purines in DNA strands bonded to each other instead of to the pyrimidines, they would be so wide that the pyrimidines would not be able to reach other pyrimidines or purines on the other side! The space between them would be so large that the DNA strand would not be able to be held together. Likewise, if the pyrimidines in DNA bonded together, there would not be enough space for the purines.
DNA has a double helix shape, which is like a ladder twisted into a spiral. Each step of the ladder is a pair of nucleotides.
A nucleotide is a molecule made up of:
DNA is made of four types of nucleotide:
The 'rungs' of the DNA ladder are each made of two bases, one base coming from each leg. The bases connect in the middle: 'A' only pairs with 'T', and 'C' only pairs with 'G'. The bases are held together by hydrogen bonds.
Adenine (A) and thymine (T) can pair up because they make two hydrogen bonds, and cytosine (C) and guanine (G) pair up to make three hydrogen bonds. Although the bases are always in fixed pairs, the pairs can come in any order (A-T or T-A similarly, C-G or G-C). This way, DNA can write 'codes' out of the 'letters' that are the bases. These codes contain the message that tells the cell what to do.
On chromosomes, the DNA is bound up with proteins called histones to form chromatin. This association takes part in epigenetics and gene regulation. Genes are switched on and off during development and cell activity, and this regulation is the basis of most of the activity which takes place in cells.
When DNA is copied, this is called DNA replication. Briefly, the hydrogen bonds holding together paired bases are broken and the molecule is split in half: the legs of the ladder are separated. This gives two single strands. New strands are formed by matching the bases (A with T and G with C) to make the missing strands.
First, an enzyme called DNA helicase splits the DNA down the middle by breaking the hydrogen bonds. Then after the DNA molecule is in two separate pieces, another molecule called DNA polymerase makes a new strand that matches each of the strands of the split DNA molecule. Each copy of a DNA molecule is made of half of the original (starting) molecule and half of new bases.
When DNA is copied, mistakes are sometimes made – these are called mutations. There are four main types of mutations:
- Deletion, where one or more bases are left out.
- Substitution, where one or more bases are substituted for another base in the sequence.
- Insertion, where one or more extra base is put in.
- Duplication, where a sequence of bases pairs are repeated.
Mutations may also be classified by their effect on the structure and function of proteins, or their effect on fitness. Mutations may be bad for the organism, or neutral, or of benefit. Sometimes mutations are fatal for the organism – the protein made by the new DNA does not work at all, and this causes the embryo to die. On the other hand, evolution is moved forward by mutations, when the new version of the protein works better for the organism.
A section of DNA that contains instructions to make a protein is called a gene. Each gene has the sequence for at least one polypeptide.  Proteins form structures, and also form enzymes. The enzymes do most of the work in cells. Proteins are made out of smaller polypeptides, which are formed of amino acids. To make a protein to do a particular job, the correct amino acids have to be joined up in the correct order.
Proteins are made by tiny machines in the cell called ribosomes. Ribosomes are in the main body of the cell, but DNA is only in the nucleus of the cell. The codon is part of the DNA, but DNA never leaves the nucleus. Because DNA cannot leave the nucleus, the cell nucleus makes a copy of the DNA sequence in RNA. This is smaller and can get through the holes – pores – in the membrane of the nucleus and out into the cell.
Genes encoded in DNA are transcribed into messenger RNA (mRNA) by proteins such as RNA polymerase. Mature mRNA is then used as a template for protein synthesis by the ribosome. Ribosomes read codons, 'words' made of three base pairs that tell the ribosome which amino acid to add. The ribosome scans along an mRNA, reading the code while it makes protein. Another RNA called tRNA helps match the right amino acid to each codon. 
DNA was first isolated (extracted from cells) by Swiss physician Friedrich Miescher in 1869, when he was working on bacteria from the pus in surgical bandages. The molecule was found in the nucleus of the cells and so he called it nuclein. 
In 1928, Frederick Griffith discovered that traits of the "smooth" form of Pneumococcus could be transferred to the "rough" form of the same bacteria by mixing killed "smooth" bacteria with the live "rough" form.  This system provided the first clear suggestion that DNA carries genetic information.
DNA's role in heredity was confirmed in 1952, when Alfred Hershey and Martha Chase in the Hershey–Chase experiment showed that DNA is the genetic material of the T2 bacteriophage. 
In the 1950s, Erwin Chargaff  found that the amount of thymine (T) present in a molecule of DNA was about equal to the amount of adenine (A) present. He found that the same applies to guanine (G) and cytosine (C). Chargaff's rules summarises this finding.
In 1953, James D. Watson and Francis Crick suggested what is now accepted as the first correct double-helix model of DNA structure in the journal Nature.  Their double-helix, molecular model of DNA was then based on a single X-ray diffraction image "Photo 51", taken by Rosalind Franklin and Raymond Gosling in May 1952. 
Experimental evidence supporting the Watson and Crick model was published in a series of five articles in the same issue of Nature.  Of these, Franklin and Gosling's paper was the first publication of their own X-ray diffraction data and original analysis method that partly supported the Watson and Crick model  this issue also contained an article on DNA structure by Maurice Wilkins and two of his colleagues, whose analysis and in vivo B-DNA X-ray patterns also supported the presence in vivo of the double-helical DNA configurations as proposed by Crick and Watson for their double-helix molecular model of DNA in the previous two pages of Nature. In 1962, after Franklin's death, Watson, Crick, and Wilkins jointly received the Nobel Prize in Physiology or Medicine.  Nobel Prizes were awarded only to living recipients at the time. A debate continues about who should receive credit for the discovery. 
In 1957, Crick explained the relationship between DNA, RNA, and proteins, in the central dogma of molecular biology. 
How DNA was copied (the replication mechanism) came in 1958 through the Meselson–Stahl experiment.  More work by Crick and coworkers showed that the genetic code was based on non-overlapping triplets of bases, called codons.  These findings represent the birth of molecular biology.
How Watson and Crick got Franklin's results has been much debated. Crick, Watson and Maurice Wilkins were awarded the Nobel Prize in 1962 for their work on DNA – Rosalind Franklin had died in 1958.
Police in the United States used DNA and family tree public databases to solve cold cases. The American Civil Liberties Union raised concerns over this practice. 
During metaphase I, the homologous chromosomes are arranged in the center of the cell with the kinetochores facing opposite poles. The homologous pairs orient themselves randomly at the equator. For example, if the two homologous members of chromosome 1 are labeled a and b, then the chromosomes could line up a-b, or b-a. This is important in determining the genes carried by a gamete, as each will only receive one of the two homologous chromosomes. Recall that homologous chromosomes are not identical. They contain slight differences in their genetic information, causing each gamete to have a unique genetic makeup.
This randomness is the physical basis for the creation of the second form of genetic variation in offspring. Consider that the homologous chromosomes of a sexually reproducing organism are originally inherited as two separate sets, one from each parent. Using humans as an example, one set of 23 chromosomes is present in the egg donated by the mother. The father provides the other set of 23 chromosomes in the sperm that fertilizes the egg. Every cell of the multicellular offspring has copies of the original two sets of homologous chromosomes. In prophase I of meiosis, the homologous chromosomes form the tetrads. In metaphase I, these pairs line up at the midway point between the two poles of the cell to form the metaphase plate. Because there is an equal chance that a microtubule fiber will encounter a maternally or paternally inherited chromosome, the arrangement of the tetrads at the metaphase plate is random. Any maternally inherited chromosome may face either pole. Any paternally inherited chromosome may also face either pole. The orientation of each tetrad is independent of the orientation of the other 22 tetrads.
This event—the random (or independent) assortment of homologous chromosomes at the metaphase plate—is the second mechanism that introduces variation into the gametes or spores. In each cell that undergoes meiosis, the arrangement of the tetrads is different. The number of variations is dependent on the number of chromosomes making up a set. There are two possibilities for orientation at the metaphase plate the possible number of alignments therefore equals 2 n , where n is the number of chromosomes per set. Humans have 23 chromosome pairs, which results in over eight million (2 23 ) possible genetically-distinct gametes. This number does not include the variability that was previously created in the sister chromatids by crossover. Given these two mechanisms, it is highly unlikely that any two haploid cells resulting from meiosis will have the same genetic composition (Figure 3).
Figure 3. Random, independent assortment during metaphase I can be demonstrated by considering a cell with a set of two chromosomes (n = 2). In this case, there are two possible arrangements at the equatorial plane in metaphase I. The total possible number of different gametes is 2n, where n equals the number of chromosomes in a set. In this example, there are four possible genetic combinations for the gametes. With n = 23 in human cells, there are over 8 million possible combinations of paternal and maternal chromosomes.
To summarize the genetic consequences of meiosis I, the maternal and paternal genes are recombined by crossover events that occur between each homologous pair during prophase I. In addition, the random assortment of tetrads on the metaphase plate produces a unique combination of maternal and paternal chromosomes that will make their way into the gametes.
Answers to all problems are at the end of this book. Detailed solutions are available in the Student Solutions Manual, Study Guide, and Problems Book. Abundance of the Different Bases in the Human Genome Results on the human genome published in Science (Science 291 :1304â€”1350 ) indicate that the haploid human genome consists of 2.91 gigabase pairs (2.91 X ]0 9 base pairs> and that 27% of the bases in human DNA are A. Calculate the number of A. T, G, and C residues in a typical human cell.
Answers to all problems are at the end of this book. Detailed solutions are available in the Student Solutions Manual, Study Guide, and Problems Book.
Abundance of the Different Bases in the Human Genome Results on the human genome published in Science (Science 291 :1304â€”1350 ) indicate that the haploid human genome consists of 2.91 gigabase pairs (2.91 X ]0 9 base pairs> and that 27% of the bases in human DNA are A. Calculate the number of A. T, G, and C residues in a typical human cell.
First life forms to pass on artificial DNA engineered by US scientists
The latest study moves life beyond the DNA code of G, T, C and A – the molecules or bases that pair up in the DNA helix.
The latest study moves life beyond the DNA code of G, T, C and A – the molecules or bases that pair up in the DNA helix.
The first living organism to carry and pass down to future generations an expanded genetic code has been created by American scientists, paving the way for a host of new life forms whose cells carry synthetic DNA that looks nothing like the normal genetic code of natural organisms.
Researchers say the work challenges the dogma that the molecules of life making up DNA are “special”. Organisms that carry the beefed-up DNA code could be designed to churn out new forms of drugs that otherwise could not be made, they have claimed.
“This has very important implications for our understanding of life,” said Floyd Romesberg, whose team created the organism at the Scripps Research Institute in La Jolla, California. “For so long people have thought that DNA was the way it was because it had to be, that it was somehow the perfect molecule.”
From the moment life gained a foothold on Earth the diversity of organisms has been written in a DNA code of four letters. The latest study moves life beyond G, T, C and A – the molecules or bases that pair up in the DNA helix – and introduces two new letters of life: X and Y.
Romesberg started out with E coli, a bug normally found in soil and carried by people. Into this he inserted a loop of genetic material that carried normal DNA and two synthetic DNA bases. Though known as X and Y for simplicity, the artificial DNA bases have much longer chemical names, which themselves abbreviate to d5SICS and dNaM.
In living organisms, G, T, C and A come together to form two base pairs, G-C and T-A. The extra synthetic DNA forms a third base pair, X-Y, according to the study in Nature. These base pairs are used to make genes, which cells use as templates for making proteins.
Romesberg found that when the modified bacteria divided they passed on the natural DNA as expected. But they also replicated the synthetic code and passed that on to the next generation. That generation of bugs did the same.
“What we have now, for the first time, is an organism that stably harbours a third base pair, and it is utterly different to the natural ones,” Romesberg said. For now the synthetic DNA does not do anything in the cell. It just sits there. But Romesberg now wants to tweak the organism so that it can put the artificial DNA to good use.
“This is just a beautiful piece of work,” said Martin Fussenegger, a synthetic biologist at ETH Zurich. “DNA replication is really the cream of the crop of evolution which operates the same way in all living systems. Seeing that this machinery works with synthetic base pairs is just fascinating.”
The possibilities for such organisms are still up for grabs. The synthetic DNA code could be used to build biological circuits in cells which do not interfere with the natural biological function scientists could make cells which use the DNA to manufacture proteins not known to exist in nature. The development could lead to a vast range of protein-based drugs.
The field of synthetic biology has been controversial in the past. Some observers have raised concerns that scientists could create artificial organisms which could then escape from laboratories and spark an environmental or health disaster.
More than 10 years ago, the scientist Eckard Wimmer, at Stony Brook University, in New York, recreated the polio virus from scratch to highlight the dangers.
Romesberg said that organisms carrying his “unnatural” DNA code had a built-in safety mechanism. The modified bugs could only survive if they were fed the chemicals they needed to replicate the synthetic DNA. Experiments in the lab showed that without these chemicals, the bugs steadily lost the synthetic DNA as they could no longer make it.
“There are a lot of people concerned about synthetic biology because it deals with life, and those concerns are completely justified,” Romesberg said. “Society needs to understand what it is and make rational decisions about what it wants.”
Ross Thyer, at the University of Texas, in Austin, suggested the synthetic DNA could become an essential part of an organism’s own DNA. “Human engineering would result in an organism which permanently contains an expanded genetic alphabet, something that, to our knowledge, no naturally occurring life form has accomplished.
“What would such an organism do with an expanded genetic alphabet? We don’t know. Could it lead to more sophisticated storage of biological information? More complicated or subtle regulatory networks? These are all questions we can look forward to exploring.”
REPRODUCTION IN ORGANISMS
Whatever be the life span, death of every individual organism is a certainty, i.e., no individual is immortal, except single-celled organisms.
- There is no natural death in single-celled organisms as they divide and form 2 new cells.
- it is defined as a biological process in which an organism gives rise to young ones (offspring) similar to itself.
- The offspring grow, mature and in turn produce new offspring. Thus, there is a cycle of birth, growth and death.
- Reproduction enables the continuity of the species, generation after generation.
- genetic variation is created and inherited during reproduction.
- There is a large diversity in the mechanism of reproduction of organisms. The organism’s habitat, its internal physiology and several other factors are collectively responsible for how it reproduces.
Reproduction is of two types–
When offspring is produced by a single parent with or without the involvement of gamete formation, the reproduction is Asexual.
When two parents (opposite sex) participate in the reproductive process and also involve fusion of male and female gametes, it is called sexual reproduction.
- Asexual Reproduction–
- In this method, a single individual (parent) is capable of producing offspring.
- The offspring that are produced are not only identical to one another but are also exact copies of their parent.These offspring are also genetically identical to each other. The term clone is used to describe such morphologically and genetically similar individuals.
- Asexual reproduction is common among single-celled organisms, and in plants and animals with relatively simple organisations.
- Binary Fission – In many single-celled organisms cell divides into two halves and each rapidly grows into an adult (e.g., Amoeba, Paramecium).
- Budding – In yeast, the division is unequal and small buds are produced that remain attached initially to the parent cell which, eventually gets separated and mature into new yeast organisms (cells).
- Special reproductive structures –Members of the Kingdom Fungi and simple plants such as algae reproduce through special asexual reproductive structures. The most common of these structures are zoospores that usually are microscopic motile structures. Other common asexual reproductive structures are conidia (Penicillium), buds (Hydra) and gemmules (sponge).
- Vegetative propagation –vegetative reproduction is also asexual process as only one parent is involved. in plants, the term vegetative reproduction is frequently used. e.g., the units of vegetative propagation in plants –runner, rhizome, sucker, tuber, offset, bulb. These structures are called vegetative propagules.In Protists and Monerans, (All unicellular) the organism or the parent cell divides into two to give rise to new individuals. Thus, in these organisms cell division is itself a mode of reproduction.
Water hyacinth, an aquatic weed, also known as ‘terror of Bengal’ propagate vegetatively. Earlier this plant was introduced in India because of its beautiful flowers and shape of leaves. Since it can propagate vegetatively at a phenomenal rate and spread all over the water body in a short period of time, it drain oxygen from water body and cause death of fishes. (Eutrophication)
Bryophyllumshow vegetative propagation from the notches present at margins of leaves.
- A sexual reproduction is the common method of reproduction in organisms that have a relatively simple organisation, like algae and fungi.
- These organisms shift to sexual method of reproduction just before the onset of adverse conditions.
- In higher plants both Asexual (vegetative) as well as sexual modes of reproduction are exhibited.
- In most of the animals only sexual mode of reproduction is present.
- Sexual reproduction involves formation of the male and female gametes, either by the same individual or by different individuals of the opposite sex. These gametes fuse to form the zygote which develops to form the new organism.
- It is an elaborate, complex and slow process as compared to asexual reproduction.
- Because of the fusion of male and female gametes, sexual reproduction results in offspring that are not identical to the parents or amongst themselves.
- Plants, animals, fungishow great diversity in external morphology, internal structure and physiology, but in sexual reproduction they share a similar pattern.
- Juvenile / vegetative phase – All organisms have to reach a certain stage of growth and maturity in their life, before they can reproduce sexually. That period of growth is called the juvenile phase. It is known as vegetative phase in plants.
- Reproductive phase –the beginning of the reproductive phase can be seen easily in the higher plants when they come to flower.
- In some plants, where flowering occurs more than once, inter-flowering period is also known as juvenile period.
- Plants-the annual and biennial types, show clear cut vegetative, reproductive and senescent phases, but in the perennial species it is very difficult to clearly define these phases.
- Bamboo species flower only once in their life time, generally after 50-100 years, produce large number of fruits and die.
- Strobilanthus kunthiana (neelakuranji), flowers once in 12 years. It is found in hilly areas in Kerala, Karnataka and Tamil Nadu.
- In animals, the juvenile phase is followed by morphological and physiological changes prior to active reproductive behaviour.
- birds living in nature lay eggs only seasonally. However, birds in captivity (as in poultry farms) can be made to lay eggs throughout the year. In this case, laying eggs is not related to reproduction but is a commercial exploitation for human welfare.
- The females of placental mammals exhibit cyclical changes in the activities of ovaries and accessory ducts as well as hormones during the reproductive phase.
- In non-primate mammals like cows, sheep, rats, deers, dogs, tiger, etc., such cyclical changes during reproduction are called oestrus cycle where as in primates (monkeys, apes, and humans) it is called menstrual cycle.
- Many mammals, especially those living in natural, wild conditions exhibit such cycles only during favourable seasons in their reproductive phase and are therefore called seasonal breeders. Many other mammals are reproductively active throughout their reproductive phase and hence are called continuous breeders.
- Senescent phase – The end of reproductive phase can be considered as one of the parameters of senescence or old age. There are concomitant changes in the body (like slowing of metabolism, etc.) during this last phase of life span. Old age ultimately leads to death.
- In both plants and animals, hormones are responsible for the transitions between the three phases. Interaction between hormones and certain environmental factors regulate the reproductive processes and the associated behavioural expressions of organisms.
- Events in sexual reproduction
- Sexual reproduction is characterised by the fusion (or fertilisation) of the male and female gametes, the formation of zygote and embryo
- These sequential events may be grouped into three distinct stages namely, the pre-fertilisation, fertilisation and the post-fertilisation events.
- These include all the events of sexual reproduction prior to the fusion of gametes.
- The two main pre-fertilisation events aregametogenesisandgamete transfer.
- It refers to the process of formation of the two types of gametes – male and female.
- Gametes are haploid cells.
- In some algae the two gametes are so similar in appearance that it is not possible to categorise them into male and female gametes.They are hence, are calledhomogametes (isogametes).
- However, in a majority of sexually reproducing organisms the gametes produced are of two morphologically distinct types (heterogametes). In such organisms the male gamete is called theantherozoid or sperm and the female gamete is called the egg or
Sexuality in organisms:
- Plants may have both male and female reproductive structures in the same plant (bisexual) or on different plants (unisexual).
- In several fungi and plants, terms such as homothallic and monoecious are used to denote the bisexual condition and heterothallic and dioecious are the terms used to describe unisexual condition.
- In flowering plants, the unisexual male flower is staminate, e., bearing stamens, while the female ispistillate or bearing pistils.
- e.g., examples of monoecious plants – cucurbitsand coconuts
- dioecious plants – Papayaand date palm.
- Earthworms, sponge, tapeworm and leech are examples of bisexual animals (hermaphrodite). Cockroach is an example of a unisexual species.
- Cell division during gamete formation:
- Gametes in all heterogametic species are of two types namely, male and Gametes are haploid though the parent plant body from which they arise may be either haploid or diploid.
- A haploid parent produces gametes by mitotic division like in monera, fungi, algae and bryophytes
- In pteridophytes, gymnosperms, angiosperms and most of the animals including human beings, the parental body isIn these, specialised cells calledmeiocytes (gamete mother cell) undergo meiosis.
- At the end of meiosis, only one set of chromosomesgets incorporated into each
- Gamete Transfer:
- After formation, male and female gametes must be physically brought together to facilitate fusion (fertilisation).
- In most of organisms, male gamete is motile and the female gamete is stationary.
- Exceptions – few fungi and algae in which both types of gametes are motile.
- For transfer of male gametes, a medium is needed. In several simple plants like algae, bryophytes and pteridophytes, water is the medium for gamete transfer.
- A large number of the male gametes, however, fail to reach the female gametes. To compensate this loss of male gametes during transport, the number of male gametes produced is very high.
- In seed plants, pollen grains are the carriers of male gametes and ovule have the egg. Pollen grains produced in anthers therefore, have tobe transferred to the stigma before it can lead to fertilization.
- In bisexual, self-fertilising plants, e.g., peas, transfer of pollen grains to the stigma is relatively easy as anthers and stigma are located close to each other pollen grains soon after they are shed, come in contact with the stigma.
- in cross pollinating plants (including dioecious plants), a specialised event called pollination facilitates transfer of pollen grains to the stigma.
- Pollen grains germinate on the stigma and the pollen tubes carrying the male gametes reach the ovule and discharge male gametes near the egg.
- In dioecious animals, since male and female gametes are formed in different individuals, the organism must evolve a special mechanism for gamete transfer. Successful transfer and coming together of gametes is essential for the most critical event in sexual reproduction, the fertilisation.
- The most vital event of sexual reproduction is perhaps the fusion of gametes. This process is also calledsyngamyresults in the formation of a diploid
- in some organisms like rotifers, honeybees and even some lizards and birds (turkey), the female gamete undergoes development to form new organisms without fertilisation. This phenomenon is called
- In most aquatic organisms, such as a majority of algae and fishes as well as amphibians, syngamy occurs in the external medium (water), i.e., outside the body of the organism. This type of gametic fusion is called external fertilisation.
Organisms exhibiting external fertilisation show great synchrony between the sexes and release a large number of gametes into the surrounding medium (water) in order to enhance the chances of syngamy. This happens in the bony fishes and frogs where a large number of offspring are produced. A major disadvantage is that the offspring are extremely vulnerable to predators threatening their survival up to adulthood.
- In many terrestrial organisms, belonging to fungi, higher animals such as reptiles birds, mammals and in a majority of plants (bryophytes, pteridophytes, gymnosperms and angiosperms), syngamy occurs insidethe body of the organism, hence the process is called internal fertilisation.
In all these organisms, egg is formed inside the female body where they fuse with the male gamete. In organisms exhibiting internal fertilisation, the male gamete is motile and has to reach the egg in order to fuse with it. In these even though the number of sperms produced is very large, there is a significant reduction in the number of eggs produced. In seed plants, however, the non-motile male gametes are carried to female gamete by pollen tubes.
- Post-fertilisation Events
- Events in sexual reproduction after the formation of zygote are called post-fertilisation events.
- Zygote :
- Formation of the diploid zygote is universal in all sexually reproducing organisms.
- In organisms with external fertilisation, zygote is formed in the external medium (usually water), whereas in those exhibiting internal fertilisation, zygote is formed inside the body of the organism.
- Further development of the zygote depends on the type of life cycle the organism has and the environment it is exposed to.
- In organisms belonging to fungi and algae, zygote develops a thick wall that is resistant to dessication and damage. It undergoes a period of rest before germination.
- In organisms with haplontic life cycle, zygote divides by meiosis to form haploid spores that grow into haploid individuals.
- Zygote is the vital link that ensures continuity of species between organisms of one generation and the next.
- Every sexually reproducing organism, including human beings begin life as a single cell-the zygote.
- It refers to the process of development ofembryo from the zygote.
- During embryogenesis, zygote undergoes cell division (mitosis) and cell differentiation. While cell divisions increase the number of cells in the developing embryo cell differentiation helps groups of cells to undergo certain modifications to form specialised tissues and organs to form an organism.
- Animals are categorised into oviparous and viviparous based on whether the development of the zygote take place outside the body of the female parent or inside, i.e., whether they lay fertilised/unfertilised eggs or give birth to young ones.
- In oviparous animals like reptiles and birds,the fertilised eggs covered by hard calcareous shell are laid in a safe place in the environment after a period of incubation young ones hatch out.
- in viviparous animals (majority of mammals including human beings), the zygote develops into a young one inside the body of the female organism. After attaining a certain stage of growth, the young ones are delivered out of the body of the female organism. Because of proper embryonic care and protection, the chances of survival of young ones is greater in viviparous organisms.
- In flowering plants, the zygote is formed inside the ovule. After fertilisation the sepals, petals and stamens of the flower wither and fall off.
- The pistil however, remains attached to the plant. The zygote develops into the embryo and the ovules develop into the seed. The ovary develops into the fruit which develops a thick wall called pericarp that is protective in function. After dispersal, seeds germinate under favourable conditions to produce new plants.downloadble pdf file is available…please click on the link below…
Mating Ability and Fertility of Diploid Males. We established 39 F2 pairings with haploid or diploid males. On the basis of the microsatellite criteria described above, we identified 19 males as diploids and 18 as haploids. Two of the males had pedigrees lacking adequate allelic variation to confirm a diploid or haploid status their families were not included in our analysis. The mating abilities of haploid and diploid males were comparable with regard to their ability to mount females and engage in copulation. All 19 diploid males and 16 of 18 haploid males mated, and for the males that mated, we found no differences between the groups in the time required for courtship (t = 1.30, P = 0.20) or to complete copulation (t = 0.02, P = 0.98). Of the 19 females that mated with diploid males, three never attempted to nest, and similarly, four of the 16 females mated to haploid males failed to nest (χ 2 = 0.27, P = 0.60). These nonnesting wasps are not included in further analyses.
Data comparing the reproductive output of nesting females mated to diploid versus haploid males are presented in Table 1. Females mated to diploid males and females mated to haploids provisioned comparable numbers of nest cells (x̄ = 37.3 vs. x̄ = 32.3, t = 1.61, P = 0.12). The proportion of immature mortality in the two groups was the same: 0.25. Thus, regardless of their mate's ploidy, females produced similar numbers of offspring.
There were however significant differences between the groups in the numbers of male offspring. Females mated to diploids averaged significantly more sons (x̄ = 11.7) than females mated to haploids (x̄ = 3.2, t = 2.97, P = 0.01). However, for male reproductive success, the critical factor is whether their sperm are used in fertilizations to make daughters. Diploid males produced, on average, 16.1 daughters versus the 21.1 daughters achieved by haploid males diploid males thus have 76% the fertility of haploids. Even so, the two-tailed test does not indicate a significant difference (t = 1.60, P = 0.12). The range in the number of daughters for diploid males (0–36) is greater than for haploids (7–32) the fertility of diploid males spans the range from zero to levels indistinguishable from that of normal haploid males. Because of the high variability in our sample, there may be undetected differences between diploid and haploid male reproduction. However, clearly some diploid males have fertility comparable to or exceeding that of some haploid males.
Reproductive Capabilities and Ploidy of Daughters of Diploid Males. The behavior of daughters of diploid males did not differ from that of daughters of haploid males with regard to courtship (t = 0.53, P = 0.60) or total time required for mating (t = 1.76, P = 0.09), nor did daughters of diploid and haploid males show any differences in nesting and reproduction. Of the 30 F3 females, 11 of 18 that had diploid fathers nested, and 10 of 12 with haploid fathers nested (χ 2 = 0.80, P = 0.37). Nesting females with diploid versus haploid fathers provisioned an average of 23.8 versus 24.8 cells respectively (t = 0.25, P = 0.81), and mortality among the offspring of the two kinds of females was also similar (0.41 versus 0.43, χ 2 = 0.01, P = 0.93) (Table 2). Because the daughters of diploid males had normal fertility, we would expect them to be diploid rather than triploid. By using microsatellites, we were able to test the ploidy of these females. Thirteen diploid males and their mates had microsatellite allelic combinations that allowed us to unequivocally determine whether their daughters were diploid or triploid. We genotyped 47 daughters from these crosses. In all cases, the daughters were diploid with one distinctive allele from each parent, and their diploid fathers could pass either allele at a locus to these daughters (Fig. 3).
Male Reproductive System
Spermatogonia are rounded cells that lie in contact with the basement membrane of the seminiferous tubules. They are the stem cells of the system, dividing to maintain their own numbers and to produce the cells that embark on the process of development into spermatozoa. Spermatogonia divide by mitosis into types A and B. Type A are the replacement cells and type B are the cells that develop into spermatocytes. Early type B spermatogonia cannot be distinguished from the spermatogonia in general, but they soon enlarge and begin the complex process of meiosis.
Meiosis is a two stage maturation process. The first division produces secondary spermatocytes and the second maturation division produces spermatids, which undergo no more division and develop into spermatozoa. Many readers will know that meiosis leads to a reduction in the number of chromosomes from the diploid number to the haploid number, which means a reduction from 42 to 21. Essentially the same process occurs in the development of ova in the female rat. By way of example, Table 18.2 explains the process in man.
Table 18.2 . Gamete Production in Man
Stage Events, etc. Male Cells Female Cells Ploidy n Number Number of Chromosomes Resting stage of cells: oogonia and spermatogonia Normal cellular metabolism Spermatogonia Oogonia Diploid 2 46 A, A′, Each chromosome contains one strand of DNA Two A chromosomes are present, A and A’, hence the cell is diploid Mitosis preparatory phase DNA replication, centromere replication occurs Spermatogonia Oogonia Diploid 4 46 AA, A′A′, The number of strands of DNA has doubled Two A chromosomes are present: AA and A′A′ Cell division Spermatogonia Oogonia Diploid 2 Back to original state 46 A, A′, Back to original state Meiosis I. Preparatory stage with a long prophase DNA replication Primary spermatocytes Primary oocyte Diploid 4 46 The cells change from spermatogonia to primary spermatocytes on duplication of the DNA The cells change from oogonia to primary oocytes on duplication of the DNA. Long delay in women: process does not proceed until puberty AA, A′A′, Crossing over occurs and the cells divide Secondary spermatocytes produced Secondary oocyte plus 1st polar body produced Haploid 2 23 AA, This is a different sort of 2 from that seen above: A, A’, Only one A chromosome (AA) is present: hence the cell is haploid Meiosis II No DNA replication takes place, Cells divide Spermatids produced 1st definitive oocyte produced plus further polar body. Haploid n 23 A, Only one A chromosome is present: hence the cell is haploid
Notes: Ploidy refers to the number of copies of each chromosome present in the cell, the n number refers to the number of copies of each strand of DNA. Recall that humans have 46 chromosomes: 44+XX or 44+XY. The 44 non-sex chromosomes comprise two sets of 22: there are two number 15 s, etc. We could call these 15 and 15′. Let A and A′ be chromosomes and let us follow them through the process. If the amount of DNA in a chromosome doubles we shall show this by AA or A′A′.
Note the ploidy/n number combinations
In man, the first meiotic division begins with a 22 day prophase that is divided into five stages: leptotene, zygotene, pachytene, diplotene and diakinesis (mnemonic: let zoologists pet dangerous dingoes). Thread like chromosomes appear during leptotene and duplicate during zygotene, shorten and thicken during pachytene (pachyderms have thick skins), when crossing over occurs. Further shortening and preparation for the next stage occurs during diplotene and diakinesis. Once prophase is over, things move more quickly. The nuclear membrane disappears and the chromosomes line up along the equator of a recently formed spindle before moving apart to produce haploid secondary spermatocytes (see Table 18.2 ). This sequence is described as metaphase, anaphase and telophase.
Secondary spermatocytes exist for a brief interphase before a second meiotic division, during which no duplication of DNA occurs, and the secondary spermatocytes move quickly through prophase, metaphase, anaphase and telophase to produce the spermatids. Cell division now complete, each spermatid can develop into a spermatozoon.
An added complication is that the cells taking part in the sequence from primary spermatocytes to spermatids never actually separate when they divide, and the daughter cells of each ‘division’ remain connected by cytoplasmic bridges. Separation only occurs when the spermatids develop into spermatozoa, thus for a large part of their development the germ cells exist as a syncytium. This cannot be seen at light microscopy, but is very apparent at electron microscopy ( Fawcett, 1994 ) ( Fig. 18.2 ).
Figure 18.2 . Testes composite – top left and right stages 4 and 8, bottom left and right stages 10 and 14.
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