Heredity and Accumulation of Variation During Reproduction: Evolution is one of the guiding principles that govern our world. Hence, it is one of the most important things for us as students of biology, to understand where all this variation comes from. In our article on meiosis, you may remember that when the sister chromatids from the two parents cross over, we get new variations of the same characteristics that we inherited from our parents.

After meiosis, this DNA ends up in our respective reproductive cells, for eg. sperms in males and ova in females. Thus, over multiple generations, more and more variations or ‘factors’ accumulate for all organisms. This process is called the accumulation of variation during reproduction.

Heredity and Accumulation of Variation During Reproduction

What is Heredity? The genetic inheritance passed down by our biological parents is referred to as heredity. That’s why we resemble them! It is the transmission of features from one generation to the next in more detail.

These characteristics can be physical or behavioural, such as eye colour, blood type, or an illness. Honeybees, for example, have an innate hygiene behaviour that causes them to remove unhealthy larvae from the nest.

Genes determine hereditary characteristics, and a single gene might have several alleles. Each gene in our cells is duplicated twice (with the exception of genes located on sex chromosomes). One copy comes from the sperm, while the other comes from the egg.

These two copies (or alleles) are not always identical in an individual. When both copies of a gene are identical, the person is said to be homozygous for that gene. The gene is heterozygous if the two copies are not the same.

A dominant or recessive relationship might exist between alleles of the same gene. When both alleles are distinct (heterozygous) and at least one of them is dominant, the dominant allele will be expressed (i.e., that we will observe as a trait in an individual).

If both parents pass down the same gene, a recessive allele (non-dominant) will not be expressed in an individual (homozygote). As a result, even though a recessive allele is present in a genotype (an individual’s genetic composition), it will not be visible in the phenotype (an individual’s collection of observable qualities) if the other copy of the gene is dominant.

Biological parents’ genes combine during reproduction to create a new unique individual. This genetic shuffle is what makes every one of us unique!

Gregor Mendel, through a series of experiments, postulated the existence of such factors that offspring inherit from their parents. The nature of those ‘factors’ governing the pattern of inheritance was not clear at the time of Mendel.

The nature of the supposed genetic material was explored during the next hundred years, ending in the discovery that DNA – deoxyribonucleic acid – is the genetic material, at least for the majority of creatures.

In biological systems, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are the two forms of nucleic acids and are the primary route via which heritable characters are passed through to the offspring. In reality, DNA is the genetic substance of almost all living things.

Although some viruses employ RNA as genetic material, it is primarily used as a messenger. RNA also serves a variety of other purposes such as an Adapter, structural, and catalytic properties. Purines (Adenine and Guanine) and Pyrimidines (Cytosine, Uracil and Thymine) are the two types of nitrogenous bases .

Purines are present in DNA and RNA. For the Pyrimidines, both DNA and RNA contain Cytosine, but DNA contains Thymine, while in RNA, Uracil takes the place of Thymine.

Molecular Basis of Inheritance (DNA and RNA)

Sugar and phosphates combine to form the backbone of a polynucleotide chain. From the backbone, nitrogenous bases are connected to the sugar moiety project. Every nucleotide residue in RNA has an extra –OH group at the 2′ position in the ribose.

In addition, uracil takes the role of thymine in RNA (5-methyl uracil, another chemical name for thymine). Friedrich Meischer discovered DNA as an acidic material found in the nucleus for the first time in 1869.

 The salient features of Double-helix structure of DNA are as follows:
(i)  It is made up of two polynucleotide chains with a sugar-phosphate backbone and bases that protrude inside.
(ii) The polarity of the two chains is anti-parallel. It indicates that if one chain is polarised 5′ → 3′, the other is polarised 3′ → 5′
(iii) Base pairs are formed when two strands’ bases are linked together by hydrogen bonds (H-bonds) (bp). Adenine and Thymine from the opposing strand make two hydrogen bonds, and vice versa. Guanine and Cytosine are also linked by three H-bonds. As a result, a purine always comes after a pyrimidine. This results in an almost constant spacing between the helix’s two strands.
(iv) The two chains are coiled in the same direction. Each double-stranded polynucleotide chain DNA double helix turn has a pitch of 3.4 nm (a nanometre is one billionth of a metre, or 10^-9 m), and each double-stranded polynucleotide chain DNA double helix turn has around 10 bp. As a result, the space between two bp in a helix is about 0.34 nm.
(v) In a double helix, one base pair’s plane is stacked on top of the other. This, in addition to H-bonds, ensures the helical structure’s resilience.

Summary

As students of biology, it is crucial to understand where variation comes from. Heredity is the transmission of features from one generation to the next in more detail. These characteristics can be physical or behavioural, such as eye colour, blood type, or an illness.

DNA and RNA are the two forms of nucleic acids, and are the primary route via which heritable characters are passed through to the offspring.

DNA is made up of two polynucleotide chains with a sugar-phosphate backbone and bases that protrude inside. Every nucleotide residue in RNA has an extra –OH group at the 2′ position in the ribose, and uracil takes the role of thymine in RNA.

Frequently Asked Questions (FAQs)

Below are the frequently asked questions on Heredity and Accumulation of Variation During Reproduction:

Q.1. What is hereditary and variation?
Ans: The genetic inheritance passed down by our biological parents is referred to as heredity.

Q.2. What are the salient features of DNA?
Ans.
(i) It is made up of two polynucleotide chains with a sugar-phosphate backbone and bases that protrude inside
(ii) Base pairs are formed when two strands’ bases are linked together by hydrogen bonds (H-bonds) (bp). Adenine and Thymine from the opposing strand make two hydrogen bonds, and vice versa. Guanine and Cytosine are also linked by three H-bonds.
(iii) The two chains are coiled in the same direction. Each double-stranded polynucleotide chain DNA double helix turn has a pitch of 3.4 nm.

Q.3. What is the molecular basis of inheritance?
Ans: DNA and RNA are the molecules that transfer genetic information across generations in living organisms, and are thus referred to as the molecular basis of inheritance.

Q.4. Where does variation arise in the cell cycle?
Ans: Meiosis is the stage in the cell cycle where variation arises because of the crossing over between homologous sister chromatids.

Q.5. Which stage of the cell cycle is involved in the transmission of genetic material?
Ans: Meiosis is the stage when the genetic information from parents is transferred to the sperm and ova, which in turn get passed on to the offspring.

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