Week 1
Objectives
Describe the essential features of transcription and translation in cells of both prokaryotes (bacteria) and eukaryotes (fungi, plants and animals)
Gene Expression (transcription + translation)
The process by which DNA directs synthesis of proteins
Genes provide the information but they don’t build the protein directly o This is done by RNA
Gene expression has two stages o Transcription
o Translation
In eukaryotes, these two processes occur separately at different times
In prokaryotes, they can occur at the same time and place because there is no nuclear membrane to separate
RNA
Similar to DNA however;
o Single stranded
o Uracil instead of thymine Transcription
The synthesis of RNA from DNA
Promoter – the DNA sequence where RNA polymerase attaches and initiates transcription
Transcription factor – a collection of proteins that mediate the binding of RNA polymerase and the initiation of transcription ineukaryotes
Terminator - the DNA sequence where RNA polymerase ends transcription
Transcription unit – The stretch of DNA between the promoter and terminator that is transcribed
In gene expression, the resulting RNA transcript is called pre messenger RNA (pre- mRNA) o Further processing yields mRNA
Occurs in nucleus of eukaryotic cells
the gene determines the sequence of bases on the RNA
mRNA is created complementary to template DNA strand (rather than identical) o synthesised in an anti-parallel direction
Process
o An enzyme called RNA polymerase pries apart the two strands of DNA and joins together the complementary RNA nucleotides
Assembles the polynucleotide in a 5’ 3’ direction
Unlike DNA polymerase it is able to start the chain from scratch (doesn’t need a primer)
o A single gene can be transcribed simultaneously by several molecules of RNA polymerase following each other in a convoy
o There are three stages of transcription
Initiation
Elongation o Termination
Post transcription
o Before the pre- mRNA is used in the cytoplasm for translation, it must be converted into mRNA
o The ends are modified
The 5’ end receives and 5’cap
This is a modified version of guanine
The 3’ end receives a poly – A tail
Made up of adenine
These two additions have important functions
They facilitate the export of the mRNA from the nucleus
Protect it from degradation by enzymes
Help ribosomes attach for translation o RNA splicing takes place
Several large portions of non-coding RNA are removed
Non coding regions are called introns
Coding regions are then joined back together
Coding regions are exons
This is done by spliceosomes What’s the use of introns?
A single gene can code for more than one polypeptide as some introns are treated as exons during RNA processing (alternative RNA splicing)
They may facilitate the evolution of new and potentially beneficial proteins due to a process called exon shuffling
o They increase the probability of crossing over simply by providing larger SA
Codons
There are only 4 bases to code for 20 amino acids
Therefore nucleotide bases code in triplets
mRNA triplets are called codons o And DNA non template triplets
‘coding strand’
Identical to mRNA except for U/T
Written 5’ 3’
Translation
The synthesis of a polypeptide using information from the mRNA
Codon sequence is translated into a sequence of amino acids making up the polypeptide chain
Three times are many nucleotide bases as amino acids
Occurs at ribosomes
61 of the codons code for amino acids
o The remaining 3 code for ‘stop’ signals that mark the end o AUG codes for ‘start’ signal (as well as an amino acid)
Most polypeptides therefore begin with AUG
Some however remove it later
Understand how point and frame-shift mutations can affect production of functional gene products/proteins
Gene – A region of DNA that can be expressed to produce a final functional product that is either a polypeptide of an RNA molecule
Point mutations – a change in a single nucleotide pair of a gene Substitutions
The replacement of one nucleotide and its partner with another pair of nucleotides
May or may not have an effect due to genetic code redundancy
If it has not effect, it is called a silent mutation
If it has an effect and codes for another amino acid it is called a missense mutation o Even still this may not have a massive effect
If it codes for a stop codon, it is called a nonsense mutation
o This type of substitution mutation usually has the greatest effect as it nearly always leads to a non-functional protein
Insertions and deletions
Nucleotide pairs are added or loss from the gene
Worse than substitution as they may alter the reading frame o This is called a frameshift mutation
o Occurs when the number of nucleotides inserted or deleted isn’t a multiple of 3 o All nucleotides downstream from the altered section will be improperly grouped
into codons How mutations arise
Spontaneous mutations
o Errors during DNA replication or recombination
Mutagens
o Radiation
o Chemical mutagens
