The Code of Life: Topic 3

Gene expression (protein synthesis)

What's in your genes?• Genes are sequences of

nucleotides along DNA strands.

• Genes (100s-1000s of nucleotides long) code for polypeptides.

• Your genotype is the actual sequence of DNA that you inherited from your parents.

• The expression of those genes results in your phenotype, or how you look.

How do we go from this to this?

The big question…

DNA An organism

The Central DogmaDNA

RNA

Polypeptide

Protein

Universal genetic code of life

RNA• There are 3 major

structural differences between RNA and DNA.

• There are 3 types of RNA involved in gene expression (protein synthesis):– mRNA– rRNA– tRNA

Types of RNA

mRNA rRNA tRNA

Gene expression (protein synthesis) Overview

Transcription: copying DNA's message

• DNA is stored safely in the nucleus, but proteins are made in the cytoplasm.

• RNA carries the instructions for proteins out of the nucleus to the cytoplasm.

• The first step is transcription.

Stages of transcription

• Three stages:– Initiation

– Elongation

– Termination

• Important notes:– RNA polymerase

does all the work

– The end product is called a primary RNA transcript

– The primary transcript is then modified before leaving the nucleus.

Practice transcribing…If your DNA strand says this…

TACAGGTCAGACTTC

What will your complementary RNA strand say?

AUGUCCAGUCUGAAG

5’ cap (GTP):•Provides stability to the mRNA

•Point of attachment for a small subunit of the ribosome during translation.

3’ poly-A tail:•Provides stability to the mRNA

•Appears to control the movement of mRNA across the nuclear membrane.

Before the RNA transcript can leave the nucleus, it has to be processed.

Step 1: Cap and tail

1. removes introns 2. joins exons, creating an mRNA molecule with a continuous coding sequence.

Step 2: RNA splicing

After transcription comes translation (protein synthesis)

Transcription Translation

Review: Types of RNA

mRNA rRNA tRNA

Translation• During translation, the code

carried in mRNA is "translated" into amino acids.

• There are 4 nucleotides and 20 amino acids.

• Clearly, the nucleotides must be combined somehow in 20 different ways.

• mRNA is "read" as codons, combinations of 3 nucleotides.

• One codon, AUG, always signals that start of a gene sequence.

• Three codons (UAA, UAG, and UGA) are stop signals, ending the formation of a polypeptide.

Take a moment to look at your chart. What observations can you make about this code?

Translating practice

• What amino acid does each of the following codons correspond to?

AAA

GCA

UGU

CAG

Lysine

Alanine

Cysteine

Glutamine

• Try these with the wheel:

CCC

AGU

UCA

GUC

Proline

Serine

Serine

Valine

tRNA: a closer look

Translation: Initiation• The 5' cap of mRNA attaches to a a small ribosome subunit. • The initiator tRNA has the anticodon for the start codon (AUG)

on mRNA.• The initiator tRNA always carries the amino acid methionine

(MET). • After the initiator tRNA hydrogen bonds to the mRNA, a large

ribosomal subunit also attaches.

Translation: Elongation

• Amino acids are added on sequentially when the appropriate tRNA matches with the next mRNA codon.

• Each new tRNA bonds its anticodon to the complementary codon on the mRNA.

• The amino acid from the old tRNA gets passed to the new amino acid on the new tRNA. They form a peptide bond.

Translation: Termination

• Once the stop codon of a sequence is reached, the whole complex comes apart and there is now a new polypeptide.

Summary

Imagine the first line is the message in DNA and the rest of the lines are the same message carried in RNA. What is going on? Pinpoint SPECIFIC errors. How do those errors affect the message?

thesunwashotbuttheoldmandidnotgethishat

the sun was hot but the old man did not get his hat

the sun was hot but the ole man did not get his hat

the sun was hot but the old man did not get his cat

the sun was hot but the old ma. did not get his hat

thd esu nwa sho tbu tth eol dma ndi dno tge thi sha t

DNA message

mRNA message

versions

(as codons)

Mutations• Mutation - any change in the DNA sequence• Causes of mutations:

– Errors in DNA replication (permanent error; 1 in every 100,000,000 bases)

– Mutagens• UV light• Radiation• Chemicals

• There are 2 major categories of mutations:– Point mutations– Frameshift mutations

Point mutations are base pair substitutions.

A base pair substitution is when the wrong nucleotide is substituted for the correct one.

These mutations may be:silent (no effect, usually last nucleotide in a codon)

missense (results in a different amino acid, effect depends on the properties of the new amino)

nonsense (premature stop codon)

mRNA transcribed from normal DNA

mR

NA

tra

nsc

ribe

d fr

om

mut

ate

d D

NA

• Frameshift mutation– Insertion– Deletion

• In this case the entire sequence after the insertion or deletion is shifted by a whole nucleotide.

• After the mutation, none of the amino acids are correct**.

• Result: non-functioning polypeptide.

** RARE case of three

nucleotides being

deleted.

mRNA transcribed from normal DNA

mR

NA

tra

nsc

ribe

d fr

om

mut

ate

d D

NA

http://www-mic.ucdavis.edu/sklab/genetic%20recomb.htm