MOLECULAR BASIS OF INHERITANCE - 5.1 THE DNA - DNA结构与多核苷酸链

Sep 2, 2026

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DNA is introduced as the genetic material for most organisms and contrasted with RNA’s messenger, adapter, structural, and catalytic roles. The chapter focuses on DNA as a long deoxyribonucleotide polymer, defining length via nucleotide count or base pairs and linking it to organism-specific characteristics. It explains the three-part nucleotide components, purines and pyrimidines, nucleoside-to-nucleotide formation, phosphodiester bond linking, and the 5′-end/3′-end orientation. It also recalls historical discovery and the Watson-Crick double-helix model with base pairing and complementarity.

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MOLECULAR BASIS OF INHERITANCE

5.1 The DNA

5.2 The Search for Genetic Material

5.3 RNA World

5.4 Replication

5.5 Transcription

5.6 Genetic Code

5.7 Translation

5.8 Regulation of Gene Expression

5.9 Human Genome Project

5.10 DNA Fingerprinting

WsCndkmodlfSenkconservatlve DCA np&aa # MOLECULAR BASIS OFINHERITANCE ## 5.1 The DNA In the previous chapter,you have learnt the inheritancepatterns and the genetic basis of such patterns.At thetime of Mendel,the nature of those ‘factors'regulatingthe pattern of inheritance was not clear.Over the nexthundred years,the nature of the putative genetic materialwas investigated culminating in the realisation thatDNA-deoxyribonucleic acid-is the genetic material,atleast for the majority of organisms.In class XI you havelearnt that nucleic acids are polymers of nucleotides. Deoxyribonucleic acid (DNA)and ribonucleic acid(RNA)are the two types of nucleic acids found in livingsystems.DNA acts as the genetic material in most of theorganisms.RNA though it also acts as a genetic materialin some viruses,mostly functions as a messenger.RNAhas additional roles as well.It functions as adapter,structural,and in some cases as a catalytic molecule.InClass XI you have already learnt the structures ofnucleotides and the way these monomer units are linkedto form nucleic acid polymers.In this chapter we are goingto discuss the structure of DNA,its replication,the processof making RNA from DNA(transcription),the genetic codethat determines the sequences of amino acids in proteins,the process of protein synthesis (translation)andelementary basis of their regulation.The determination 5.2 The Searchfor GeneticMaterial 5.3 RNAWorld 5.4 Replication 5.5 Transcription 5.6 Genetic Code 5.7 Translation 5.8 Regulation ofGeneExpression 5.9 Human Genome Project 5.10 DNA Fingerprinting of complete nucleotide sequence of human genome during last decadehas set in a new era of genomics.In the last section,the essentials ofhuman genome sequencing and its consequences will also be discussed. Let us begin our discussion by first understanding the structure ofthe most interesting molecule in the living system,that is,the DNA.Insubsequent sections,we will understand that why it is the most abundantgenetic material,and what its relationshipis with RNA. 5.1 THE DNA DNA is a long polymer of deoxyribonucleotides.The length of DNA isusually defined as number of nucleotides (or a pair of nucleotide referredto as base pairs)present in it.This alsois the characteristic ofan organism.For example,a bacteriophage known as φ×174 has 5386 nucleotides,Bacteriophage lambda has 48502 base pairs (bp),Escherichia coli has 4.6×10⁶bp,and haploid content of human DNA is 3.3×10⁹bp.Let usdiscuss the structure of such a long polymer. 5.1.1 Structure of Polynucleotide Chain Let us recapitulate the chemical structure ofa polynucleotide chain(DNAor RNA).A nucleotide has three components -a nitrogenous base,apentose sugar (ribose in case of RNA,and deoxyribose for DNA),and aphosphate group.There are two types of nitrogenous bases -Purines(Adenine and Guanine),and Pyrimidines(Cytosine,Uracil and Thymine).Cytosine is common for both DNA and RNA and Thymine is present inDNA.Uracil is present in RNA at the place of Thymine.A nitrogenousbase is linked to the OH of 1'C pentose sugar through a N-glycosidiclinkage to form a nucleoside,such as adenosine or deoxyadenosine,guanosine or deoxyguanosine,cytidine or deoxycytidine and uridine ordeoxythymidine.When a phosphate group is linked to OH of 5'C of anucleoside through phosphoester linkage,a corresponding nucleotide(or deoxynucleotide depending upon the type of sugar present)is formed.Two nucleotides are linked through 3-5 phosphodiester linkage to forma dinucleotide.More nucleotides can bejoined in such a manner to forma polynucleotide chain.A polymer thus formed has at one end a free Figure 5.1 A Polynucleotide chain MOLECULAR BASIS OF INHERITANCE phosphate moiety at 5′-end of sugar,which is referred to as 5'-end ofpolynucleotide chain.Similarly,at the other end of the polymer the sugarhas a free OH of 3'C group which is referred to as 3'-end of thepolynucleotide chain.The backbone of a polynucleotide chain i

FAQs

What roles does RNA play compared with DNA?

DNA serves as the genetic material in most organisms. RNA acts as a genetic material in some viruses and primarily functions as a messenger, adapter, structural component, and in some cases a catalytic molecule.

How is the length of DNA commonly defined?

DNA length is usually defined by the number of nucleotides, or by the number of base pairs when a pair of nucleotides is considered as one base-pair unit.

What determines base pairing and complementarity in DNA’s double helix?

Base pairing allows one strand’s sequence to predict the other strand’s sequence, and daughter DNA produced from each template strand is identical to the parental DNA. Chargaff’s observations show constant ratios between adenine-thymine and guanine-cytosine in double-stranded DNA.

Sep 2, 2026

Upload by Gloria