dna: the molecule of heredity - gavilan...

Copyright © 2009 Pearson Education, Inc.

Lectures byGregory Ahearn

University of North Florida

Chapter 10

DNA: The Molecule of Heredity

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.



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.



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



DNA has four nitrogen-containing bases.• Thymine• Cytosine• Adenine• Guanine



Fig. 10-1

sugar

phosphate

base = adenine sugar

phosphate

base = guanine

sugar

phosphate

base = thymine

sugar

phosphate

base = cytosine

H

HH

HH

H

N

N

N

NN

HHH

OH

CH2

OP

O O–

–O

OH

OOP

O O–

–O

H

HH

H

H

N

N

N

N

N

HHH

OH

CH2 O

H

O

H

O

OP

O O–

–O

H

H

CH3

N

N

HHH

OH

CH2 OH

O

H

OP

O O–

–O

H

H

HH

N

N

N

HHH

OH

CH2 O



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.



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.



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 ladder- like double helix, with a sugar-phosphate backbone and nucleotide pairs forming the rungs.



The Watson-Crick model of DNA structure

Fig. 10-2

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 nucleotide

(a) (b) (c)



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.


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.


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.


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.



DNA replication produces two DNA double helices, each with one original strand and 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



The basic features of DNA replication

Fig. 10-3

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

1

2

4

3



DNA replication produces two DNA double helices, each with one original strand and 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.



DNA replication keeps, or conserves, one parental DNA strand and produces one new daughter strand.• This process is called semiconservative

replication.



DNA replication is semiconservative.

Fig. 10-4

Two identical DNA double helices, each with one parental strand (blue) and one new strand (red)

One DNA double helix

DNA replication


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.



The mechanism of DNA replication, step (1)

Fig. 10-5(1)

DNAreplication bubbles



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.




Fig. 10-5(2)

replication forks

DNA helicase DNA helicase



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 phosphate of the incoming nucleotide with the sugar 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.• 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



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



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.




Fig. 10-5(5)

DNA polymerase #4

DNA polymerase #3 leaves

DNA ligase joins the daughter DNA strands together



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 mutations.


Mistakes Do Happen

DNA is damaged in a number of ways

Spontaneous chemical breakdown at body temperature

Certain chemicals (some components of cigarette smoke)


Mistakes Do Happen

UV light from the sun causes DNA damage

• DNA damage leads to uncontrollable cell division and skin cancer


Types of Mutations

Point mutation - individual nucleotide in the DNA sequence is changed

Insertion mutation - one or more nucleotide pairs are inserted into the DNA double helix

Deletion mutation - one or more nucleotide pairs are removed from the double helix


Types of Mutations

Inversion - piece of DNA is cut out of a chromosome, turned around, and re- inserted into the gap

Translocation - chunk of DNA (often very large) is removed from one chromosome and attached to another

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