transcription and translation Flashcards
gene expression
the process by which DNA directs protein synthesis, includes 2 stages:
- transcription
- translation
gene expression sequence
DNA -> transcription -> RNA (mRNA) -> translation -> protein
Beadle and Tatum
the one gene-one enzyme hypothesis
- bread mold Neurospora by changing genes, bread couldn’t grow a certain protein
- different classes of these mutants were blocked at a different step in the biochemical pathway for arginine biosynthesis
- revises to one gene- one protein/polypeptide hypothesis
RNA
single stranded, uracil, ribose sugar
transcription
synthesis of RNA using information in DNA
- produces messenger RNA (mRNA)
translation
the synthesis of a polypeptide, using information in the mRNA
- ribosomes are site of translation
transcription and translation: bacteria
both can happen at the same time
transcription and translation: eukaryotes
nucleur envelope separates the two; translation happens in cytoplasm
primary transcript
the initial RNA transcript from any gene prior to processing
central dogma
cells are governed by a cellular chain of command
triplet code
a series of non overlapping, 3 -nucleotide words
- 20 amino acids, 64 codons
template strand
provides a template for ordering the sequence of complementary nucleotides in a RNA transcript
codons
base triplets; read in 5’ to 3’ direction
- 64 codons, 61 for amino acids, 3 STOP codon’s
- redundant but not ambiguous
RNA polymerase
pries DNA strands apart and joins together RNA nucleotides
- assembles in 5’ to 3’ direction
- doesn’t need a primer
- attaches to the promoter (starting DNA sequence)
- terminator (is ending sequence)
transcription unit
stretch of DNA that is transcribed
transcription factors, TATA box and terminator in eukaryotic transcription
transcription factor: mediate the binding of RNA polymerase and the initiation of transcription
promoter has TATA box, transcription initiation complex forms (transcription factors + RNA polymerase II bound to promoter)
terminator in eukaryotes: RNA polymerase II transcribes polyadenylation signal sequence (AAUAAA) - RNA released 10-35 nucleotides after
transcription: DNA -> RNA steps
- initiation
- RNA polymerase attaches to promoter - Elongation
- RNA polymerase unwinds DNA and elongates RN transcript - Termination
- RNA polymerase detaches and RNA is made
RNA processing
both ends of primary transcript are altered, enzymes in the eukaryotic nucleus modify pre-mRNA
- introns cut out, exons spiced together
- addition of 5’ cap (5’ end) and poly A-tail (3’ end)
5’ cap and poly A-tail
- facilitates export of mRNA to cytoplasm
- protects mRNA from hydrolytic enzymes
- helps ribosomes attach to 5’ end
RNA splicing
removes introns (noncoding) and joins exons (coding)
- different segments of exons used alternative RNA splicing
- carried out by splicesomes (proteins and small RNA)
Ribozymes
RNA molecules that function as enzymes
- introns can catalyze own splicing
transfer RNA (tRNA)
helps translate mRNA message into protein
- can translate a particular mRNA code into a amino acid
- has amino acid on one end and nucleotide triplet that can base-pair with complementary codon on mRNA (anticodon)
Translation two steps: tRNA
- aminoacetyl-tRNA synthesase enzyme matches tRNA with amino acid
- tRNA anticodon matched with mRNA codon
- wobble: flexible pairing of third base of a codon
Ribosomes
facilitate coupling of tRNA anticodons with mRNA codons
- large and small subunits made of proteins and ribosomal RNAs (rRNAs)
- 3 binding sites for tRNA
3 binding sites for tRNA
- P site: holds the tRNA that carries growing polypeptide chai
- A site: holds tRNA that has next AA
- E site: exit site, tRNA discharges
Three Stages of translation
initiation, elongation, termination
- energy by hydrolysis of GPT
- require protein “factors” to aid
1. intiation
-mRNA and tRNA and ribosomal subunits bind (large binds last)
2. elongation
- AA’s added to previous AA at C-terminus
- elongation factors, occur in 3 steps: codon recognition, peptide bond formation and translocation
- 5’ to 3’ direction
3. termination
- when stop codon reaches A site (accept a release factor)
elongation factors steps
- codon recognition
- peptide bond formation
- translocation
release factor
addition of water molecule instead of AA so it ends
Two types of ribosomes
free (make proteins for cytosol) and bound (make proteins for ER)
signal peptide
mark polypeptides destined for ER or for secretion
- signal recognition partial (SRP): binds to signal peptide, brings it and ribosome to ER
polyribosomes
string of ribosomes
mutations
changed in the genetic material of a cell or virus
point mutations
chemical changes in just one nucleotide pair of a gene (ex: sickle-cell disease)
- single nucleotide-pair substitutions: replaces one nucleotide and its partner with another pair of nucleotides
- nucleotide-pair insertions or deletions: additions or losses of nucleotide pairs in a gene (disastrous effect)
Substitutions
- silent: no effect on AA because of redundancy of codons
- missense: still code for an AA, but the wrong way
- nonsense: change a AA codon into a stop codon, not functioning protein
Insertion/deletion
frameshift: altered reading from genetic message
- can be missense and nonsense
mutagens
physical/chemical agents that can cause mutations
