Protein Synthesis and Selection Flashcards
Nucleic acids involved, Protein synthesis, Transcription, Translation, The genetic code, Gene mutaions (Substitution and Deletion), Causes of mutations, Chromosome mutations, Natural selection, Directional selection, Stabilising selection, Disruptive selection
Where does transcription occur?
In the nucleus of eukaryotic cells and the cytoplasm of prokaryotic cells
Introns
Non-coding, intervening sequences within a gene of DNA
Exons
Coding sequences within a gene of DNA
What is the first step in transcription?
DNA Helicase attaches to and unwinds the DNA double helix at site to be transcribed
What is the second step in transcription?
H bonds between the two strands are broken so the DNA strands separate
What is the third step in transcription?
One strand is used as a template. This is called the template strand
What is the fourth step in transcription?
Complimentary free RNA nucleotides are attracted to the exposed DNA bases and join via base pairing on the template strand
What is the fifth step in transcription?
RNA Polymerase joins the free RNA nucleotides together
What is the sixth step in transcription?
The sequence formed is pre-mRNA and is complimentary to the DNA. It contains introns and exons
What is the seventh step in transcription?
Introns are removed from the pre-mRNA by splicing
What is the eighth step in transcription?
The exons are spliced back together to produce mRNA containing only coding regions of the gene
What is the ninth step in transcription?
The mRNA travels out of the nucleus via the nuclear pore
Where does translation take place?
In the cytoplasm of a cell
What is the first step in translation?
The 1st two codons of mRNA bind to the ribosome. (The first codon is the start codon AUG)
What is the second step in translation?
tRNA with the complimentary anticodon (UAC) moves to the ribosome and binds to the codon
What is the third step in translation?
The tRNA carries a specific amino acid, for the start codon this is methionine
What is the fourth step in translation?
The 2nd tRNA aniticodon binds to the second complimentary codon on the mRNA
What is the fifth step in translation?
A peptide bond forms between the two amino acids
What is the sixth step in translation?
The ribosome moves along to the third codon on the mRNA
What is the seventh step in translation?
The first tRNA leaves the ribosome to collect another methionine from the ‘pool’ of amino acids in the cell
What is the eighth step in translation?
The process continues until a stop codon is reached
What is the genetic code?
. It is universal - the same codons code for teh same amino acid in all organisms
. It is non-overlapping - the ribosome reads each base only once in the codon, the first three are read then the next three etc..
. It is degenerate - More than one triplet codes for an amino acid
Change in the base/nucleotide sequence of a gene
Mutation
What are the two ways a mutation in DNA replication could occur?
. Substitution - one DNA molecule is exchanged for another
. Deletion - a DNA molecule is lost
What are the consequences of a substitution mutation?
. A silent mutation would cause no change in the amino acid produced due to the degenerate code
. On the other hand a silent mutation could cause a different amino acid to be produced ultimately changing the protein produced
. It could also cause an early stop codon prematurely ending the polypeptide chain so it will probably not be able to perform it’s proper function
What are the consequences of a deletion mutation/frame shift mutation?
. If the mutation occurs in an exon it will result in an alteration of the base triplets from the mutation onwards
. If the deletion occurs in an intron there will be no effect because the intron will be removed before it becomes mRNA
What are the causes of mutations?
They can occur spontaneously at a set rate. Mutagenic agents increase the likelihood of mutations
Name 5 mutagenic reagents
- High energy radiation
- Ionising radiation
- UV light
- Tar in cigarettes
- Asbestos/cadmium
What is a chromosome mutation?
. Sometimes the reduction in chromosome number from diploid to haploid goes wrong and daughter cells are produced containing too many chromosomes
. A pair of homologous chromosomes may fail to separate during anaphase 1 or sister chromatids fail to separate in anaphase 2
The number of different allelles in a population
Genetic diveresity/gene pool
What do mutations do to the gene pool?
Mutations introduce new alleles so increase the size of the gene pool
Name three mutations could do:
. Provide a new selective advantage
. Be neutral
. Be disadvantageous
What is natural selection?
Process by which organisms that are better adapted to their environment survive and and reproduce in greater numbers, resulting in the increase of the frequency of the advantageous allele within the population
How does natural selection occur?
. There is variation in the gene pool because there are mutations within the DNA
. Better adapted individuals are more likely to survive and reproduce
. These organisms are more likely to pass on their advantageous alleles
. Organisms are therefore subject to selection pressure due to the abiotic/biotic factors environment they live in
. Selection pressure determines the spread of an allele within the gene pool
Which type of selection tends to occur when environmental conditions change?
Directional selection
Which value is more likely to survive?
The extreme/The best suited for the new change
Give an example of directional selection
Antibiotic resistance
Which type of selection tends to occur when environmental conditions are stable for long periods of time
Stabilising
What happens to the mode in directional selection?
It moves toward the extreme
What happens to the mode in stabilising selection?
It stays the same
Which value is favoured?
The mean
What does stabilising selection do the gene pool?
It reduces the size of the gene pool
Give an example of stabilising selection
Birth mass in humans
Which value is favoured in disruptive selection?
The two extremes
Give an example of disruptive selection
Coho salmon
