Transcript Protein Synthesis-Part Two - Halton District School Board

Protein Synthesis
Mutations
Mutations
Mutation: a change in the DNA sequence that is
inherited
Mutagens: factors that can produce gene
mutations…
a) UV radiation
b) X rays
c) Chemicals such
as pesticides
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 noncoding 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.