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

Copyright © 2009 Pearson Education Inc 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.  Individual traits of an organism are transmitted from parent to offspring in discrete units of DNA called ________.  Genes are located on chromosomes found within the _________ of cells.  What makes all organisms different from each other is the arrangement and molecular composition of its genes.

Copyright © 2009 Pearson Education Inc What Is The Structure Of DNA?  DNA is composed of four different subunits, called ___________. Each nucleotide has three parts: A __________ group A sugar, called ___________ One of four different nitrogen-containing ______ Thymine Cytosine Adenine Guanine

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

Copyright © 2009 Pearson Education Inc What Is The Structure Of DNA?  A DNA molecule contains _____ nucleotide strands composed 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.  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 What Is The Structure Of DNA?  _________ bonds between bases hold the two DNA strands together in a double helix. The combined strands of DNA form a ladder-like double ______, with a sugar-phosphate backbone and nucleotide pairs forming the rungs.  Nucleotide rungs only result in specific pair combinations. _________ only pairs with thymine. _________ only pairs with cytosine. These A–T and G–C pairs are called complementary base pairs.

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

Copyright © 2009 Pearson Education Inc 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. 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 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. Fig free nucleotides The parental DNA is unwound New DNA strands are synthesized with bases complementary to the parental strands Each new double helix is composed of one parental strand (blue) and one new strand (red) Parental DNA double helix  DNA replication produces two DNA double helices, each with one original strand and one new strand. This 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 How Is DNA Copied? Fig Two identical DNA double helices, each with one parental strand (blue) and one new strand (red) One DNA double helix DNA replication  DNA replication keeps, or conserves, one parental DNA strand and produces one new daughter strand. This process is called _______________ replication.

Copyright © 2009 Pearson Education Inc What Are The Mechanisms Of DNA Replication? Fig. 10-5(1) DNA replication bubbles  STEP 1: DNA _____ase separates the parental DNA strands by breaking the hydrogen bonds between complementary bases. This activity separates the two strands and forms a ___________ bubble where the parental strands are no longer paired. Replication then proceeds.

Copyright © 2009 Pearson Education Inc What Are The Mechanisms Of DNA Replication?  STEP 2: Fig. 10-5(2) replication forks DNA helicase 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 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 and matches them up with free nucleotides. It then links up the __________ of the incoming nucleotide with the ________ of the previously added nucleotide, thereby contributing to the growing molecule backbone.  DNA helicase and DNA polymerase work together to copy each strand of separated parental DNA.

Copyright © 2009 Pearson Education Inc What Are The Mechanisms Of DNA Replication?  STEP 3: 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. Fig. 10-5(3) DNA polymerase #1 free sugar end of the parental DNA free phosphate end of the parental DNA strand DNA polymerase #2

Copyright © 2009 Pearson Education Inc What Are The Mechanisms Of DNA Replication?  STEP 4: 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. Fig. 10-5(4) DNA polymerase #1 continues along the parental DNA strand DNA polymerase #3 DNA polymerase #2 leaves

Copyright © 2009 Pearson Education Inc What Are The Mechanisms Of DNA Replication?  STEP 5: 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 __________ joins together the separate segments of DNA. Fig. 10-5(5) DNA polymerase #4 DNA polymerase #3 leaves DNA ligase joins the daughter DNA strands together

Copyright © 2009 Pearson Education Inc 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. Mistakes that remain in the DNA nucleotide sequence are called ___________.