3.4 - DNA to protein Flashcards
transcription
production of messenger RNA using DNA template and ribonucleoside triphosphates as substrates
in which direction does RNA synthesis occur?
5’-3’
difference between DNA replication and transcription? (2)
- replication - identical copies of entire genome
- transcription - selective copies of certain parts of genome into RNA
role of RNA polymerase (3)
- DNA strands separate at active site of enzyme, polymerase transiently unwinds DNA base pairs
- ribonucleotides added to 3’ end of new RNA
- newly formed RNA forms RNA-DNA double helix (about 12 base pairs in length)
3 phases of transcription (3)
- initiation
- elongation
- termination
where does transcription occur?
the nucleus
promoters
RNA polymerase recognises and binds to promoters to start transcription
promoter in eukaryotes
TATA box (binding initiated by TATA-binding protein)
how is the first transcribed nucleotide numbered?
+1, regulatory sequences upstream have -ve numbers
terminator sequences (2)
- palindromic sequences that result in formation of hairpin, causing RNA polymerase to drop off DNA
- tell RNA polymerase where to stop
prokaryotic transcription (4)
- simpler production of mRNA molecules
- prokaryotic cells lack nucleus
3.transcription/translation/degradation take place in common compartment - translation of mRNA can begin before synthesis is completed
mRNA processing (eukaryotes) (3)
- adding 5’-cap of 7-methylguanosine (protects from degradation by nucleases)
- adding 3’-poly(A) tail
- splicing out introns and rejoining exons
what performs splicing reactions in the processing of pre-mRNA?
spliceosome
alternative splicing (2)
- important means if generating protein diversity in eukaryotes
- different combinations of exons removed or retained in mature mRNA (can generate 2+ proteins from one gene)
what % of genetic diseases are caused by defects in splicing?
15%
how can defects in splicing cause genetic disease?
mutation of splice site in gene results in missing exon, giving truncated (missing functional domain) protein
retrovirus replication (5)
- virus enters host cell by binding to a cellular envelope protein
- reverse transcriptase makes DNA from RNA
- degrades RNA from DNA-RNA hybrid and replaces it with DNA
- DNA incorporated into host DNA
- host cell machinery transcribes viral genome and new viruses assembled before budding and infecting new cells
why does HIV evolve quickly?
error-prone reverse transcriptase, high rates of mutation
where does translation occur?
ribosome
codon
sequence of 3 nucleotides that encode an amino acid
what is the correct frame for most proteins?
frame initiated by the most 5’AUG start codon
tRNA function
adaptor molecule between codon and amino acid
how does tRNA bind to mRNA? (2)
- mRNA codon sequence complementary to tRNA anticodon sequence
- codon and anticodon bond via hydrogen bonding (antiparallel allignment)
what enzyme attaches amino acids to tRNA
aminoacyl tRNA synthase
what are the 4 types of mutation? (4)
- silent - dont change amino acid sequence
- missense - change resulting amino acid
- nonsense - introduce stop codon
- frameshift - insert/delete bases causing ribosome to read “out of frame”, usually will eventually hit stop codon
2 major sub-units of ribosomes (2)
- large subunit - contains peptidyl transferase centre (small subunit containing decoding centre)
- catalytic site - for peptide bond formation (is in 23S rRNA, not the protein) - ribozyme
3 tRNA binding sites (3)
- A - for aminoacyl
- P - for peptidyl
- E - for exit
(only 2 of which occupied at any one time)
which sites of tRNA are base-paired with mRNA?
A and P
polysome
each mRNA can be translated by more than one ribosome (polysome)
how many bases can 1 ribosome translate?
80
rate of translation in bacteria and eukaryotes? (2)
- bacteria - 20 amino acids/second
- eukaryotes - 2-4. amino acids/second
role of mRNA
encode amino acid sequences of proteins
tRNA
match their anticodon to mRNA while carrying specific amino acid for protein synthesis
rRNA
constituent of large and small ribosomal subunits
microRNA
regulate expression of genes
ribozymes
catalytic RNA molecules that act as enzymes
prions
- misfolded versions of normal cellular proteins
what can prions cause?
can cause normal proteins to also misfold (essentially copy themselves), can also be infectious
infectious beta sheet structure
aggregates and converts cellular prion into infectious prion
