Transcription and Translation
DNA and RNA
RNA -- ribose, uracil
DNA -- deoxyribose, thymine
The linear sequence of nucleotides in DNA determines the linear sequence of amino
acids in a protein.
Transcription -- The synthesis of RNA using DNA as a template.
A gene’s nucleotide sequence is transcribed from DNA to a complementary
sequence in messenger RNA (mRNA)
Translation -- Synthesis of a polypeptide (protein) from mRNA
The sequence of bases in mRNA is translated into the linear sequence of amino
acids in a protein.
Occurs on ribosomes
Prokaryotes Transciption and translation are coupled
Eukaryotes
pre-mRNA (primary transcript), 2 step process
Fig 17.2
Triplet codes
Codons -- A three nucleotide sequence in mRNA that specifies an amino acid or
signals termination of a polypeptide
4 nucleotides = 43 or 64 triplet (codons) possibilities
Fig 17.3
61 of 64 triplets code for amino acids
AUG signals “start” and codes for methionine
Three codons do not code for amino acids but signal
termination (UAA, UAG, and UGA)
Fig 17.4
Reading Frame
Fig 17.5
Redundancy (several codons with variations in the third base can code for the
same amino acid)
but no ambiguity (codons code for only one amino acid)
Transcription of mRNA from DNA is catalyzed by RNA polymerases which:
Separates the two DNA strands and link RNA nucleotides
Add nucleotides only to the 3” end
Prokaryotes have only one type of RNA polymerase for mRNA,
Eukaryotes have three
tRNA, and rRNA
RNA polymerase II catalyzes mRNA synthesis
Transcription Unit = Initiation sequence, termination sequence, and nucleotides in
between.
Eukaryotes -- Transcription unit = one gene, one polypeptide
Prokaryotes -- Transcription unit can = several genes
several proteins
Fig 17.6
Three steps to transcription
1. Binding and initiation
2. Elongation
3. Termination
Binding and Initiation
Promoter = region on DNA where RNA polymerase binds and
(initiation site)
Start Point = where transcription begins
transcription begins
In eukaryotes, RNA polymerases cannot recognize the promoter without the help of
transcription factors - DNA binding proteins that bind to specific DNA nucleotide
sequences at the promoter.
Transcription initiation complex-- transcription factors and RNA ploymerase bound to
the promoter.
TATA box = A short nucleotide sequence of A’s and T’s located upstream from the
initiation site
Fig 17.7
RNA polymerase II recognizes the complex between TATA box, transcription
factors, and DNA binding site.
Elongation Fig 17.6
1. RNA polymerase untwists and opens DNA
2. Links incoming RNA nucleotides to the 3” end
3. mRNA peels away from DNA template
4. DNA reseals
5. Several RNA polymerases can act at once
Termination
Transcription proceeds until RNA polymerase reaches a termination site on the
DNA
Terminator sequence--RNA sequence
In eukaryotes the most common is AAUAAA
Prokaryotes mRNA is ready for translation
Eukaryotes mRNA is processed before it leaves the nucleus
RNA Processing
RNA modifications in eukaryotes
Before eukaryotic mRNA is exported from the nucleus:
1.
both ends are covalently altered
2.
intervening sequences are removed and the
remainder is spliced together
Fig 17.8
5' cap--added to 5' end
Protects the growing mRNA from degradation
functions as "attach here" signal for ribosomes
Poly A tail--added to 3' end
inhibits degradation of mRNA
facilitates export to cytoplasm
mRNA splicing
Fig 17.9
RNA Splicing = removes introns and joins exons
Introns = noncoding sequences in DNA
Exons = Coding sequences in DNA
snRNPs ("Snurps") = small nuclear ribonucleoproteins = small nuclear RNA
(snRNA) and protein that are involved in RNA splicing
Spliceosome = large molecular complex that catalyzes RNA splicing
Fig 17.10
Ribozymes-- self-splicing, intron RNA catalyzing its own splicing
Translation
Transfer RNA (tRNA)--transfers amino acids from the cytoplasmic pool to a
ribosome.
Fig 17.11
Anticodon =
triplet in tRNA that base pairs with triplet
in mRNA
Fig 17.12
tRNA is about 80 nucleotides long
hydrogen bonded
anticodon
L shaped
amino acid binding site -- 3’ end
Wobble = relaxation in base-pairing rules, third base in mRNA codon can vary with
corresponding base in tRNA (U of tRNA can pair with either A or G in 3 rd position of
mRNA)
some tRNAs contain inosine (I can pair with U, C or A)
45 distinct types of tRNA
Aminoacyl-tRNA synthetase = type of enzyme that catalyzes the attachment of an
amino acid to its tRNA
Fig 17.13
Ribosomes = 60% rRNA and 40% protein
17.14
two subunits, large and small
constructed in nucleolus
Passed through nuclear pores to cytoplasm
assemble into functional ribosomes only when
attached to an mRNA
P site
A site
E site
Building a polypeptide
Initiation
17.15
Initiation complex
1.
Small subunit binds initiator tRNA (with methionine)
Small subunit binds special recognition sequence of
mRNA complementary to sequence on rRNA
2.
Large subunit binds to small one
initiator tRNA fits into the P site
powered by GTP
Elongation
17.16
Termination
16.15
Termination codon (stop codon)
UAA, AUG, UGA
When stop codon reaches the A site a protein release factor binds to the
codon and initiates:
1. hydrolysis of the bond between tRNA and the polypeptide
2. Subunits, mRNA dissociate
Polyribosome -cluster of ribosomes simultaneously translating an mRNA molecule
17.18
Post-translational modifications to polypetides
attachments e.g. sugars, lipids, phosphates groups
cleaved
quaternary structure
Signal peptides
Free ribosomes in cytosol
Bound ribosomes attached to cytosolic side of ER--proteins for endomembrane system
and for secretion
Signal recognition particle
17.19
Other signal peptides are used to target polypeptides to mitochondria, chloroplasts, etc.
Prokaryotic protein synthesis
Fig 17.20
Mutations = permanent change in DNA
Point mutations = one or two nucleotides in a single gene
Base pair substitutions = replacement of one base
with another
Missense mutation = substitution that alters an
amino acid codon to a new codon for a different
amino acid
Nonsense mutation = changes amino acid codon
to a stop codon or vice versa-protein usually
nonfunctional
17.21
Insertions or deletions-additions or losses of one or more
nucleotide pairs in a gene
Frameshift mutation = insertion or deletion
that causes a shift in the reading frame
17.22
Spontaneous mutations- errors in DNA replication, repair, recombination
Mutagens- physical (e.g. x-rays) or chemical agents (e.g. base analogues, distortions
of the double helix) that cause mutations
Summary fig 17.23