The Molecular Basis of Inheritance

Overview: Life’s Operating Instructions

•  In 1953, James Watson and Francis Crick introduced an elegant double-helical model for the structure of deoxyribonucleic acid, or DNA

•  DNA, the substance of inheritance, is the most celebrated molecule of our time

•  Hereditary information is encoded in DNA and reproduced in all cells of the body

•  This DNA program directs the development of biochemical, anatomical, physiological, and (to some extent) behavioral traits

Fig. 16-1

Additional Evidence That DNA Is the Genetic Material

•  It was known that DNA is a polymer of nucleotides, each consisting of a nitrogenous base, a sugar, and a phosphate group

•  In 1950, Erwin Chargaff reported that DNA composition varies from one species to the next

•  Chargaff’s rules state that in any species there is an equal number of A and T bases, and an equal number of G and C bases

Fig. 16-5 Sugar–phosphate

backbone 5ʹ end

Nitrogenous bases

Thymine (T)

Adenine (A)

Cytosine (C)

Guanine (G)

DNA nucleotide Sugar (deoxyribose)

3ʹ end

Phosphate

Fig. 16-7

(c) Space-filling model

Hydrogen bond 3ʹ end

5ʹ end

3.4 nm

0.34 nm 3ʹ end

5ʹ end

(b) Partial chemical structure (a) Key features of DNA structure

1 nm

Fig. 16-7a

Hydrogen bond 3ʹ end

5ʹ end

3.4 nm

0.34 nm

3ʹ end

5ʹ end

(b) Partial chemical structure (a) Key features of DNA structure

1 nm

Fig. 16-8

Cytosine (C)

Adenine (A) Thymine (T)

Guanine (G)

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

•  In DNA replication, the parent molecule unwinds, and two new daughter strands are built based on base-pairing rules

Fig. 16-9-1

A T

G C

T A

T A

G C

(a) Parent molecule

Fig. 16-9-2

A T

G C

T A

T A

G C

A T

G C

T A

T A G C

(a) Parent molecule (b) Separation of strands

Fig. 16-9-3

A T

G C

T A

T A

G C

(a) Parent molecule

A T

G C

T A

T A G C

(c) “Daughter” DNA molecules, each consisting of one parental strand and one new strand

(b) Separation of strands

A T

G C

T A

T A

G C

A T

G C

T A

T A G C

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

Circular (mitochondria) versus linear DNA (nucleus)

Getting Started

•  Replication begins at special 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

Fig. 16-12 Origin of replication Parental (template) strand

Daughter (new) strand

Replication fork

Replication bubble

Two daughter DNA molecules

(a) Origins of replication in E. coli

Origin of replication Double-stranded DNA molecule

Parental (template) strand Daughter (new) strand

Bubble Replication fork

Two daughter DNA molecules

(b) Origins of replication in eukaryotes

0.5 µm

0.25 µm

Double- stranded DNA molecule

•  At the end of each replication bubble is a replication fork, a Y-shaped region where new DNA strands are elongating

•  Helicases are enzymes that untwist the double helix at the replication forks

•  Single-strand binding protein binds to and stabilizes single-stranded DNA until it can be used as a template

•  Topoisomerase corrects “overwinding” ahead of replication forks by breaking, swiveling, and rejoining DNA strands

Fig. 16-13

Topoisomerase

Helicase

Primase Single-strand binding proteins

RNA primer

5ʹ 5ʹ

5ʹ 3ʹ

3ʹ

3ʹ

Fig. 16-14

A

C

T

G

G

G

G C

C C

C

C

A

A

A

T

T

New strand 5ʹ end

Template strand 3ʹ end 5ʹ end 3ʹ end

3ʹ end

5ʹ end 5ʹ end

3ʹ end

Base Sugar

Phosphate

Nucleoside triphosphate

Pyrophosphate

DNA polymerase

Fig. 16-15

Leading strand

Overview Origin of replication

Lagging strand

Leading strand Lagging strand

Primer

Overall directions of replication

Origin of replication

RNA primer “Sliding clamp”

DNA poll III Parental DNA

5 ́

3 ́

3 ́

3 ́

3 ́

5 ́

5 ́

5 ́

5 ́

5 ́

•  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

Fig. 16-16 Overview

Origin of replication Leading strand

Leading strand

Lagging strand

Lagging strand

Overall directions of replication

Template strand

RNA primer

Okazaki fragment

Overall direction of replication

1 2

3ʹ

2

1

1

1

1

2

2

5ʹ 1

3ʹ

3ʹ

3ʹ

3ʹ

3ʹ

3ʹ

3ʹ

3ʹ

3ʹ

5ʹ

5ʹ

5ʹ

5ʹ

5ʹ

5ʹ

5ʹ

5ʹ

5ʹ

5ʹ

5ʹ 3ʹ

3ʹ

Fig. 16-16a

Overview Origin of replication

Leading strand

Leading strand

Lagging strand

Lagging strand

Overall directions of replication

1 2

Table 16-1

Fig. 16-17

Overview Origin of replication

Leading strand

Leading strand

Lagging strand

Lagging strand Overall directions

of replication

Leading strand

Lagging strand

Helicase

Parental DNA

DNA pol III

Primer Primase

DNA ligase

DNA pol III

DNA pol I

Single-strand binding protein

5ʹ 3ʹ

5ʹ

5ʹ

5ʹ

5ʹ

3ʹ

3ʹ

3ʹ

3ʹ 1 3 2

4

Concept 16.3 A chromosome consists of a DNA molecule packed together with proteins

•  The bacterial chromosome is a double-stranded, circular DNA molecule associated with a small amount of protein

•  Eukaryotic chromosomes have linear DNA molecules associated with a large amount of protein

•  In a bacterium, the DNA is “supercoiled” and found in a region of the cell called the nucleoid

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

•  Chromatin is a complex of DNA and protein, and is found in the nucleus of eukaryotic cells

•  Histones are proteins that are responsible for the first level of DNA packing in chromatin

Fig. 16-21a

DNA double helix (2 nm in diameter)

Nucleosome (10 nm in diameter)

Histones Histone tail H1

DNA, the double helix Histones Nucleosomes, or “beads on a string” (10-nm fiber)

Fig. 16-21b

30-nm fiber

Chromatid (700 nm)

Loops Scaffold

300-nm fiber

Replicated chromosome (1,400 nm)

30-nm fiber Looped domains (300-nm fiber)

Metaphase chromosome

Fig. 16-UN2

Sugar-phosphate backbone

Nitrogenous bases

Hydrogen bond

G

C

A T

G G

G

A

A

A

T

T

T

C

C

C

Fig. 16-UN3

DNA pol III synthesizes leading strand continuously

Parental DNA DNA pol III starts DNA

synthesis at 3ʹ end of primer, continues in 5ʹ → 3ʹ direction

Lagging strand synthesized in short Okazaki fragments, later joined by DNA ligase

Primase synthesizes a short RNA primer

5ʹ 3ʹ

5ʹ

5ʹ

5ʹ

3ʹ

3ʹ

Fig. 16-UN5

PCR Animations

https://www.youtube.com/watch?v=3XPAp6dgl14

https://www.youtube.com/watch?v=eEcy9k_KsDI

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