DNA Structure

3 Key excerpts on "DNA Structure"

• 

  eBook - ePub

  Medical Genetics at a Glance

  • Dorian J. Pritchard, Bruce R. Korf(Authors)
  • 2013(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  19 DNA Structure

  Overview

  The chromosomes are composed essentially of DNA, which contains coded instructions for synthesis of every protein in the body. DNA consists of millions of nucleotides within two interlinked, coiled chains. Each nucleotide contains one of four bases and it is the sequence of these bases that contains the coded instructions (see Chapter 24 ). Each base on one chain is matched by a complementary partner on the other, and each sequence provides a template for synthesis of a copy of the other. Synthesis of new DNA is called replication (see Chapter 20 ).

  The unit of length of DNA is the base pair (bp) with 1000 bp in a kilobase (kb) and one million base pairs in a megabase (Mb). A typical human body cell contains nearly 7000 Mb of DNA.

  The structure of DNA

  We can visualize DNA as an extremely long, flexible ladder that has been twisted right-handed (like a corkscrew) by coiling around a telegraph pole. Each ‘upright’ of the ladder is a series of deoxyribose sugar molecules linked together by phosphate groups attached to their 3′ (‘three prime’) and 5′ (‘five prime’) carbon atoms. At the bottom of one upright is a 3′ carbon atom carrying a free hydroxyl (−OH) group and at the top, a 5′ carbon carrying a free phosphate group. On the other upright this orientation is reversed.

  The ‘rungs’ of the ladder are pairs of nitrogenous bases of two types, purines and pyrimidines. The purines are adenine (A) and guanine (G) and the pyrimidines are cytosine (C) and thymine (T). The bases are attached to the 1′ carbon of each sugar. Each unit of purine or pyrimidine base together with one attached sugar and one phosphate group constitute a nucleotide. A section of double-stranded DNA is therefore essentially two linked, coiled chains of nucleotides. This double helix has a major groove corresponding to the gap between adjacent sections of the sugar–phosphate chains and a minor groove

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  eBook - ePub

  Molecular Biology

  • David P. Clark(Author)
  • 2009(Publication Date)
  • Academic Cell

    (Publisher)

  Chapter Three

  DNA, RNA and Protein

  Nucleic Acid Molecules Carry Genetic Information Chemical Structure of Nucleic Acids DNA and RNA Each Have Four Bases Nucleosides Are Bases Plus Sugars; Nucleotides Are Nucleosides Plus Phosphate Double Stranded DNA Forms a Double Helix Base Pairs are Held Together by Hydrogen Bonds Complementary Strands Reveal the Secret of Heredity Constituents of Chromosomes The Central Dogma Outlines the Flow of Genetic Information Ribosomes Read the Genetic Code The Genetic Code Dictates the Amino Acid Sequence of Proteins Various Classes of RNA Have Different Functions Proteins, Made of Amino Acids, Carry Out Many Cell Functions The Structure of Proteins Has Four Levels of Organization Proteins Vary in Their Biological Roles

  Nucleic Acid Molecules Carry Genetic Information

  Chapter 1 discussed how the fundamentals of modern genetics were laid when Mendel found that hereditary information consists of discrete fundamental units now called genes. Each gene is responsible for a single inherited property or characteristic of the organism. Just as the discovery that atoms are made of subatomic particles ushered in the nuclear age, so the realization that genes are made up of DNA molecules opened the way both to a deeper understanding of life and to its artificial alteration by genetic engineering.

  Genetic information is encoded by molecules named nucleic acids because they were originally isolated from the nucleus of eukaryotic cells. There are two related types of nucleic acid, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA)

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  eBook - ePub

  Molecular Biology

  Structure and Dynamics of Genomes and Proteomes

  • Jordanka Zlatanova(Author)
  • 2023(Publication Date)
  • Garland Science

    (Publisher)

  Nucleic Acids

  DOI: 10.1201/9781003132929-4

  Learning objectives

  Information storage and transmission involve two kinds of nucleic acids: ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). Nucleic acids are polymers built on a repetitive backbone of sugar moieties linked by phosphodiester bonds. In RNA the sugar is ribose; in DNA the sugar is 2′-deoxyribose. Attached to each sugar is a basic unit, either a purine or a pyrimidine. In DNA, the purines are adenine (A) and guanine (G), and the pyrimidines are cytosine (C) and thymine (T); in RNA uracil (U) substitutes for T. The sequence of bases along the polynucleotide chain provides the information for protein structure. Here, we present the Watson and Crick double-helical structure of the DNA, with base pairing of A with T and G with C in the complementary strands. This structure of DNA allows for copying and information transfer from one cell generation to the next.

  We also describe the possible structures of circular DNA molecules, introducing the concepts of supercoiling and linking number, Lk, the total number of times one strand crosses the other. We also introduce the classes of enzymes (topoisomerases) that can change the linking number of DNA molecules in the cell.

  RNA molecules are usually found in vivo as single-stranded molecules that have been copied (transcribed) from one strand of genomic DNA. RNAs are intermediaries in the transfer of genetic information from DNA to proteins. They can also perform many other functions, from regulation of transcription to the enzyme functions of ribozymes.

  4.1 Introduction

  Protein sequences are dictated by nucleic acids

  We have seen that proteins, in their enormous variety, can play a host of roles in the cell, both structural and functional. Each protein accomplishes this by having a unique amino acid sequence, which determines its secondary, tertiary, and quaternary structures. The information that dictates these sequences must somehow be stored in the cell, expressed in proteins, and transmitted through generations of cells and organisms. These vital functions are provided by biopolymers called nucleic acids, or polynucleotides, of which there are two kinds: ribonucleic acid (RNA) and deoxyribonucleic acid (DNA

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