ch 17: gene expression - from gene to protein

gene expression

the process by which DNA directs the synthesis of protein

proteins

link between genotypes and phenotypes (DNA → RNA → protein → phenotype)

what are the two stages of gene expression?

1. transcription

2. translation

transcription

DNA → RNA

• produces messenger RNA (mRNA)

• primary transcript: initial RNA transcript prior to processing

translation

mRNA → polypeptide

where does translation take place?

in ribosomes

in a prokaryotic cell, which comes first, transcription or translation?

because there is no nucleus, these processes can happen simultaneously

in a prokaryotic cell, where does transcription and translation take place?

both in the cytoplasm

in a eukaryotic cell, what separates transcription from translation?

the nuclear envelope

what processes take place in the nucleus in a eukaryotic cell?

transcription & RNA processing

what process takes place in the cytoplasm in a eukaryotic cell?

translation

codon

a 3-nucleotide sequence (triplet) in mRNA that specifies an amino acid

template strand

used to synthesize complementary mRNA

coding strand

same sequence as mRNA, except T replaces U

how is the genetic code redundant?

multiple codons specify for the same amino acid

how many codons exist?

61 codes for amino acids + 3 stop codons = 64

which are the three stop codons?

UAA, UGA, UAG

what synthesizes RNA using a DNA template strand?

RNA polymerase

how is RNA synthesized?

synthesized in the 5’ → 3’ using complementary base pairings

promoter

DNA sequence where transcription begins

terminator

DNA sequence where transcription ends

how many stages of transcription are there?

1. initiation (start)

2. elongation (build)

3. termination (stop)

what helps to guide the binding of RNA polymerase and the initiation of transcription?

transcription factors

transcription factors + RNA polymerase II bound to a promoter to form _____

transcription initiation complex

TATA box

common promoter crucial in forming the initiation complex in eurkaryotes

to which end are RNA nucleotides added to?

3’ end

how does RNA grow?

5’ → 3’

how does RNA polymerase move?

along the template strand

polyadenylation signal

triggers release of RNA transcription

RNA processing

eukaryotic pre-mRNA is processed in the nucleus before translation (pre-mRNA → RNA processing → mature mRNA)

what are the three major modifications made to pre-mRNA to get it ready for translation?

1. 5’ cap - the 5’ end receives a modified nucleotide 5’ cap

2. 3’ poly-A tail - 3’ end gets a poly-A tail

3. RNA splicing

what functions do these modifications share?

1. facilitates the export of mRNA to the cytoplasm

2. protects mRNA from hydrolytic enzymes

3. helps ribosomes attach to the 5’ end

what is the process that removes introns and joins exons to produce mature mRNA for translation?

RNA splicing

introns

noncoding sequences between coding regions that are removed from pre-mRNA

exons

sequences retained in mature mRNA that include protein-coding regions

how are introns removed from pre-mRNA?

by spliceosomes

spliceosomes

consist of a variety of proteins + small RNAs that recognize splice sites (spliceosome → removes introns → joins exons)

alternative RNA splicing

different exons can be joined to produce different polypeptides from the same gene; allows organisms to produce more proteins than they have genes (same gene → diff exon combos → diff mRNA → diff polypeptides)

mRNA

provides codon sequence (messenger)

transfer RNA (tRNA)

carries specific amino acids

ribosomes

matches tRNA with mRNA codons to build polypeptides; consists of large and small subunits made of ribosomal RNA (rRNA) and proteins

domains

distinct functional regions in proteins

in translation, how does genetic information flow?

from mRNA sequence → proteins/amino acid sequence

1. ribosomes read codons in mRNA

2. tRNAs bring corresponding amino acids

3. amino acids are joined to form a polypeptide

what does each tRNA connect an mRNA codon to?

a specific amino acid

• each carries a specific amino acid on one end w/ an anticodon on the other end

  • the anticodon base pairs w/ a complementary codon on mRNA

what attaches the correct amino acid to its tRNA?

aminoacyl-tRNA synthetase

what does aminoacyl-tRNA synthetase require?

ATP

how many binding sites does tRNA have?

1. A site

2. P site

3. E site

A(rrival) site

incoming tRNA that carries the next amino acid

P(olypeptide) site

holds tRNA that carries growing polypeptide chain

E(xit) site

where discharged tRNAs leave the ribosome

what are the three steps of translation?

1. initiation - assemble the translation machinery

2. elongation - add amino acids to growing chain

3. termination - stop and release polypeptide

what do all three stages of translation need?

protein “factors” that aid in the translation process

initiation