Chapter 10 DNA The Molecule of Heredity Lectures

Chapter 10 DNA: The Molecule of Heredity Lectures by Gregory Ahearn University of North Florida Copyright © 2009 Pearson Education, Inc.

10. 1 What Is The Structure Of DNA? § Deoxyribonucleic acid (DNA) is the blueprint of life. § DNA carries the information in its molecular structure, which codes for all the special features of a given life form. Copyright © 2009 Pearson Education Inc.

10. 1 What Is The Structure Of DNA? § Individual traits of an organism are transmitted from parent to offspring in discrete units of DNA called genes. § Genes are located on chromosomes found within the nucleus of cells. § What makes all organisms different from each other is the arrangement and molecular composition of its genes. Copyright © 2009 Pearson Education Inc.

10. 1 What Is The Structure Of DNA? § DNA is composed of four different subunits, called nucleotides. • Each nucleotide has three parts: • A phosphate group • A sugar, called deoxyribose • One of four different nitrogen-containing bases Copyright © 2009 Pearson Education Inc.

10. 1 What Is The Structure Of DNA? § DNA has four nitrogen-containing bases. • • Thymine Cytosine Adenine Guanine Copyright © 2009 Pearson Education Inc.

10. 1 What Is The Structure Of DNA? O O O– P –O phosphate CH 2 O H H H O– O phosphate CH 2 H H phosphate H O N OH H sugar Copyright © 2009 Pearson Education Inc. H O H H H O base = thymine N H base = guanine H O– H O phosphate H N H P –O N H OH H sugar O O N H CH 2 N O N H O N CH 3 P –O –O H base = adenine OH H sugar O N H P H N H O O– CH 2 H H N H H O N N H H O base = cytosine OH H sugar Fig. 10 -1

10. 1 What Is The Structure Of DNA? § A DNA molecule contains two nucleotide strands. • A DNA molecule consists of two DNA strands of linked nucleotides. • Within each strand, the phosphate group of one nucleotide binds to the sugar group of another nucleotide. • The sugar-phosphate bonding produces a sugar-phosphate backbone to the DNA molecule. Copyright © 2009 Pearson Education Inc.

10. 1 What Is The Structure Of DNA? § All the nucleotides in a single DNA strand are oriented in the same direction. • The ends of the two DNA strands are different. • One strand ends in an unbonded sugar. • One strand ends in an unbonded phosphate. Copyright © 2009 Pearson Education Inc.

10. 1 What Is The Structure Of DNA? § Hydrogen bonds hold the two DNA strands together in a double helix. • The two DNA strands are held together by hydrogen bonding between the protruding bases of the separate strands. • The combined strands of DNA form a ladderlike double helix, with a sugar-phosphate backbone and nucleotide pairs forming the rungs. Copyright © 2009 Pearson Education Inc.

10. 1 What Is The Structure Of DNA? § The Watson-Crick model of DNA structure nucleotide free phosphate base (cytosine) sugar free sugar (a) Hydrogen bonds hold complementary base pairs together in DNA Copyright © 2009 Pearson Education Inc. (b) Two DNA strands form a double helix (c) Four turns of a DNA double helix Fig. 10 -2

10. 1 What Is The Structure Of DNA? § Nucleotide rungs only result in specific pair combinations. • Adenine only pairs with thymine. • Guanine only pairs with cytosine. • These A–T and G–C pairs are called complementary base pairs. Copyright © 2009 Pearson Education Inc.

10. 1 What Is The Structure Of DNA? PLAY Animation—DNA Structure Copyright © 2009 Pearson Education Inc.

10. 2 How Does DNA Encode Information? § It is NOT the number of different subunits that code for all the diversity of characteristics among organisms, but it is rather the sequence in which they are arranged along the molecule. § Within a DNA molecule, the bases can be arranged in any sequence. § Each sequence is a unique set of genetic instructions. Copyright © 2009 Pearson Education Inc.

10. 2 How Does DNA Encode Information? § A stretch of DNA only 10 nucleotides long can have more than 1 million possible sequences of the four bases. § Since a typical organism has millions (e. g. , a bacterium) or billions (e. g. , a plant or animal) of nucleotides, DNA molecules can encode an incredible amount of information. Copyright © 2009 Pearson Education Inc.

10. 3 How Is DNA Copied? § Cells reproduce themselves by making two daughter cells from each parental cell, each with a complete copy of all the parental cell’s genetic information. § During cell reproduction, the parental cell synthesizes two exact copies of its DNA through a process called DNA replication. § One copy goes into each daughter cell. Copyright © 2009 Pearson Education Inc.

10. 3 How Is DNA Copied? § DNA replication produces two DNA double helices, each with one original strand one new strand. • DNA replication requires three ingredients: • The parental DNA strands • Free nucleotides that were synthesized in the cytoplasm and then imported to the nucleus • A variety of enzymes that unwind the parental DNA double helix and synthesize new DNA strands Copyright © 2009 Pearson Education Inc.

10. 3 How Is DNA Copied? § The basic features of DNA replication 1 Parental DNA double helix 2 The parental DNA is unwound 3 New DNA strands are synthesized with bases complementary to the parental strands free nucleotides 4 Each new double helix is composed of one parental strand (blue) and one new strand (red) Copyright © 2009 Pearson Education Inc. Fig. 10 -3

10. 3 How Is DNA Copied? § DNA replication produces two DNA double helices, each with one original strand one new strand (continued). • The first step involves enzymes called DNA helicases, which pull apart the parental DNA double helix. • Next, enzymes called DNA polymerases move along each separated parental DNA strand, matching each base on the strand with free nucleotides. Copyright © 2009 Pearson Education Inc.

10. 3 How Is DNA Copied? § DNA replication keeps, or conserves, one parental DNA strand produces one new daughter strand. • This process is called semiconservative replication. Copyright © 2009 Pearson Education Inc.

10. 3 How Is DNA Copied? § DNA replication is semiconservative. One DNA double helix DNA replication Two identical DNA double helices, each with one parental strand (blue) and one new strand (red) Copyright © 2009 Pearson Education Inc. Fig. 10 -4

10. 4 What Are The Mechanisms Of DNA Replication? § DNA helicase separates the parental DNA strands by breaking the hydrogen bonds between complementary bases. • This activity separates the two strands and forms a replication bubble where the parental strands are no longer paired. • Replication then proceeds. Copyright © 2009 Pearson Education Inc.

10. 4 What Are The Mechanisms Of DNA Replication? § The mechanism of DNA replication, step (1) replication bubbles DNA Copyright © 2009 Pearson Education Inc. Fig. 10 -5(1)

10. 4 What Are The Mechanisms Of DNA Replication? § DNA helicase separates the parental DNA strands. • There is a replication fork on each end of the bubble, where replication is taking place and the original DNA strand is unzipping. • The unzipping and replication continues in both directions until the new strands are completely formed. Copyright © 2009 Pearson Education Inc.

10. 4 What Are The Mechanisms Of DNA Replication? § The mechanism of DNA replication, step (2) DNA helicase replication forks Copyright © 2009 Pearson Education Inc. Fig. 10 -5(2)

10. 4 What Are The Mechanisms Of DNA Replication? § DNA polymerase synthesizes new DNA strands. • At the replication forks, DNA polymerase recognizes unpaired nucleotide bases in the parental strand matches them up with free nucleotides. • It then links up the phosphate of the incoming nucleotide with the sugar of the previously added nucleotide, thereby contributing to the growing molecule backbone. Copyright © 2009 Pearson Education Inc.

10. 4 What Are The Mechanisms Of DNA Replication? § DNA helicase and DNA polymerase work together to copy each strand of separated parental DNA. • Polymerase # 1 lands on one strand of DNA and follows behind the helicase toward the free phosphate end of the DNA, making a continuous new DNA strand. • DNA polymerase # 2 on the other parental strand moves away from the helicase and makes only part of the new DNA strand. Copyright © 2009 Pearson Education Inc.

10. 4 What Are The Mechanisms Of DNA Replication? § The mechanism of DNA replication, step (3) DNA polymerase #1 free sugar end of the parental DNA polymerase #2 Copyright © 2009 Pearson Education Inc. free phosphate end of the parental DNA strand Fig. 10 -5(3)

10. 4 What Are The Mechanisms Of DNA Replication? § As the helicase continues to unwind more of the double helix, additional DNA polymerase (# 3, # 4, etc. ) must land on this strand to synthesize more pieces of DNA. § Therefore, DNA synthesis on the second parental strand is discontinuous. Copyright © 2009 Pearson Education Inc.

10. 4 What Are The Mechanisms Of DNA Replication? § The mechanism of DNA replication, step (4) DNA polymerase #1 continues along the parental DNA strand DNA polymerase #2 leaves DNA polymerase #3 Copyright © 2009 Pearson Education Inc. Fig. 10 -5(4)

10. 4 What Are The Mechanisms Of DNA Replication? § Multiple DNA polymerases make many pieces of DNA of varying lengths that need to be tied together to form a single continuous DNA polymer. § DNA ligase joins together the separate segments of DNA. Copyright © 2009 Pearson Education Inc.

10. 4 What Are The Mechanisms Of DNA Replication? § The mechanism of DNA replication, step (5) DNA polymerase #3 leaves DNA polymerase #4 Copyright © 2009 Pearson Education Inc. DNA ligase joins the daughter DNA strands together Fig. 10 -5(5)

10. 4 What Are The Mechanisms Of DNA Replication? PLAY Animation—DNA Replication Copyright © 2009 Pearson Education Inc.

10. 4 What Are The Mechanisms Of DNA Replication? § Proofreading produces almost error-free replication of DNA. • DNA polymerase is almost 100% perfect in matching free nucleotides with those on the original parental strands. • Once in every 10, 000 base pairs, there is an error in replication. • Some types of DNA polymerase recognize errors when they are made and correct them. • This keeps the total errors in a complete DNA molecule to one mistake in every billion base pairs. Copyright © 2009 Pearson Education Inc.

10. 4 What Are The Mechanisms Of DNA Replication? § Mistakes that remain in the DNA nucleotide sequence are called mutations. Copyright © 2009 Pearson Education Inc.