The molecular basis of inheritance. (Chapter 16)

Overview: Life’s Operating Instructions

In 1953, James Watson and Francis Crick introduced

Figure 16.1

Concept 16.1: DNA is the genetic material

Early in the 20th century,

The Search for the Genetic Material: Scientific Inquiry

When T. H. Morgan’s

Evidence That DNA Can Transform Bacteria

The discovery of the genetic role

Living S cells (control)

Living R cells (control)

Heat-killed S cells (control)

Mixture of heat-killed S cells and living R cells

Mouse

In 1944, Oswald Avery, Maclyn McCarty, and Colin MacLeod announced that

Evidence That Viral DNA Can Program Cells

More evidence for DNA as

Animation: Phage T2 Reproductive Cycle Right-click slide / select “Play”

Figure 16.3

Phage head

Tail sheath

Tail fiber

DNA

Bacterial cell

100 nm

In 1952, Alfred Hershey and Martha Chase performed experiments showing that

Animation: Hershey-Chase Experiment
Right-click slide / select “Play”

Figure 16.4-1

Bacterial cell

Phage

Batch 1: Radioactive sulfur (35S)

DNA

Batch 2: Radioactive phosphorus (32P)

Radioactive DNA

EXPERIMENT

Radioactive protein

Figure 16.4-2

Bacterial cell

Phage

Batch 1: Radioactive sulfur (35S)

Radioactive protein

DNA

Batch 2: Radioactive phosphorus (32P)

Radioactive DNA

Empty protein shell

Phage DNA

EXPERIMENT

Figure 16.4-3

Bacterial cell

Phage

Batch 1: Radioactive sulfur (35S)

Radioactive protein

DNA

Batch 2: Radioactive phosphorus (32P)

Radioactive DNA

Empty protein shell

Phage DNA

Centrifuge

Centrifuge

Radioactivity (phage protein) in liquid

Pellet (bacterial cells and contents)

Pellet

Radioactivity (phage DNA) in

Additional Evidence That DNA Is the Genetic Material

It was known that

Animation: DNA and RNA Structure Right-click slide / select “Play”

Two findings became known as Chargaff’s rules
The base composition of DNA

Figure 16.5

Sugar–phosphate backbone

Nitrogenous bases

Thymine (T)

Adenine (A)

Cytosine (C)

Guanine (G)

Nitrogenous base

Phosphate

DNA nucleotide

Sugar (deoxyribose)

3′ end

5′ end

Building a Structural Model of DNA: Scientific Inquiry

After DNA was accepted

Figure 16.6

(a) Rosalind Franklin

Figure 16.6a

(a) Rosalind Franklin

Figure 16.6b

Franklin’s X-ray crystallographic images of DNA enabled Watson to deduce that

Animation: DNA Double Helix Right-click slide / select “Play”

Figure 16.7

3.4 nm

1 nm

0.34 nm

Hydrogen bond

(b) Partial chemical structure

3′ end

5′ end

3′

3.4 nm

1 nm

0.34 nm

Hydrogen bond

(b) Partial chemical structure

3′ end

5′ end

3′ end

5′

Figure 16.7b

(c) Space-filling model

Watson and Crick built models of a double helix to conform

At first, Watson and Crick thought the bases paired like with

Figure 16.UN01

Purine + purine: too wide

Pyrimidine + pyrimidine: too narrow

Purine +

Watson and Crick reasoned that the pairing was more specific, dictated

Figure 16.8

Sugar

Sugar

Sugar

Sugar

Adenine (A)

Thymine (T)

Guanine (G)

Cytosine (C)

Concept 16.2: Many proteins work together in DNA replication and repair

The

The Basic Principle: Base Pairing to a Template Strand

Since the two

Animation: DNA Replication Overview
Right-click slide / select “Play”

Figure 16.9-1

(a) Parent molecule

A

A

A

T

T

T

C

C

G

G

Figure 16.9-2

(a) Parent molecule

A

A

A

A

A

A

T

T

T

T

T

T

C

C

C

C

G

G

G

G

Figure 16.9-3

(a) Parent molecule

A

A

A

A

A

A

A

A

A

A

A

A

T

T

T

T

T

T

T

T

T

T

T

T

C

C

C

C

C

C

C

C

G

G

G

G

G

G

G

G

Watson and Crick’s semiconservative model of replication predicts that when a

Figure 16.10

(c) Dispersive model

Parent cell

First replication

Second replication

Experiments by Matthew Meselson and Franklin Stahl supported the semiconservative model

The first replication produced a band of hybrid DNA, eliminating the

Figure 16.11

Bacteria cultured in medium with 15N (heavy isotope)

Bacteria transferred to medium with 14N (lighter isotope)

DNA sample centrifuged after first replication

DNA sample centrifuged after

Figure 16.11a

Bacteria cultured in medium with 15N (heavy isotope)

Bacteria transferred to medium with 14N (lighter isotope)

DNA sample centrifuged after first replication

DNA sample centrifuged after

Figure 16.11b

Predictions:

First replication

Second replication

Conservative model

Semiconservative model

Dispersive model

CONCLUSION

DNA Replication: A Closer Look

The copying of DNA is remarkable in

Getting Started

Replication begins at particular sites called origins of replication, where

Animation: Origins of Replication
Right-click slide / select “Play”

Figure 16.12

(a) Origin of replication in an E. coli cell

(b) Origins

Figure 16.12a

(a) Origin of replication in an E. coli cell

Origin of replication

Parental

Figure 16.12b

(b) Origins of replication in a eukaryotic cell

Origin of replication

Double-stranded DNA

Figure 16.12c

0.5 μm

Figure 16.12d

0.25 μm

At the end of each replication bubble is a replication fork,

Figure 16.13

Topoisomerase

Primase

RNA primer

Helicase

Single-strand binding proteins

5′

3′

5′

5′

3′

3′

DNA polymerases cannot initiate synthesis of a polynucleotide; they can only

An enzyme called primase can start an RNA chain from scratch

Synthesizing a New DNA Strand

Enzymes called DNA polymerases catalyze the elongation

Each nucleotide that is added to a growing DNA strand is

Figure 16.14

New strand

Template strand

Sugar

Phosphate

Base

Nucleoside triphosphate

DNA polymerase

Pyrophosphate

5′

5′

5′

5′

3′

3′

3′

3′

OH

OH

OH

P

P i

2 P i

P

P

P

A

A

A

A

T

T

T

T

C

C

C

C

C

C

G

G

G

G

Antiparallel Elongation

The antiparallel structure of the double helix affects replication
DNA polymerases

Along one template strand of DNA, the DNA polymerase synthesizes a

Animation: Leading Strand Right-click slide / select “Play”

Figure 16.15

Leading strand

Lagging strand

Overview

Origin of replication

Lagging strand

Leading strand

Primer

Overall directions of replication

Origin of replication

RNA primer

Sliding clamp

DNA pol

Figure 16.15a

Leading strand

Lagging strand

Overview

Origin of replication

Lagging strand

Leading strand

Primer

Overall directions of replication

Origin of replication

RNA primer

Sliding clamp

DNA pol III

Parental DNA

3′

5′

5′

3′

3′

5′

3′

5′

3′

5′

3′

5′

Figure 16.15b

To elongate the other new strand, called the lagging strand, DNA

Animation: Lagging Strand Right-click slide / select “Play”

Origin of replication

Overview

Leading strand

Leading strand

Lagging strand

Lagging strand

Overall directions of replication

Template strand

RNA primer for fragment 1

Okazaki fragment 1

RNA primer for

Figure 16.16a

Origin of replication

Overview

Leading strand

Leading strand

Lagging strand

Lagging strand

Overall directions of replication

1

2

Figure 16.16b-1

Template strand

3′

3′

5′

5′

Figure 16.16b-2

Template strand

RNA primer for fragment 1

3′

3′

3′

3′

5′

5′

5′

5′

1

Figure 16.16b-3

Template strand

RNA primer for fragment 1

Okazaki fragment 1

3′

3′

3′

3′

3′

3′

5′

5′

5′

5′

5′

5′

1

1

Figure 16.16b-4

Template strand

RNA primer for fragment 1

Okazaki fragment 1

RNA primer for fragment 2

Okazaki fragment 2

3′

3′

3′

3′

3′

3′

3′

3′

5′

5′

5′

5′

5′

5′

5′

5′

2

1

1

1

Figure 16.16b-5

Template strand

RNA primer for fragment 1

Okazaki fragment 1

RNA primer for fragment 2

Okazaki fragment 2

3′

3′

3′

3′

3′

3′

3′

3′

3′

3′

3′

5′

5′

5′

5′

5′

5′

5′

5′

5′

5′

5′

2

2

1

1

1

1

Figure 16.16b-6

Template strand

RNA primer for fragment 1

Okazaki fragment 1

RNA primer for fragment 2

Okazaki fragment 2

Overall direction

Figure 16.17

Overview

Leading strand

Origin of replication

Lagging strand

Leading strand

Lagging strand

Overall directions of replication

Leading strand

DNA pol III

DNA pol III

Lagging

Figure 16.17a

Overview

Leading strand

Origin of replication

Lagging strand

Leading strand

Lagging strand

Overall directions of replication

Leading strand

DNA pol III

Primer

Primase

Parental DNA

5′

5′

3′

3′

3′

Overview

Leading strand

Origin of replication

Lagging strand

Leading strand

Lagging strand

Overall directions of replication

Leading strand

Primer

DNA pol III

DNA pol I

Lagging strand

DNA

The DNA Replication Complex

The proteins that participate in DNA replication form

Animation: DNA Replication Review
Right-click slide / select “Play”

Figure 16.18

Parental DNA

DNA pol III

Leading strand

Connecting protein

Helicase

Lagging strand

DNA pol III

Lagging strand template

5′

5′

5′

5′

5′

5′

3′

3′

3′

3′

3′

3′

Proofreading and Repairing DNA

DNA polymerases proofread newly made DNA, replacing any

Figure 16.19

Nuclease

DNA polymerase

DNA ligase

5′

5′

5′

5′

5′

5′

5′

5′

3′

3′

3′

3′

3′

3′

3′

3′

Evolutionary Significance of Altered DNA Nucleotides

Error rate after proofreading repair is

Replicating the Ends of DNA Molecules

Limitations of DNA polymerase create problems

Figure 16.20

Ends of parental DNA strands

Leading strand

Lagging strand

Last fragment

Next-to-last fragment

Lagging strand

RNA primer

Parental

Figure 16.20a

Ends of parental DNA strands

Leading strand

Lagging strand

Last fragment

Next-to-last fragment

Lagging strand

RNA primer

Parental

Figure 16.20b

Second round of replication

Further rounds of replication

New leading strand

New lagging strand

Shorter and

Eukaryotic chromosomal DNA molecules have special nucleotide sequences at their ends

Figure 16.21

1 μm

If chromosomes of germ cells became shorter in every cell cycle,

The shortening of telomeres might protect cells from cancerous growth by

Concept 16.3 A chromosome consists of a DNA molecule packed together

Chromatin, a complex of DNA and protein, is found in the

Animation: DNA Packing Right-click slide / select “Play”

Figure 16.22a

DNA double helix (2 nm in diameter)

DNA, the double helix

Nucleosome (10 nm

Figure 16.22b

30-nm fiber

30-nm fiber

Loops

Scaffold

300-nm fiber

Chromatid (700 nm)

Replicated chromosome (1,400 nm)

Looped domains (300-nm fiber)

Metaphase chromosome

Figure 16.22c

DNA double helix (2 nm in diameter)

Figure 16.22d

Nucleosome (10 nm in diameter)

Figure 16.22e

30-nm fiber

Figure 16.22f

Loops

Scaffold

Figure 16.22g

Chromatid (700 nm)

Chromatin undergoes changes in packing during the cell cycle
At interphase, some

Figure 16.23

5 μm

Figure 16.23a

Figure 16.23b

Figure 16.23c

5 μm

Most chromatin is loosely packed in the nucleus during interphase and

Histones can undergo chemical modifications that result in changes in chromatin

Figure 16.UN02

Sugar-phosphate backbone

Nitrogenous bases

Hydrogen bond

G

G

G

G

C

C

C

C

A

A

A

A

T

T

T

T

Figure 16.UN03

DNA pol III synthesizes leading strand continuously

Parental DNA

DNA pol III starts DNA synthesis

Figure 16.UN04

Figure 16.UN05

Figure 16.UN06