Protein SynthesisMutations

Mutagens: factors that can produce gene mutations…

a) UV radiationb) X raysc) Chemicals such

as pesticides

Mutation: a change in the DNA sequence that is inherited

Mutations

Point Mutations

• Change in one base of the DNA sequence.

Nonsense mutation: A mutation that converts a codon from an amino acid into a STOP codon

An example of a Nonsense mutation: Cystic fibrosis. •A huge gene encodes a protein of 1480 amino acids called the cystic fibrosis transmembrane conductance regulator (CFTR).• The protein is responsible for transporting chloride ions out of cells.

• In some cases of cystic fibrosis, the substitution of a T for a C at nucleotide 1609 converts a glutamine codon (CAG) to a STOP codon (TAG).

• The protein produced has only the first 493 amino acids of the normal chain of 1480 and cannot function.

• Over 1000 possible mutations have been found in individuals with cystic fibrosis

Missense mutation: A mutation that results in the substitution of one amino acid in the resulting polypeptide

A nonsense mutation results in premature termination of translation, therefore the protein is inactive.

A missense mutation interferes with the normal 3D shape of the protein, and makes it completely or partially inactive.

Silent mutation: has no effect on the polypeptide sequence of the protein

Frame shift mutations

• Mutations that change the reading frame of the DNA sequence.

• Insertions: extra base pairs are added to the DNA

• Deletions: base pairs are removed from the DNA

• Usually result in different amino acids being incorporated into the polypeptide

• They often create new STOP codons.• If the insertions or deletions are in groups of

three nucleotides, then extra amino acids may be added to the protein, or amino acids may be lost from the protein.

• Huntington’s Disease: In this disorder, the repeated trinucleotide is CAG, which adds a string of glutamines to the encoded protein (called huntingtin).

• The abnormal protein interferes with synaptic transmission in parts of the brain and leads to the death of these brain cells.

Chromosomal mutations

1. Duplications:• Doubling of a section of the genome. Usually

harmless, and may lead to evolution because the duplication is free to mutate.

2. Deletion: may result in the loss of one or more genes. Effect is usually lethal.

3. Translocation: This is where information from one of two homologous chromosomes breaks and binds to the other. Usually this sort of mutation is lethal

4. Inversion: The order of the genes is changed. • The new sequence may not produce a viable

organism, depending on which genes are reversed.

• Advantageous characteristics from this mutation are also possible

FREQUENCY OF MUTATIONS:

• Mutations are rare events. • Humans inherit 3 x 109 base pairs of DNA from each

parent. • This means that each cell has 6 billion (6 x 109)

different base pairs that can be the target of a point mutation. Point mutations are most likely to occur when DNA is being copied (S phase of cell cycle)

• It has been estimated that in humans and other mammals, mutations occur at the rate of about 1 in every 50 million (5 x 107) nucleotides.

• With 6 x 109 base pairs in a human cell, that means that each new cell contains some 120 new mutations.

• But as much as 97% of our DNA does not encode anything. The wobble effect also results in many silent mutations.

• Determine whether or not the following mutations would be harmful. Translate the mRNA sequence into protein to help you decide. The mutation is indicated in red.

a) AUG UUU UUG CCU UAU CAU CGU

AUG UUU UUG CCU UAC CAU CGU

What kind of mutation is this? What is the effect on the polypeptide?

b) AUG UUU UUG CCU UAU CAU CGU

AUG UUU UUG CCU UAA CAU CGU

What kind of mutation is this? What is the effect on the polypeptide?

c) AUG UUU UUG CCU UAU CAU CGU

AUG UUU CUU GCC UUA UCA UCG U

What kind of mutation is this? What is the effect on the polypeptide?

The process of Protein synthesis

Transcription

Genetic Code

• Reads as a long series of codons that have no spaces and never overlap.

• Each sequence of nucleotides has a correct reading frame, or grouping of codons. This means that knowing where to start transcription and translation is essential.

• There is no mechanism for re-setting transcription or translation if they do not start at the right place.

Transcription

• A section of DNA is copied as messenger RNA • Occurs in the nucleus• Involves four steps:– Initiation: finding the right place to start– Elongation: adding ribonucleotides– Termination: finding the right place to stop– Processing: getting the mRNA ready to go into

cytoplasm..

Messenger RNA

• Like all RNAs, Uracil replaces thymine• Is copied from one DNA strand• Is linear and single stranded

Initiation

• Enzyme: RNA polymerase• Attaches ribonucleotides in the 5’ to 3’

direction. • Only reads the template strand of DNA

(3’ to 5’ strand)• Attaches to the promoter site on the DNA

strand

Promoter site

I) lies upstream of a DNA sequence that represents a gene, therefore, it acts as a signal for RNA polymerase to bind and transcribe the gene found downstream;

II) it is also an area where DNA can be unwound more easily due to its high concentration of adenine and thymine.

• Promoter site: contains a high concentration of adenine and thymine bases.

• Often called a TATA box. • Since adenine and thymine only

share two double bonds between them, RNA polymerase will expend less energy in opening up the double helix at this point.

• RNA polymerase recognizes and attaches to these promoter sites. This ensures that transcription begins at the right place.

Elongation• RNA polymerase attaches new nucleotides to

the 3’-OH group of the previous nucleotide• RNA Polymerase opens the DNA double helix

one section at a time. As the polymerase molecule passes, The DNA helix re-forms and the mRNA strand separates from the DNA

• A new RNA polymerase can bind to the promoter site and begin transcription before the first is done. This speeds up the process.

• RNA polymerase has no proofreading function.

• Transcription is less accurate than replication but errors only affect one protein molecule.

• Lack of proofreading speeds up the process of transcription.

Termination

• RNA polymerase continues along the DNA strand until it reaches the terminator sequence.

• These sequences are very specific and cause the RNA polymerase to release the mRNA and to dissociate (fall off) the DNA strand.

RNA processing

• Occurs only in EUKARYOTES• The mRNA that is released from the DNA

strand is called the RNA transcript.• Before it can move into the cytoplasm and

undergo translation:– It will be protected by capping and tailing– The mRNA will be spliced to remove any non-

coding area.

Capping

• The 5’ end of the mRNA transcript is capped by the addition of a modified form of the Guanine nucleotide.

Poly A tail

• At the 3’ end of the mRNA transcript, a song series of A nucleotides are added by the enzyme Poly A polymerase.

• This forms a long POLY A tail.

• Functions of the cap and tail:–1. Protect the mRNA from enzymes in the

nucleus that break down nucleic acids. The longer the Poly A tail, the greater the life span of the mRNA.–2. Serve as signals that help bind the

molecules that synthesize proteins.

mRNA splicing

• The eukaryotic genome has exons (expressed regions) and introns (intervening non-coding sections)

• The introns must be removed before the mRNA proceeds to translation.

• The spliceosome is a large molecule formed from proteins and snRNAs. (small nuclear RNAs)

• The spliceosome cleaves the mRNA transcript at the ends of each introns and then splices the remaining exons.

• Different splicing patterns can occur and exons can be assembled in different orders.

mRNA export

• The mRNA is now exported from the nucleus into the cytoplasm, through the nuclear pores

• The rate of export can be controlled by the nuclear pores, since it occurs as a form of active transport.

• This level of gene regulation is poorly understood.