Transcription and translation Flashcards
transcription
genetic information is transmitted from DNA to RNA
info stored in genetic code is transmitted from the DNA of a gene to messenger RNA
mRNA
synthesised by RNA polymerase II - adds appropriate complementary ribonucleotide to 3’ end
every base in molecule is complementary to a corresponding base in the DNA
template
in a gene, only one DNA strand of the double helix acts as the template strand - sense strand
antisense strand
template strand
processes involved in RNA processing
splicing
5’ capping
polyadenylation
mRNA splicing
non-coding introns in precursor mRNA are excised during/after transcription, and non-contiguous coding exons are spliced together to form shorter mature mRNA before its transportation to ribosomes
boundary between introns and exons
5’ donor GT dinucleotide
3’ acceptor AG dinucleotide
what is necessary for the splicing process?
boundary between introns and exons surrounding short splicing consensus sequences intronic sequence (branch site) small nuclear RNA molecules associated proteins
5’ capping
facilitates mRNA transport and ribosome attachment
protects RNA transcript from degradation by endogenous cellular exonucleases
after 20-30 nucleotides have been transcribed, guanine nucleotide is added to the 5’ end by 5’ to 5’ triphosphate linkage
methyltransferase enzyme methylates N7 position of guanine - final 5’ cap
polyadenylation
transcription continues until specific nucleotide sequences are transcribed that cause mRNA to be cleaved and RNA polymerase II to be released from the DNA template
20 adenylate residues (poly(A) tail) added to mRNA - nuclear export and translation
translation
transmission of genetic information from mRNA to protein
mRNA becomes associated with ribosomes
ribosomes
2 different sized subunits consisting of 4 different types of ribosomal RNA molecules and ribosomal specific proteins
polyribosomes
groups of ribosomes associated with same molecule of mRNA
what happens in the ribosomes?
mRNA forms template for producing specific sequence of amino acids for a particular polypeptide
transfer RNA
in cytoplasm
incorporation of amino acids into a polypeptide chain requires amino acids to be covalently bound by reacting with ATP to specific tRNA molecule by aminoacyl tRNA synthetase
amino acids link up through peptide bonds by peptidyl transferase
enzyme linking amino acids to tRNA
aminoacyl tRNA synthetase
enzyme linking amino acids to eachother
peptidyl transferase
post translational modification of proteins examples
chemical modification of amino acid side chains (hydroxylation, methylation)
addition of carbohydrate or lipid moieties (glycosylation)
proteolytic cleavage of polypeptides (conversion of proinsulin to insulin)
localisation sequences
certain short amino acid sequences in newly synthesised proteins
what does post translational modification and localisation sequences lead to?
transport to specific cellular locations or secretion from the cell
how many amino acids are found in proteins?
20
how many bases specify an amino acid?
3
4^3 = 64, more than enough
codon
triplet of nucleotide bases in the mRNA that codes for a specific amino acid
genetic code is non-overlapping
reading frame
order of triplet codons in a gene
degenerate code
some amino acids are coded for by more than one triplet
anticodon
each tRNA species for a specific amino acid has a specific trinucleotide sequence that’s complementary to the codon of the mRNA
number of codons and cytoplasmic tRNAs
64 codons
30 cytoplasmic tRNAs, the anticodons of a number of the tRNAs recognising codons differing at the position of the 3rd base, guanine being able to pair with uracil and cytosine
how is termination of translation of the mRNA signalled?
one of three stop codons
UAA
UAU
UGA
human RNA vs DNA
DNA is double stranded with a complementary chain. RNA is single stranded, any double stranding within itself
mRNA, rRNA, tRNA
DNA present in cells at all times, many mRNA species only accumulate after cell stimulation
subunits of ribosome
60S and 40S combine to form 80S ribosomes
subunits of subunits of ribosomes
40S: 18S + 33 proteins
60S: 28S-5.8S, 5S + 50 proteins
tRNA function and structure
carry amino acids to ribosomes, and ensure they’re incorporated into the right position
carries 1 aa each
small molecules
tRNA binding
contains an aa at its 3’ end corresponding to the codon on mRNA to which the anticodon of the tRNA can pair
genetic code is
degenerate, but unambiguous
almost universal
non-overlapping and without punctuation
degenerate and unambiguous
many amino acids specified by more than one codon
each codon specifies only one amino acid
almost universal
all organisms use same code
fewer than 10 exceptions
non-overlapping and without punctuation
codons don’t overlap
each nucleotide is only read once
start codon is AUG
stop codons are UGA, UAG, UAA
factors initiating gene expression
proteins called transcription factors find their way into specific sequences 5’ of 1st exon (promoter)
transcription complex forms around TATA box 5’ of 1st exon
helix opens, DNA strand separation
RNA pol II starts building mRNA
heterochromatin to euchromatin
DNA inaccessible to transcription factors
recruitment of histone acetyltransferases
recruitment of chromatin remodelling complexes
recruitment of DNA demethylases
hyperaccessible chromatin, genes activate
euchromatin to heterochromatin
unmethylated DNA
methylated DNA
recruitment of methyl-DNA Binding Domains (MBD)
recruitment of deacetylase complexes
silent genes
factors turning off expression
activation of repressors (inhibitors of RNA polymerase binding)
no longer actively produced transcription or processing proteins
complexes don’t form due to lack of phosphorylation
enzymes no longer activated
RNA stability
