Genetics Exam 3: Genetic Code & Transcription

What are general features of the genetic code

Written in linear form using ribonucleotide bases that compose mRNA

What is a codon

three ribonucleotides created for one specific amino acid

What is the triplet code made of

64 codons to specify 20 amino acids

What does it mean by the genetic code being unambiguous and degenerate

unambiguous = Each triplet specifies only one amino acid

degenerate = an amino acid can be specified by more than one triplet codon

What does it mean by the genetic code being comma-less?

once translation begins, codons are read with no break

Which amino acids are coded by a single codon

tryptophan and methionine

What is the role of mRNA (messenger RNA)

Serves as intermediate messenger in transferring genetic information from DNA to proteins

What is the wobble hypothesis

The initial two ribonucleotides of triplet codes are often more critical than the third

Which codon is the initiator

AUG/methionine

-AUG only codes for methionine

What are the termination codons?

UAG, UAA, and UGA

What is a nonsense mutation?

a premature stop codon where translation is terminated, producing a partial polypeptide

What is an example of a nonsense mutation?

dwarfism in American Angus

Which organelle is an exception to the universal genetic code?

Mitochondria

-UGA codes tryptophan and AUA codes methionine

What can overlapping genes do

create multiple proteins from the same RNA strand

What is an open reading frame (ORF)

–DNA sequence that produces RNA with start and stop

–Series of triplet codons specify amino acids to make polypeptide

What happens during transcription

RNA is synthesized from DNA template

What is the role of RNA polymerase

directs synthesis of RNA using DNA template

-does not require a primer

What is reverse transcription

making DNA from RNA, which needs reverse transcriptase as the enzyme

What does transcription result in?

ssRNA (singled-stranded RNA)

How does transcription begin

template binding by RNA polymerase at promoter

What is a promoter?

Specific DNA sequences in 5′ region upstream of initial transcription point

What happens at the transcription start site?

DNA double helix is denatured and unwound to make template strand accessible for RNA polymerase

What happens during chain elongation?

Ribosomes are added to RNA chain and σ subunit dissociates from holoenzyme

What directs chain elongation

the core enzyme

What happens during termination in bacteria

termination transcribed into RNA causes newly formed transcript to fold back on itself (hairpin)

What does rho-dependent termination depend on?

the rho termination factor

How does transcription occur in eukaryotes

-happens in nucleus

-mRNA must leave

-chromatin uncoils to become accessible to RNA polymerase

What controls transcription regulation in eukaryotes

enhancers & silencers

Which forms of RNA polymerase do eukaryotes have

I, II and III

What is the role of RNA polymerase II

Responsible for transcription of wide range of genes in eukaryotes

-activity depends on cis and trans acting factors

What is the TATA box

A core-promoter element

Binds TATA-binding protein (TBP) of transcription factor TFIID: determines start transcription start site

What are enhancers and silencers

-Enhancers increase transcription levels; silencers decrease them

-Modulate transcription from a distance; can either increase or decrease it

What are general transcription factors

Required for all RNAP II-mediated transcription

What are transcription activators and repressors

Influence efficiency or rate of RNAP II transcription initiation

What are the 3 posttranscriptional modifications that occurs in eukaryotes

-Adding 7mG cap to 5’ position

-Add poly A tail to 3’

-Splicing of introns

What are introns

Regions of initial RNA transcript not expressed in amino acid sequence of protein

What are exons

sequences that are retained and expressed

What happens when introns are removed?

Exons join in together in mature mRNA

T/F: mature mRNA is smaller than initial RNA

True, because mature mRNA has its introns removed (lost “stuffing”)