Review
Retrieved: 2/1/2007
Source:http://faculty.clintoncc.suny.edu/faculty/Michael.Gregory/files/Bio%20101/Bio%20101%20Lectures/Protein%20Synthesis/protein.htm
Central Dogma of Biology
Protein synthesis requires two steps: transcription
and translation.
DNA contains codes
Three bases in DNA code for one amino acid. The
DNA code is copied to produce mRNA. The order of amino acids in the polypeptide
is determined by the sequence of 3-letter codes in mRNA.
DNA vs RNA
|
|
DNA |
RNA |
|
Sugar: |
deoxyribose |
ribose |
|
Bonds with Adenine: |
thymine |
uracil |
|
# of Strands: |
two |
one |
Kinds of RNA
Messenger RNA (mRNA)
Messenger RNA contains genetic information. It is
a copy of a portion of the DNA.
It carries genetic information from the gene (DNA)
out of the nucleus, into the cytoplasm of the cell where it is translated to
produce protein.
Ribosomal RNA (rRNA)
This type of RNA is a structural component of the ribosomes. It does not contain a genetic message.
Transfer RNA (tRNA)
Transfer RNA functions to transport amino acids to
the ribosomes during protein synthesis.
Transcription
Transcription is the synthesis of mRNA from a DNA template.
It is like DNA replication in that a DNA strand is
used to synthesize a strand of mRNA.
Only one strand of DNA is copied.
A single gene may be transcribed thousands of times.
After transcription, the DNA strands rejoin.
Steps involved in transcription
DNA unwinds.
RNA polymerase recognizes a specific base sequence in the DNA called a promoter
and binds to it. The promoter identifies the start of a gene, which strand is
to be copied, and the direction that it is to be copied.
Complementary bases are assembled (U instead of
T).
A termination code in the DNA
indicates where transcription will stop.
The mRNA produced is called a
mRNA transcript.
Processing the mRNA Transcript
In eukaryotic cells, the newly-formed mRNA
transcript (also called heterogenous nuclear RNA or hnRNA) must be further modified before it can be used.
A cap is added to the 5’ end and a poly-A tail
(150 to 200 Adenines) is added to the 3’end of the molecule.
The newly-formed mRNA has regions that do not
contain a genetic message. These regions are called introns
and must be removed. Their function is unknown.
The remaining portions of mRNA are called exons. They are spliced together to form a mature
mRNA transcript.
The Nucleus
DNA is located in an organelle called the nucleus.
Transcription and mRNA processing occur in the
nucleus.
The nucleus is surrounded by a double membrane.
After the mature mRNA transcript is produced, it moves out of the nucleus and
into the cytoplasm
through pores in the nuclear membrane.
Translation
Translation is the process where ribosomes synthesize proteins using the mature mRNA
transcript produced during transcription.
Overview
The diagram below shows a
ribosome attach to mRNA, and then move along the mRNA adding amino acids
to the growing polypeptide chain.
Translation - Details
A mature mRNA transcript, a ribosome, several tRNA molecules and amino acids are shown. There is a
specific tRNA
Genetic Code
The table below can be used to determine what
amino acid corresponds to any 3-letter codon.
|
First Base |
Second Base |
Third Base |
|||
|
U |
C |
A |
G |
||
|
U |
UUU phenylalanine |
UCU serine |
UAU tyrosine |
UGU cysteine |
U |
|
UUC phenylalanine |
UCC serine |
UAC tyrosine |
UGC cysteine |
C |
|
|
UUA leucine |
UCA serine |
UAA stop |
UGA stop |
A |
|
|
UUG leucine |
UCG serine |
UAG stop |
UGG tryptophan |
G |
|
|
C |
CUU leucine |
CCU proline |
CAU histidine |
CGU arginine |
U |
|
CUC leucine |
CCC proline |
CAC histidine |
CGC arginine |
C |
|
|
CUA leucine |
CCA proline |
CAA glutamine |
CGA arginine |
A |
|
|
CUG leucine |
CCG proline |
CAG glutamine |
CGG arginine |
G |
|
|
A |
AUU isoleucine |
ACU threonine |
AAU asparagine |
AGU serine |
U |
|
AUC isoleucine |
ACC threonine |
AAC asparagine |
AGC serine |
C |
|
|
AUA isoleucine |
ACA threonine |
AAA lysine |
AGA arginine |
A |
|
|
AUG (start) methionine |
ACG threonine |
AAG lysine |
AGG arginine |
G |
|
|
G |
GUU valine |
GCU alanine |
GAU aspartate |
GGU glycine |
U |
|
GUC valine |
GCC alanine |
GAC aspartate |
GGC glycine |
C |
|
|
GUA valine |
GCA alanine |
GAA glutamate |
GGA glycine |
A |
|
|
GUG valine |
GCG alanine |
GAG glutamate |
GGG glycine |
G |
|
Mutation
Mutations are changes in the DNA.
Frameshift
A frameshift mutation is
usually severe, producing a completely nonfunctional protein.
The priniciple of a frameshift can be explained using the sentence below. If
the letters are read three at a time and one is deleted, the second sentence
becomes meaningless.
|
Original
DNA: |
THE BIG RED ANT ATE ONE FAT BUG |
Point Mutation
Point mutations involve a single nucleotide, thus
a single amino acid.
In the sentence below, eliminating one letter does
not change in the remaining three-letter words and therefore may not cause a
significant change in the meaning of the sentence.
|
Original
DNA: |
THE BIG RED ANT ATE ONE FAT BUG |
Silent, Missense, and Nonsense Mutations
Three kinds of point mutations can occur. A
mutation that results in an amino acid substitution is called a missense mutation.
A mutation that results in a stop codon so that incomplete proteins are produced, it is
called a nonsense mutation.
A mutation that produces a functioning protein is
called a silent mutation.