The Living World - Chapter 9nvhshonorsbio.weebly.com/uploads/1/3/3/4/13346597/chapt09_lectu… · Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction

The Living World Fourth Edition

GEORGE B. JOHNSON

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

PowerPoint® Lectures prepared by Johnny El-Rady

9 How Genes Work


9.1 The Griffith Experiment

Mendel’s work left a key question unanswered:

What is a gene?

The work of Sutton and Morgan established that

genes reside on chromosomes

But chromosomes contain proteins and DNA

So which one is the hereditary material

Several experiments ultimately revealed the

nature of the genetic material


9.1 The Griffith Experiment

In 1928, Frederick Griffith discovered transformation while working on Streptococcus pneumoniae

The bacterium exists in two strains

S Forms smooth colonies in a culture dish

Cells produce a polysaccharide coat and can cause disease

R Forms rough colonies in a culture dish

Cells do not produce a polysaccharide coat and are therefore harmless


Fig. 9.1 How Griffith discovered transformation

Thus, the dead S bacteria

somehow “transformed” the live R

bacteria into live S bacteria


9.2 The Avery and Hershey-Chase

Experiments

Two key experiments that demonstrated conclusively

that DNA, and not protein, is the hereditary material

Oswald Avery and his coworkers Colin MacLeod and

Maclyn McCarty published their results in 1944

Alfred Hershey and Martha Chase published their

results in 1952


Avery and his colleagues prepared the same mixture

of dead S and live R bacteria as Griffith did

They then subjected it to various experiments

All of the experiments revealed that the properties of

the transforming principle resembled those of DNA

1. Same chemistry and physical properties as DNA

2. Not affected by lipid and protein extraction

3. Not destroyed by protein- or RNA-digesting enzymes

4. Destroyed by DNA-digesting enzymes

The Avery Experiments


Viruses that infect bacteria have a simple structure

DNA core surrounded by a protein coat

Hershey and Chase used two different radioactive

isotopes to label the protein and DNA

Incubation of the labeled viruses with host bacteria

revealed that only the DNA entered the cell

Therefore, DNA is the genetic material

The Hershey-Chase Experiment


Fig. 9.2 The

Hershey-Chase

Experiment

Thus, viral DNA

directs the

production of

new viruses


9.3 Discovering the Structure of DNA

DNA is made up of nucleotides

Each nucleotide has a central sugar, a

phosphate group and an organic base

The bases are of two main types

Purines – Large bases

Adenine (A) and Guanine (G)

Pyrimidines – Small bases

Cytosine (C) and Thymine (T)


Fig. 9.3 The four nucleotide subunits that make up DNA

5-C sugar

Nitrogenous

base


Erwin Chargaff made key DNA observations that

became known as Chargaff’s rule

Rosalind Franklin’s

X-ray diffraction

experiments

revealed that DNA

had the shape of a

coiled spring or helix

Purines = Pyrimidines A = T and C = G

Rosalind

Franklin

(1920-1958)

Fig. 9.4


In 1953, James Watson and Francis Crick deduced

that DNA was a double helix

Fig. 9.4

James Watson

(1928- )

Francis Crick

(1916-2004)

They came to their conclusion using Tinkertoy

models and the research of Chargaff and Franklin


Fig. 9.4 The DNA

double helix Dimensions

suggested by

X-ray diffraction

The two

possible

basepairs


9.4 How the DNA Molecule Replicates

The two DNA strands are held together by weak

hydrogen bonds between complementary base pairs

A and T

C and G

ATACGCAT If the sequence on one strand is

The other’s sequence must be TATGCGTA

Each chain is a complementary mirror image of the

other

So either can be used as template to reconstruct the other


There are 3 possible methods for

DNA replication

Fig. 9.5

Original DNA

molecule is

preserved

Daughter DNAs

contain one old

and one new

strand

Old and new

DNA are

dispersed in

daughter

molecules


These three mechanisms were tested in 1958 by

Matthew Meselson and Franklin Stahl

Fig. 9.6


Fig. 9.6

Thus, DNA replication

is semi-conservative


How DNA Copies Itself

The process of DNA replication can be summarized

as such

The enzyme helicase first unwinds the double

helix

The enzyme primase puts down a short piece of

RNA termed the primer

DNA polymerase reads along each naked single

strand adding the complementary nucleotide


Fig. 9.7 How nucleotides are added in DNA replication

Template strand New strand

Sugar-

phosphate

backbone

C G

T A

A T

C G

A

A

HO 3’

O

O

O

O

O

O

O

O

O

O

5’

3’ OH

5’

P

P

P

P

P

P

P

P

P

P

T

O

OH

P P P

DNA polymerase

Template strand New strand

Pyrophosphate

G C

T A

T A

C G

A T

A

HO 3’

O

O

O

O

O

O

5’

3’ OH

O

O

O

O

O

P

P

P

P

P

P

P

P

P

P

P

5’


DNA polymerase can only build a strand of DNA in

one direction

The leading strand is made continuously from one primer

The lagging strand is assembled in segments created

from many primers

Fig. 9.8


RNA primers are removed and replaced with DNA

Ligase joins the ends of newly-synthesized DNA

Mechanisms exist for DNA proofreading and repair

Fig. 9.9


9.5 Transcription

The path of genetic information is often called the

central dogma

DNA RNA Protein

A cell uses three kinds of RNA to make proteins

Messenger RNA (mRNA)

Transfer RNA (tRNA)

Ribosomal RNA (rRNA)


9.5 Transcription

Gene expression is the use of information in DNA to

direct the production of proteins

It occurs in two stages

Fig. 9.10


9.5 Transcription

The transcriber is

RNA polymerase

It binds to one DNA

strand at a site

called the promoter

It then moves along

the DNA pairing

complementary

nucleotides

It disengages at a

stop signal

Fig. 9.11


9.6 Translation

Translation converts the order of the nucleotides of

a gene into the order of amino acids in a protein

The rules that govern translation are called the

genetic code

mRNAs are the “blueprint” copies of nuclear genes

mRNAs are “read” by a ribosome in three-

nucleotide units, termed codons

Each three-nucleotide sequence codes for an

amino acid or stop signal


The genetic code is (almost) universal

Only a few exceptions have been found

Fig. 9.12


Ribosomes

The protein-making factories of cells

Fig. 9.13

Sites play key

roles in

translation

They use mRNA to direct the assembly of a protein

A ribosome is

made up of two

subunits

Each of which

is composed

of proteins

and rRNA


Transfer RNA

tRNAs bring amino

acids to the ribosome

They have two

business ends

Anticodon which is

complementary to

the codon on

mRNA

3’–OH end to

which the amino

acid attaches Fig. 9.14

Hydrogen

bonding causes

hairpin loops

3-D shape


Making the Protein

mRNA binds to the

small ribosomal

subunit

The large subunit

joins the complex,

forming the

complete ribosome

mRNA threads

through the

ribosome producing

the polypeptide Fig. 9.16


Fig. 9.15 How translation works

The process continues until a stop codon enters the A site

The ribosome complex falls apart and the protein is released


9.7 Architecture of the Gene

In eukaryotes, genes are fragmented

They are composed of

Exons – Sequences that code for amino acids

Introns – Sequences that don’t

Eukaryotic cells transcribe the entire gene,

producing a primary RNA transcript

This transcript is then heavily processed to

produce the mature mRNA transcript

This leaves the nucleus for the cytoplasm


Different combinations of exons can generate different

polypeptides via alternative splicing

Fig. 9.17 Processing eukaryotic mRNA

Protect from

degradation

and facilitate

translation


Fig. 9.18 How

protein synthesis

works in

eukaryotes

Cytoplasm

Nuclear membrane

DNA

RNA polymerase

Primary RNA transcript

5’

3’ 5’

3’

1. In the cell nucleus, RNA polymerase transcribes RNA from DNA

5’ 3’ Cap

Poly-A tail

2. Introns are excised from the RNA transcript, and the remaining exons are spliced together, producing mRNA

Introns

3’ mRNA

Exons

5’

mRNA

Cap

Nuclear pore

Poly-A tail

3. mRNA is transported out of the nucleus. In the cytoplasm, ribosomal subunits bind to the mRNA

Small ribosomal

subunit

Large ribosomal

subunit

tRNA

Amino acid

4. tRNA molecules become attached to specific amino acids with the help of activating enzymes. Amino acids are brought to the ribosome in the order dictated by the mRNA.

5. tRNAs bring their amino acids in at the A site of the ribosome. Peptide bonds form between amino acids at the P site, and tRNAs exit the ribosome from the E site.

5’

3’

Ribosome

Ribosome moves toward 3’ end

6. The polypeptide chain grows until the protetin is completed.

Completed polypeptide

7. Phosphorylation or other

chemical modifications can

alter the activity of a protein

after it is translated.


9.7 Architecture of the Gene

Most eukaryotic genes exist in multiple copies

Clusters of almost identical sequences called

multigene families

As few as three and as many as several

hundred genes

Transposable sequences or transposons are DNA

sequences that can move about in the genome

They are repeated thousands of times, scattered

randomly about the chromosomes


9.8 Turning Genes Off and On

Genes are typically controlled at the level of

transcription

In prokaryotes, proteins either block or allow the

RNA polymerase access to the promoter

Repressors block the promoter

Activators make the promoter more accessible

Most genes are turned off except when needed


The lac Operon

An operon is a segment of DNA that contains a cluster of genes that are transcribed as a unit

The lac operon contains

Three structural genes Encode enzymes involved in lactose metabolism

Two adjacent DNA elements

Promoter Site where RNA polymerase binds

Operator Site where the lac repressor binds


The lac Operon

In the absence of lactose, the lac repressor binds to

the operator

RNA polymerase cannot access the promoter

Therefore, the lac operon is shut down

Fig. 9.19


The lac Operon

In the presence of lactose, a metabolite of lactose

called allolactose binds to the repressor

This induces a change in the shape of the

repressor which makes it fall off the operator

RNA polymerase can now bind to the

promoter

Transcription of the lac operon is ON


Fig. 9.19


The lac Operon

What if the cell encounters lactose, and it already

has glucose?

The bacterial cell actually prefers glucose!

The lac operon is also regulated by an activator

The activator is a protein called CAP

It binds to the CAP-binding site and gives the

RNA polymerase more access to the promoter

However, a “low glucose” signal molecule has to

bind to CAP before CAP can bind to the DNA


Fig. 9.20 Activators

and repressors of

the lac operon


Enhancers

DNA sequences that make the promoters of genes

more accessible to many regulatory proteins at the

same time

Usually located

far away from

the gene they

regulate

Common in

eukaryotes; rare

in prokaryotes

Fig. 9.21


9.9 Mutation

The genetic material can

be altered in two ways

Recombination

Change in the

positioning of the

genetic material

Mutation

Change in the

content of the

genetic material Fig. 9.22

Bithorax mutant


9.9 Mutation

Mutation and recombination provide the raw material for evolution

Evolution can be viewed as the selection of particular combinations of alleles from a pool of alternatives

The rate of evolution is ultimately limited by the rate at which these alternatives are generated

Mutations in germ-line tissues can be inherited

Mutations in somatic tissues are not inherited

They can be passed from one cell to all its descendants


Kinds of Mutation

Mutations are caused in one of two ways

Errors in DNA replication

Mispairing of bases by DNA polymerase

Mutagens

Agents that damage DNA


Kinds of Mutation

The sequence of DNA can be altered in one of two

main ways

Point mutations

Alteration of one or a few bases

Base substitutions, insertion or deletion

Frame-shift mutations

Insertions or deletions that throw off the

reading frame


Fig. 9.23


Kinds of Mutation

The position of genes can be altered in one of two

main ways

Transposition

Movement of genes from one part of the

genome to another

Occurs in both eukaryotes and prokaryotes

Chromosomal rearrangements

Changes in position and/or number of large

segments of chromosomes in eukaryotes




Mutation, Smoking and Lung Cancer

Agents that cause cancer are called carcinogens

These are typically mutagens

The hypothesis that chemicals cause cancer was

first advanced in the 18th century

Many investigations since then have determined

that chemicals can cause cancer in both animals

and humans

For example, tars and other chemicals in

cigarette smoke can cause cancer of the lung

Documents

The Living World - Chapter 9nvhshonorsbio.weebly.com/uploads/1/3/3/4/13346597/chapt09_lectu… · Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction