Molecular Basis of Inheritance (1) PDF

# Cytology and General Biology ## DNA is the Genetic Material - DNA is a polymer of nucleotides, each consisting of a nitrogenous base, a sugar, and a phosphate group - The nitrogenous bases can be adenine (A), thymine (T), guanine (G), or cytosine (C) - In 1950, Erwin Chargaff reported that DNA...

# Cytology and General Biology ## DNA is the Genetic Material - DNA is a polymer of nucleotides, each consisting of a nitrogenous base, a sugar, and a phosphate group - The nitrogenous bases can be adenine (A), thymine (T), guanine (G), or cytosine (C) - In 1950, Erwin Chargaff reported that DNA composition varies from one species to the next - This evidence of molecular diversity among organisms made DNA a more credible candidate for the genetic material - Two findings became known as Chargaff's rules - The base composition of DNA varies between species - In any species the number of A and T bases is equal and the number of G and C bases is equal - The basis for these rules was not understood until the discovery of the double helix ## Building a Structural Model of DNA - After DNA was accepted as the genetic material, the challenge was to determine how its structure accounts for its role in inheritance - Maurice Wilkins and Rosalind Franklin used a technique called X-ray crystallography to study molecular structure - Franklin produced a picture of the DNA molecule using this technique - Franklin's X-ray crystallographic images of DNA allowed James Watson to deduce that DNA was helical - The X-ray images also enabled Watson to deduce the width of the helix and the spacing of the nitrogenous bases - The pattern in the photo suggested that the DNA molecule was made up of two strands, forming a **double helix** - Watson and Crick built models of a double helix to conform to the X-rays and chemistry of DNA - Franklin had concluded that there were two outer sugar-phosphate backbones, with the nitrogenous bases paired in the molecule’s interior - Watson built a model in which the backbones were **antiparallel** (their subunits run in opposite directions) - At first, Watson and Crick thought the bases paired like with like (A with A, and so on), but such pairings did not result in a uniform width - Instead, pairing a purine (A or G) with a pyrimidine (C or T) resulted in a uniform width consistent with the X-ray data - Watson and Crick reasoned that the pairing was more specific, dictated by the base structures - They determined adenine (A) paired only with thymine (T), and guanine (G) paired only with cytosine (C) - The Watson-Crick model explains Chargaff's rules: in any organism the amount of A = T, and the amount of G = C ## DNA Replication - The copying of DNA is called DNA replication ### The Basic Principle: Base Pairing to a Template Strand - Since the two strands of DNA are complementary, each strand acts as a template for building a new strand in replication - This yields two exact replicas of the "parental" molecule - Watson and Crick’s **semiconservative model** of replication predicts that when a double helix replicates, each daughter molecule will have one old strand (derived or “conserved” from the parent molecule) and one newly made strand - Competing models were the conservative model (the two parent strands rejoin) and the dispersive model (each strand is a mix of old and new) ## DNA Replication: A Closer Look - The copying of DNA is remarkable in its speed and accuracy - More than a dozen enzymes and other proteins participate in DNA replication - Replication in bacteria is best understood, but evidence suggests that the replication process in eukaryotes and prokaryotes is fundamentally similar ### Initiation of DNA replications - Replication begins at particular sites called **origins of replication**, where the two DNA strands are separated, opening up a replication “bubble” - A eukaryotic chromosome may have hundreds or even thousands of origins of replication - Replication proceeds in both directions from each origin, until the entire molecule is copied - At the end of each replication bubble is a **replication fork**, a Y-shaped region where parental DNA strands are being unwound - **Helicases** are enzymes that untwist the double helix at the replication forks - **Single-strand binding proteins** bind to and stabilize single-stranded DNA - **Topoisomerase** relieves the strain of twisting of the double helix by breaking, swiveling, and rejoining DNA strands ### Synthesizing a New DNA Strand - DNA polymerases require a primer to which they can add nucleotides - The initial nucleotide chain is a short **RNA primer** - This is synthesized by the enzyme **primase** - The completed primer is five to ten nucleotides long - The new DNA strand will start from the 3’ end of the RNA primer - Enzymes called **DNA polymerases** catalyze the synthesis of new DNA at a replication fork - Most DNA polymerases require a primer and a DNA template strand - The rate of elongation is about 500 nucleotides per second in bacteria and 50 per second in human cells - Each nucleotide that is added to a growing DNA strand is a **nucleoside triphosphate** - **dATP** supplies adenine to DNA and is similar to the ATP of energy metabolism - The difference is in their sugars: dATP has deoxyribose while ATP has ribose - As each monomer joins the DNA strand, via a dehydration reaction, it loses two phosphate groups as a molecule of pyrophosphate ### Antiparallel Elongation - The **antiparallel** structure of the double helix affects replication - DNA polymerases add nucleotides only to the free 3’ end of a growing strand; therefore, a new DNA strand can elongate only in the 5’ → 3’ direction - Along one template strand of DNA, the DNA polymerase synthesizes a **leading strand** continuously, moving toward the replication fork - To elongate the other new strand, called the **lagging strand**, DNA polymerase must work in the direction away from the replication fork - The lagging strand is synthesized as a series of segments called **Okazaki fragments**, which are joined together by **DNA ligase** ### The DNA Replication Complex - The proteins that participate in DNA replication form a large complex, a “DNA replication machine” - The DNA replication machine may be stationary during the replication process - Recent studies support a model in which DNA polymerase molecules “reel in” parental DNA and extrude newly made daughter DNA molecules - The exact mechanism is not yet resolved ## Proofreading and Repairing DNA - DNA polymerases **proofread** newly made DNA, replacing any incorrect nucleotides - In **mismatch repair** of DNA, repair enzymes replace incorrectly paired nucleotides that have evaded the proofreading process - DNA can be damaged by exposure to harmful chemical or physical agents such as cigarette smoke and X-rays; it can also undergo spontaneous changes - In **nucleotide excision repair**, a **nuclease** cuts out and replaces damaged stretches of DNA ### Summary of key concepts: many proteins work together in DNA replication and repair.