Gene Expression Part 1 Flashcards
What is Crick’s Central Dogma
DNA contains instructions for making a protein, which are copied by RNA. RNA then uses the instructions to make a protein
What is a gene? (3)
A stretch of DNA or locus on a chromosome
Unit of inheritance unit of transcription
Where does transcription take place
The nucleus
What happens in transcription (3)
- DNA is copied to make small single stranded mRNA molecule
- mRNA modified rand then exported through nuclear pore to be translated in ribosome
- Ribosome complex found on RER
5 types of RNA
mRNA - transition molecule between gene and protein
tRNA - read triplet code on mRNA
rRNA
miRNA
non-coding mRNA
What makes up the highest percentage of RNA species?
Ribosomal RNA (rRNA) - 80% of total RNA species
Which type of RNA has the most kinds?
MRNA - 100,000 of kinds as each corresponds to a specific gene - make up around 2% of total RNA species
3 stage process of making a polynucleotide or peptide
Initiation, elongation, termination
What 3 things are needed to make a polynucleotide or peptide?
Enzyme, activated substrates, a template
3 things needed for DNA replication (enzyme, activated substrate, template)
Enzyme : ribosome
Activated substrate : amino acid (combined to make polypeptide chain)
Template : mRNA
Describe how mRNA is synthesised in the first step of transcription
- RNA polymerase binds to template strand and selects NTP with correct base attached
- RNA polymerase links bases in a chain using phosphodiester bonds
What is gene transcription initiated by?
By promoter sequences upstream
Why are promoter sequences important in transcription? (2)
Important for RNA polymerase to bind and copy gene in correct 5’ to 3’ direction
Controls the timing of transcription
What is the initiation codon
ATG (AUG) - amino acid methionine
What is an enhancer in transcription?
Another upstream sequence that controls the location and level of gene expression - act in concert with promoter
Describe elongation in transcription
1) Polymerase unwinds small bubble of DNA (3’-5’)
2) promoter binds upstream and mRNA transcript is copied- RNA nucleotides match up (5’-3’).
3) once mRNA copied, it leaves DNA template and leaves nucleus for processing
3 processes that happen to turn pre-mRNA to mature mRNA
Capping at 5’ end
Tailing or Polyadenylation at 3’ end
Splicing in middle (removes introns)
What is capping?
A 5’ -5’ linkage is created by triphosphate bride to add a cap to 5’
What is tailing or polyadenylation at 3’ end and why is it important?
Addition of polyA tail (repeat sequences of base bait A) and protects against degradation and allows mRNA to exit nucleus
What is the mRNA made up of?
Exons - coding sequences which are translated
Where does translation happen?
Ribosome (Rough ER)
What happens to 4-letter DNA code in translation?
GATC (or GAUC) converted to 20-letter protein sequence
What molecule reads the genetic code?
Ribosome
What are the stop codons of the genetic code? What do they do?
TAA, TAG, TGA
Terminates translation of protein
What does the genetic code being degenerate mean?
Some amino acids are coded for by more than one 3bp combination
Explain why there are only a few kinds of rRNA and 100,000 kinds of mRNA
Ribosomes have the same function so dont have a lot of variation therefore don’t require many different rRNA
mRNA comes from DNA template so there will be lots of variation - DNA has lots of functions so mRNA has to code for many different proteins
Why is the capacity for proof reading in RNA polymerase quite limited compared to DNA polymerase?
If there is an error in DNA it will be passed down to offspring on the germ line and all proteins will be affected
Whereas rRNA only codes for one protein so a mutation will only affect one protein and not be passed down
Why does cordycepin (3’deoxyadenosine) inhibit the synthesis of mRNA?
3’ carbon is used to extend nucleotides but if there is no oxygen, the phosphate of another nucleotide cannot attach so dna cant be replicated
