Anjali Mishra Content Writer-SME
Content Writer-SME
Molecular Basis of Inheritance is one of the important chapters of CBSE class 12 biology that covers a detailed information of the genetic material and its importance in molecular biology. Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are the two molecules that governs the molecule basis of inheritance in living organisms. Molecular Basis of Inheritance explains how offspring acquire the characteristics of maternal or paternal parents.
DNA
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Friedrich Meischer first discovered the transforming principle in genes and named it as DNA. Deoxyribonucleic acid or DNA consists of two long polynucleotide chains which forms the molecular basis of inheritance. The polynucleotide chain coils around each other to form a helix structure. The number of nucleotide pairs determines the length of DNA.
- DNA carries genetic information from parents to their offsprings.
- The genetic code that DNA carries is very unique and varies from person to person.
- It is also called molecular basis of life as genes are the specific segment of DNA and chromosomes are composed of genes.
Structure of Polynucleotide Chain
A single nucleotide has three main components, which include a Nitrogenous base, Pentose sugar (ribose just in the case of RNA, and deoxyribose in DNA), and a phosphate group. There are two types of nitrogenous bases, namely Purines and Pyrimidines.
- Adenine and Guanine are called purines while cytosine, uracil and thymine are called pyrimidines.
- Thymine is found in DNA, and uracil is found in RNA in the place of thymine.
- A nitrogenous base is linked to the pentose sugar through a N-glycosidic linkage to make a nucleoside.
- A phosphate group is linked to five OH groups of a nucleoside through phosphodiester linkage.
- Two nucleotides are linked through 3-5 phosphodiester to make a dinucleotide.
- A polymer formed at the 5’-end of ribose sugar is referred to as the 5’-end of the polynucleotide chain.
- Similarly, at the opposite end of the polymer, the ribose features a free 3' -OH group, which is a 3'-end of the polynucleotide chain.
Polynucleotide Chain
The Search for Genetic Material
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The search for genetic material itself has a long history. According to Mendel, there were certain molecules which were responsible for the inheritance of genes which he later named factors. However, he could not explain the exact mutational molecule passed from parents to their offspring. Later, with the Hershey and Chase experiment, it was proven that DNA is the genetic material that controls the molecular basis of inheritance of traits. The experiment also concluded that RNA is found as genetic material in some viruses.
DNA as Molecular Basis of Inheritance
DNA Packaging
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DNA is tightly packed up in such a way that six feet of this long molecule fit into the microscopic nucleus of a cell. The process starts when DNA is wrapped around special protein molecules called histones. The combined loop of DNA and protein is named a nucleosome.
- Nucleosomes are packaged into a thread, usually described as "beads on a string".
- Chromatin fibres are formed as end product of DNA packaging.
- Chromatin fibre is coiled into a structure called a "solenoid".
- This fibre is then looped and coiled, leading finally to the familiar shapes referred to as chromosomes.
- It can be seen within the nucleus of dividing cells.
- They form round cells when two copies of the cell's DNA are separated.
- It is still bound up around the histones but not coiled into chromosomes.
DNA Replication
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DNA replication is a process where DNA strands make multiple copies of itself. The process of synthesis is initiated at particular points within the DNA strand, referred to as origins. There are multiple origin sites where DNA replication takes place. These sites are mentioned as replication forks.
- There are four important steps involved in the process of DNA replication: initiation, unwinding, primer synthesis, and elongation.
- Within the replication, DNA helicase is the complex enzyme, which can be used as a template for replication.
- DNA Primase is another enzyme that's important in DNA replication.
RNA
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RNA, also known as Ribonucleic acid, is a crucial biological macromolecule that is present in biological cells. It is principally involved in the synthesis of proteins, carrying the messenger instructions from DNA. RNA contains the genetic instructions required for the event and maintenance of life.
- In some viruses, RNA, instead of DNA, carries genetic information.
- m-RNA, t-RNA and r-RNA are three different types of RNA.
- It is responsible for the formation of a template for the synthesis of protein.
- RNA is used for catalyzing biological reactions.
Genetic Code
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Genetic Code is defined as the combination of three letters of nucleotides called codons. It uses four bases of DNA, namely the A, C, G, and Ts. They are strung together in a way that the cellular machinery, the ribosome, can read them and switch them into a protein.
- In the ordering ,each three nucleotides is counted in a row count as a triplet.
- It consists of a Code for one amino alkanoic acid.
- Genetic Codes translate information within genetic material into proteins.
- The codons will specify which amino acid is added during protein biosynthesis.
Central Dogma
The process by which the instructions in DNA are converted into a functional product is called Central Dogma. It was first proposed in 1958 by Francis Crick, discoverer of the structure of DNA. The central dogma of molecular biology explains the flow of genetic information from DNA to RNA. It forms a functional product and a protein.
- The central dogma suggests that DNA contains the knowledge needed to form all of our proteins.
- RNA may be a messenger that carries this information to the ribosomes.
- It is divided into general transfers, unknown transfers, and special transfers.
- The flow of central dogma is given as
DNA –> RNA –> Protein
Human Genome Project
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Human Genome Project (HGP) was an international, collaborative research program whose goal was the complete mapping and understanding of all the genes of human beings. All our genes together are known as our "genome." The main goals of the Human Genome Project were first articulated in 1988.
- A special committee of the U.S. National Academy of Sciences was formed.
- It adopted a detailed series of five-year plans jointly written by the National Institutes of Health and the Department of Energy.
- The project aims to sequence euchromatic regions of the nuclear genome.
- It was used in molecular medicine and human evolution.
Things to Remember
- Molecular Basis of Inheritance is transmitted by molecules called factors.
- The entire process of inheritance is governed by DNA and RNA.
- The pentose sugar and phosphate group are the backbone of the polynucleotide chain.
- Frederick Griffith uses the example of mice to explain the phenomenon.
- Nucleotides are made of three components found inside the DNA and RNA.
Previous Year Questions
- DNA strands are antiparallel because of the presence of
- Removal of introns and joining of exons in a defined order during transcription is called
- Alec Jeffreys developed the DNA finger printing technique. The probe he used was:
- Aminoacylation of tRNA is essential for
- The basis of DNA finger-printing is
- Histones are rich in
- In Hershey and Chase experiments, radioactive 32P was used to culture bacteriophages which resulted in radioactive:
- In the double-helical structure of DNA, the pitch of the helix is
- The 3' - 5' phosphodiester linkages inside a polynucleotide chain serve to join
- A nucleotide is made up of
- During DNA replication in prokaryotes DNA is anchored to
Sample Questions
Ques. What are genes? (2 marks)
Ans. Genes are small units of DNA that contain instructions on how to make proteins. A chromosome contains many genes. It is made up of DNA. It is responsible for making molecules called proteins. Gene conducts various chemical reactions within the body. The globin gene is responsible for the creation of haemoglobin. Each human carries two copies of the gene.
Ques. What are mutations? (2 marks)
Ans. Mutations are permanent changes to the DNA. While some mutations cause disease, many other mutations do not impact health. It will make duplicate copies of large sections of the chromosome. Mutation will create extra copies of genes. The genes are found inside the genome. During the process, half of the protein-coding genes are harmful.
Ques. What is recombination? (2 marks)
Ans. Recombination is an important process that can help repair broken DNA, and help shuffle the DNA when making eggs and sperm. The broken DNA are recombined to produce to set of alleles. It highlights the difference in DNA of different living organisms. It occur during the process of meiosis. In the first stage, alignment of maternal and paternal combies with each other.
Ques. What does dominant vs recessive traits? (3 marks)
Ans. The difference between dominant and recessive traits are as follows:
| Dominant Traits | Recessive Traits |
|---|---|
| Dominant traits are formed when one connected alleles is dominant in nature. | Recessive traits are formed when both connective alleles are recessive in nature. |
| It is denoted by capital letter. | It is denoted by small letter. |
| For example: dark hair of human | For example: blond hair of human |
Ques. What is peptide bond? How is it formed? (3 marks)
Ans. Peptide bond is formed between carboxylic group (COOH) of first amino acid & amino group (-NH2) of second amino acid. This reaction is catalysed by peptidyltransferase. It links two alpha amino acid with only one amino acid. It will form a covalent bond and release water molecule as byproducts. The new product of amino acid formed in peptide bond are called dipeptide.
Ques. Give two reasons why both the strands of DNA are not copied during DNA transcription? (3 marks)
Ans. The reason why both the strands of DNA are not copied during transcription are as follows:
- If both the strands code for RNA two different RNA molecules & two different proteins wouldbe formed hence genetic machinery would become complicated
- Since the two RNA molecules would be complementary to each other, they would wind togetherto form dsRNA without carrying out translation which means process of transcription would befutile.
Ques. State the four criteria which a molecule must fulfill to act as a genetic material? (4 marks)
Ans. The four criteria which a molecule must fulfill to act as a genetic material are as follows:
- It should be able to generate its replica.
- Molecule should be chemically and structurally stable.
- It should be able to express itself in the form of Mendelian characters.
- Molecule should provide the scope for slow changes (mutations) that are necessary for evolution.
Ques. Why is it essential that tRNA binds to both amino acids & mRNA codons during protein synthesis? (2 marks)
Ans. It is essential that tRNA binds to both amino acids & mRNA codon because tRNA acts as an adapter molecule with picks up a specific activated aminoacid from the cytoplasm & transferred it to the ribosomal in the cytoplasm where proteins are synthesized. It attracts itself to ribosome with the sequence specified by mRNA & finally it transmits its amino acid to new polypeptide chain.
Ques. Give six points of difference between DNA and RNA in their structure/chemistry and function? (5 marks)
Ans. The difference between DNA and RNA are as follows:
| DNA | RNA |
|---|---|
| DNA are double stranded molecules. | RNA are single stranded molecules. |
| It uses thymine as pyrimidine base, | It uses uracil as pyrimidine base |
| In DNA, pentose sugar is deoxyribose | In RNA, sugar is ribose. |
| It is quite stable and not very reactive. | Its two-OH bond is reactive, |
| DNA dictates the synthesis of Polypeptides. | RNA perform their functions in protein synthesis. |
| It is found in the nucleus. | They are transported into the cytoplasm. |
Ques. What are the salient features of the Double-helix structure of DNA? (5 marks)
Ans. The salient features of the Double-helix structure of DNA are as follows:
- It's made from two polynucleotide chains, where the backbone is constituted by sugar-phosphate, and therefore the bases project inside.
- The two chains have antiparallel polarity. It means, if one chain has the polarity 5'→3', the other has 3’→5'.
- The bases in two strands are paired through chemical bond (H-bonds) forming base pairs (bp). Adenine forms two hydrogen bonds with Thymine from opposite strand and vice-versa. Similarly, Guanine is bonded with Cytosine with three H-bonds.
- As a result, always a purine comes opposite to a pyrimidine.
- This generates approximately uniform distance between the 2 strands of the helix
- The 2 chains are coiled during a right-handed fashion.
- The pitch of the helix is 3.4 nm (a nanometre is one billionth of a metre, that's 10-9 m) and there are roughly 10 bp in each turn.
- Consequently, the space between a bp during a helix is approximately adequate to 0.34 nm.
- The plane of 1 nucleotide stacks over the opposite in helix . This, additionally to H-bonds, confers stability of the helical structure
Ques. What is the difference between recombination and crossing over? (3 marks)
Ans. The difference between recombination and crossing over are as follows:
| Recombination | Crossing Over |
|---|---|
| Recombination refers to production of alleles in case of offspring. | Crossing over refers to exchnage of genetic material between parent cells. |
| It take place during the process of cross over. | It take place in prophase one in meiosis. |
| Recombination leads to the formation of genetic variation. | Crossing over leads to the formation of recombination. |
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