Reproduction is the reason for life’s continued existence on the face of Earth. Every organism needs to be able to reproduce so that it can classify as a living being. Even viruses, whose living nature has been debated for decades, can reproduce in the host’s body. The molecular basis of life, i.e., the DNA, needs to be replicated for the reproduction of an individual. There are two modes of cell division, namely, mitosis and meiosis, both of which lead to duplication of the DNA.
Essentially, mitosis is a form of asexual reproduction wherein two genetically and morphologically identical individuals are produced. Meiosis, on the other hand, defines sexual reproduction. If all organisms did reproduce by asexual reproduction, there would hardly be any variations among populations, and nature would not have been this diverse! Meiosis allows for variations to occur in living organisms and this is not only important to maintain biological diversity but to also help organisms adapt and evolve.
Heredity refer to the passing over of characters from the parent to the offspring. Variations in sexually-reproducing species result from three genetic processes− mutations, independent segregation of chromosomes, and genetic recombinations. Let's address these processes one by one.
Mutations − They are the random changes in the genes of any organism and can be a result of physical, chemical, or biological factors. These changes lead to the production of different forms of the same gene (i.e., alleles) among different individuals of the same species. In asexually reproducing organisms, a mutation simply passes on to the next generation during mitosis.
However, in sexually-reproducing organisms, these mutations are incorporated and then are subjected to further reshuffling during sexual reproduction.
Independent assortment of chromosomes.
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Diploid eukaryotes contain homologous pairs of chromosomes. One member of the pair is inherited from the mother and contains a particular set of alleles, the other from the father and contains a different set of alleles. During meiosis, these homologous pairs of chromosomes separate, and the members of the pair are segregated into a different daughter nucleus, resulting in haploid gametes. What each gamete receives is random and independent of the other pairs of chromosomes. The Independent assortment of chromosomes serves as the basis for classical genetics.
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Each parent has a particular set of characteristics that are encoded in the chromosomes of their gametes. The diploid individual thus formed has features that it has inherited from both the parents. There are more than 8 million possible combinations across the 23 pairs of chromosomes in humans. Hence, no two homologous pairs in an individual are ever the same! Further, this explains the genetic variations are seen in siblings, all of whom inherit their characters from the same parents, but what alleles they inherit are different.
Chromosomal crossing over.
Interestingly, just before the separation of homologous chromosomes occurs, the homologous chromosomes undergo the event of “chromosomal crossing over” which results in the recombination of genes on the chromosomes. During recombination, alleles of one chromosome are swapped with those of the other homologue. Chromosomal crossing over enhances the chances of variation in sexually reproducing organisms.
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The cumulative effect of the events of mutations, independent assortment, and crossing over is the accumulation of a plethora of variations which are not possible during asexual reproduction.
Variation is an indispensable part of expanding the gene of a population. It leads to increased genetic diversity. In fact, heredity, coupled with the variation of the inheritable genes (i.e., “descent with modification”), is the basis for the entire evolutionary history of the earth!.
Heredity and variations are the basis for.
Diversified generations of the same lineages.
The evolutionary advantage in adverse conditions.
Adaptations of organisms .
Evolution of new species.
For tracing the evolutionary history and classification of an organism's.
Mendel’s experiments with garden peas were a setting stone in genetics. He knew that some “factor” was responsible for the characteristics observed in organisms and each of these factors had two “variants”, only one of which was passed down from each parent to the offspring. However, he wasn’t aware of what this factor was.
Certain ground breaking experiments, including those by Fredrick Griffith (1928), Averty, MacLeod, and McCarty (1944), and by Hershey and Chase (1952) laid conclusive evidence that it is certainly the DNA, and not the RNA or proteins, which is the molecular basis of inheritance.
Nucleic acids (DNA, Deoxyribonucleic Acid and, RNA, Ribonucleic Acid) are the basis for the continuity of life. The DNA bears the information in the form of genes that are passed from the parent to the offspring. In eukaryotes, DNA is present in the nucleus and is organized into chromosomes. Each chromosome contains genes that encode specific characters for the individual. Depending upon what variant of a particular gene (i.e., what allele) is present on the chromosome, the phenotypic traits between individuals vary for that character.
Each DNA or RNA molecule is a polymer of single units known as nucleotides. Hence, the nucleic acids are polynucleotide molecules. Each nucleotide monomer contains.
A nitrogenous base (Adenine, Guanine, Cytosine or Thymine. Note that in RNA, Thymine is replaced by Uracil).
A ribose sugar (Deoxyribose sugar in DNA, Ribose sugar in RNA).
A phosphate group.
| DNA | RNA |
|---|---|
| Comprises two polynucleotide strands, coiled around a common axis in a right-handed manner | SIngle polynucleotide strand |
| Adenine pairs with Thymine | Adenine pairs with Uracil |
| Cytosine pairs with Guanine | Cytosine pairs with Guanine |
| Contains a 2’-deoxyribose sugar | Contains a ribose sugar |
| Carries hereditary information in the form of nucleotide segments known as genes | Translates the gene transcripts (mRNA) from DNA into proteins |
While DNA is the inheritable genetic material in all living organisms, there are few viruses which carry the RNA as their genetic material.
This is seen under the following conditions −.
Heredity is the transfer of characters from parents to offspring.
Variation refers to the differences among individuals of any species. It may due to genetic or environmental reasons.
Variations accumulate during sexual reproduction during the events of independent assortment of chromosomes and crossing over.
Independent assortment of chromosomes ensures random inheritance of characteristics from the parents.
Crossing over and recombination is imperative in constructing new genetic combinations and variations among individuals of the same species.
Variations are important for adaptation and evolution of species, for enhancing the biodiversity, and for.
DNA is the molecular basis of the inheritance of all organisms and carries information in the form of genes. Each gene has variant forms known as alleles which adds to the genetic variation across different individuals.
Q1. How does the genetic composition of gametes determine the sex of humans?.
Ans. In humans, sex is determined by the sex chromosomes. Males have the XY combination of sex chromosomes (one X from the mother and the Y from the father) while females carry the XX chromosome pair (one X chromosome from each parent).
Q2. Are both parents always involved in the inheritance of all characters?.
Ans. No. There are some traits, especially some diseases that are sex-linked and can be present on the sex chromosomes. Moreover, we know that the mitochondria have their DNA. This mitochondrial DNA is in fact, inherited from the mother ONLY. In the inheritance of all other nuclear traits, both parents are involved.
Q3. What is meant by sex-linked traits?.
Ans. The sex chromosomes don’t just determine the gender of an individual. They carry other alleles which encode traits such as colour blindness, haemophilia, etc.
Q4. What does the term somatic variation mean?.
Ans. An offspring acquires or inherits its genetic information from its parents. However, during the development of the zygote its somatic (non-gametic) cells may acquire mutational changes which are not integrated into the germline. These traits will not be inherited by the individual’s offspring.
Q5. Does crossing over always result in genetic variations?.
Ans. Crossing over may occur between the same allele of a gene, on the non-sister chromatids of a homologous pair. In such a case, crossing over may happen but it won't lead to any new variation unless the alleles on both chromosomes are different.
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