Lecture 10DNA Translation and Control

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

The genetic code is (almost) universal Only a few exceptions have been

found

Ribosomes

The protein-making factories of cells

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

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

How translation works

The process continues until a stop codon enters the A site

The ribosome complex falls apart and the protein is released

Play Protein Synthesis

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

Processing eukaryotic mRNA

Protect from degradation and facilitate translation

Play How Spliceosomes Process RNA

Cytoplasm

Nuclear membrane

DNA

RNA polymera

se

PrimaryRNA transcript

5’3

’5’

3’

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

5’3

’CapPoly-A

tail

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

Introns3’

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.

Play Control of Gene Expression

How protein synthesis works in eukaryotes

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

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

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

The lac Operon

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

Play Combination of Switches

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

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 materialBithorax mutant

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

Kinds of Mutation

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

Kinds of Mutation

Kinds of Mutation

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