RNA_processing.html: 17_09RNAProcessingCapTail.jpg
The first step in RNA processing is the addition of a
5' cap
and
3' poly-A tail
to the pre–mRNA. These modified ends and the UnTranslated Regions
facilitate ribosome attachment in the cytoplasm.
RNA_splicing.html: 17_10RNASplicing.jpg
RNA processing: RNA splicing.
The pre-mRNA has regions called
introns
which do not code for polypeptides and must be removed.
The coding regions are called
exons
are spliced together produce the
mature mRNA,
which exits the nucleus.
elongation.html: 17_07bTranscripElongation_L.jpg
Transcription: elongation.
The RNA polymerase unwinds the double helix,
adding nucleotides to the 3'
end of the growing RNA molecule.
The same base-pairing rules
as DNA replication is used, except that Uracil
(U
) substitutes for Thymine (T
).
gene.html: 17_03bGeneticInfoFlow_3-L.jpg
In an eukaryotic cell,
the nucleus provides a separate compartment for transcription.
The original RNA transcript, called
pre-mRNA,
is processed before leaving the nucleus through nuclear pores as mRNA
for translation in the cytoplasm.
gene_info.html: 17_04GeneInfoFlowTriplet_L.jpg
Central dogma of genetics: transcription followed by translation.
One
DNA
strand serves as a template for transcription of messenger RNA
(mRNA),
following the base-pairing rules
- remember A pairs with U in RNA.
During translation, sequences of base triplets
(codons)
in the
mRNA
specify
amino acids
to be added to the growing
protein
(polypeptide) chain.
gene_prokaryote.html: 17_03aGeneticInfoFlow_2-L.jpg
A prokaryotic cell lacks a nucleus, and mRNA
produced by transcription is immediately translated without additional
processing.
genetic_code.html: 17_05GeneticCode_L.jpg
The dictionary of the genetic code.
The 3 bases of a codon are read in the 5' - 3'
direction along the mRNA.
The 64 codons specify 20 different amino acids.
The codon
AUG
not only stands for the amino acid methionine
(Met)
but also functions as a
“start”
signal to begin translating the mRNA.
Three codons are
“stop”
signals, marking the end of a polypeptide.
Codons must be read in a correct reading frame to produce the correct sequence
of amino acids.
The genetic code is nearly universal
among all organisms.
initiation.html: 17_07aTranscription_1_L.jpg
Transcription: initiation.
RNA polymerase (RNA polymerase II in eukaryotes) binds to
promoter
sequences on the DNA.
mutation-frameshift.html: 17_23-PointMutationTypes-L.jpg
Base-pair insertions or deletions usually cause frameshift in reading the codon,
leading to nonsense or missense mutations.
A base-pair insertion can cause immediate nonsense
if the resulting a results in a premature stop codon.
A base-pair deletion can cause extensive missense
by changing the reading frame of the codons.
mutation-substitution.html: 17_24BasePairSubstitution_L.jpg
Base-pair substitution may lead to silent, missense, or nonsense mutations.
A silent mutation alters a codon but does not result in a change in the amino acid.
A missense mutation leads to a change in the translated amino acid.
A nonsense mutation changes a codon into a stop
codon and results in premature termination of translation.
mutation.html: 17_22MoleBasisSickleCell-L.jpg
In this point mutation, the DNA template strand has an
A where the wild-type template has a
T (base-pair substitution).
The mutant mRNA has a
U instead of an
A in one codon.
The mutant (sickle-cell) hemoglobin
has a valine (Val)
instead of a
glutamic acid (Glu).
mutation_missense.html: 17_23bPointMutationTypes-L.jpg
A missense base-pair substitution leads to a change in the translated amino
acid.
mutation_nonsense.html: 17_23cPointMutationTypes-L.jpg
A nonsense base-pair substitution changes a codon into a
stop codon and results in premature termination of
translation.
mutation_silent.html: 17_23aPointMutationTypes-L.jpg
A silent base-pair substitution alters a codon but does not result in a change in the amino
acid.
promoter.html: 17_08TranscripInitiation.jpg
The promoter includes a TATA box,
a nucleotide sequence containing many T-A base pairs.
Several transcription factors form an initiation complex with RNA polymerase II.
ribosome.html: 17_16RibosomeStructure.jpg
A ribosome (site of polypeptide synthesis) consists of a large and a small subunit.
Each subunit is an aggregate of ribosomal RNA (rRNA) molecules and proteins.
splicing.html: 17_11snRNPsSpliceosomes_L.jpg
Pre-mRNA splicing.
summary.html: 17_25GeneExpressSummary-L.jpg
Summary of eukaryotic transcription and translation.
A gene in the DNA is transcribed into
RNA
molecules, including
pre-mRNA.
RNA processing occurs in the nucleus.
Translation occurs in the cytoplasm on ribosomes in conjunction with
tRNA.
A gene is a region of DNA whose final product is either a
polypeptide or an
RNA molecule.
tRNA.html: 17_14tRNAStructure2D.jpg
Each tRNA type has a unique
anticodon
triplet, and carries a specific
amino acid
at its 3'
end.
tRNA_3D.html: 17_14tRNAStructure3D.jpg
Hydrogen bonds twist and fold the tRNA into a three-dimensional molecule.
This exposes the
anticodon
(3' - 5'
) on the tRNA to
align with a mRNA codon (5' - 3'
) on the ribosome.
term.html: 17_07aTranscriptionStag_4-L.jpg
Transcription: termination.
In eukaryotes, transcription terminates when a
polyadenylation signal
is transcribed, and the
RNA transcript
falls off from the
polymerase.
term_pacman.html: 17_RNA_pacman.jpg
transcription.html: 17_07aTranscription.jpg
Transcription.
5' to 3'
.
transgenic.html: 17_06TobaccoLuciferase.jpg
A tobacco plant expressing a firefly gene.
Because diverse forms of life share a common genetic code,
it is possible to program one species to produce proteins of another species by transplanting DNA.
Here a gene from a firefly is incorporated into the DNA of a tobacco plant.
This gene codes for an enzyme that catalyzes a chemical reaction that releases light energy.
Rabbit.
translation-elongation.html: 17_18TranslationElong_L.jpg
Translation: elongation.
A
site.
A
site and the growing polypeptide in the P
site.
A
site to the P
site.
P
site is moved to the E
site and released.
The
mRNA moves the next
codon into the A
site.
translation-initiation.html: 17_17TranslationInitiat.jpg
Translation: initiation. | |||
---|---|---|---|
A small ribosomal subunit binds to mRNA. An initiator tRNA, with the anticodon UAC and carrying the amino acid methionine (Met), base-pairs with the start codon AUG. | A large ribosomal subunit completes the initiation complex. The initiator tRNA is in the P site; the A site is available to the next tRNA. |
translation-termination.html: 17_19TranslationTerminat.jpg
Translation: termination. | ||||
---|---|---|---|---|
When a
ribosome
reaches a
stop codon,
the A site accepts a
release factor.
|
The
release factor
releases the last amino acid of the
polypeptide
from the
tRNA
in the P .
|
The two ribosomal subunits and the other components of the assembly dissociate. |
translation.html: 17_13Translation.jpg
Codons on the
mRNA
move through a ribosome, and are translated into amino acids.
The interpreters are
tRNA
molecules, each type with an
anticodon
at one end and a corresponding
amino acid at the other end.
triplets.html: 17_04GeneInfoFlowTriplet_L.jpg
The triplet code.
For each gene, one DNA strand functions as a template for transcription to produce
mRNA.
The base–pairing rules for DNA synthesis also guide transcription,
but uracil (U
) takes the place of thymine (T
) in RNA.
In translation, the
mRNA
is read in the 5' - 3'
direction as a sequence of base triplets, or
codons.
Each
codon
specifies an
amino acid
to be added to the growing
polypeptide
chain.