L19- Gene expression Flashcards
trancription
copying the code
translation
changing the language
transcription occurs in the
nucleus
translation occurs in the
cytoplasm
a gene
is a stretch of DNA found at a specific locus ona. chromsosme, which codes for a protein
- unit of inheritance and transcription
DNA synthesis overview
- Needs an enzyme: RNA polymerase
- Need activate substrates: NTPs
- Need template: DNA
- 3 stage process:
- Initiation- promoter recognition. Transcription initiation factor. RNA polymerase- reads 3’ to 5’
- Elongation- 5’ to 3’ chain growth
- Termination- sequence dependent
RNA synthesis overview (transcription)
- Needs an enzyme: RNA polymerase
- Need activate substrates: NTPs
- Need template: DNA
- 3 stage process:
- Initiation- promoter recognition. Transcription initiation factor. RNA polymerase- reads 3’ to 5’
- Elongation- 5’ to 3’ chain growth
- Termination- sequence dependent
RNA translation (translation)
- Need enzyme: in the ribosome
- Activate substrate: amino acids
- Needs a template- mRNA
- 3 stage process
- Initiation- tRNA and start codon on the mRNA within the ribosome
- Elongation- moevemtn of tRNA from P site to E site. Peptide bond formation between amino acids on the top of the tRNA
- Termination- stop codon- no tRNA carries amino acid for
transcription occurs in the
nucleus- euchromatin
function of transcription
Specific gene is read and copied on an individual mRNA
transcription requires
RNA polymerase, NTPs, DNA template
outline initation of transcription
- Transcription factor binds to promotor region (e.g. TATATATA – TATA box) which lies upstream of the gene (right at the beginning)
- Promotor and gene that is transcribed = transcription unit
- TF binds from 5’ to 3’ direction- directs direction of transcription
- TF recruit RNA polymerase (does not require primer just template strand)
- Reads Template DNA from 3’ to 5’ direction
- DNA coding strand is not involved with transcription (mRNA strand will be identical, bar having uracil instead of thymine)
- RNA polymerase breaks Hydrogen bonds, creating a transcription bubble, mRNA transcript produced via the addition of complementary nucleotides
- Transcribes RNA in 5’ to 3’ direction
elongation in transcription
- mRNA elongation occurs from left to right (5’ to 3’) via the addition of new complementary nucleotides
- As RNA polymerase passes along the DNA the hydrogen bonds between the strands reform
termination of transcription
1) When RNA reaches the termination sequence of the gene, the RNA polymerase detaches from the DNA
2) mRNA is edited (see below) and proceeds out of the nucleus
the initation site
The site on the DNA from which the first RNA nucleotide is transcribed is called the +1 site- initiation site
nucleotides that come before the initiation site are
given a negative number and said to be upstream
nucleotides that come after the intiiation site are marked with positive numbers and said to be downstream
RNA processing converts
pre-mRNA to mature mRNA
3 types of RNA processing
- Capping
- Polyadenylation
- Splicing
Capping
- At 5’ end- addition of a 5’CAP
- 5’ to 5’ linkage
- Protection against degradation
polyadenylation
- At 3’ end= polyA tail added by polyA polymerase
- Protection against degradation
splicing
- removes introns- sequence dependent (e.g. 5 to 3’ splice sites on introns)
- Not random- very precise
- Always one more exon than introns
translation involves
decoding mRNA and using its info to build a polypeptide chain, which will fold to form a protein.
location of translation
ribosomes in the cytoplasm- found on the RER
ribosomes assemble proteins from
amino acids based on the sequence of the mRNA
translation requires
ribosomes and template (mRNA)
difference between prokaryote and eukaryote ribosomes
Prokaryotes
Prokaryotes
3rRNAs + 56 proteins
30S + 50S subunit
70S ribosome (related to sedimentation)
Eukaryotes
4 rRNAs +82 proteins
40S +60S subunits
80S ribosomes
how many rRNAs in eukaryotic ribosomes
4
subinits in eukaryotic ribosome
40S and 606 = 80s ribosome
rRNA
rRNA ribosomal RNA
- >80% of all RNA
- RNA polymerase I
- Few kinds
- Many copies of each
mRNA messenger RNA
- 2% of all RNA
- RNA polymerase II
- 100,000s of kinds
- Few copies of each
tRNA
- 15%
- RNA polymerase III
- 100 kinds
- Lots of copies of each
other types of RNA
miRNA
noncoding RNA
the genetic code
From 4-letter ‘DNA language’ to 20-letter ‘protein-language’
64 possibilities-only need 20
triplet code
three bases make an amino acid
e.g. UUU- phenylalaline
initiation codon always
AUG- methionine
termination codons
UAA
UAG
UGA
–> dont code for an amino acid
triplet code is
degenrative and non overlapping
mRNA is read in the
5’ to 3’ direction
- N to C polypeptide chain extension
adaptor molecule in translation
tRNA
each tRNA molecule
- tRNA recognises 1 amino acid molecule, which is bonded to the top of the molecule.
structure of tRNA
- mostly ss
- some ds regions which forms loops
- forms clover shape
- Located on the bottom of the tRNA is the anticodon (3 nucleotide bases)
- Anticodon pairs up with complementary codon on the mRNA
- amino acid binds to the 3’ end
anti codon for methyionyl tRNA recongises
mRNA codon 5’AUG
some tRNA can recognise
different codons- for the same amino acid
- degenerative nature
- wobble position
e. g.Alanyl tRNA anticodon 5’IGC, recognises the cocoons 5’GCU, 5’GCC, 5’GCA
( I nucleotide binds with different nucleotides e.g. U, C and A)
amino acid activation by tRNA
Enzymes which recognise a specific amino acid, a specific tRNA and ATP- causes AA to become attached to tRNA —> amino acid activation
translation process: initiation
- tRNA attaches to mRNA which is found within a Ribosome – causes 60S subunit to bind
- Each tRNA recognises 1 amino acid molecule, which is bonded to the top of the molecule.
- Located on the bottom of the tRNA is the anticodon (3 nucleotide bases)
- Anticodon pairs up with complementary codon on the mRNA
- As translation begins the first codon of the mRNA, the Start codon (usually AUG- methionine) attaches to a ribosome
- .tRNA with complementary amino acid can bind to three parts of the ribosome: A- aminoacyl site, P- peptidyl site and E- exit site
- tRNA carrying an anticodon complementary to the start codon (carrying a methionine) of the mRNA binds in the P site
translation: elongation
- mRNA then slides along the ribosome to the next codon, where a new tRNA carrying another amino acid will pair with the mRNA codon (A site)
- The 2 amino acids will join via a peptide bond (peptidyl transferase)
- 3)The first tRNA molecule will be released by the ribosome (from E site) and the second will slide into the P site
translation: termination
- This process continues until the ribosome meets a stop codon – no tRNA code for a stop codon- therefore translation terminated
- Ribosome with polypeptide will translocate to the ER to be packaged
what does the APE positions in the ribososme look like
