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Forensic DNA Analysis
Protein Synthesis
Overview: The Flow of Genetic Information
• The information content of DNA is in the form of
specific sequences of nucleotides
• The DNA inherited by an organism leads to
specific traits by dictating the synthesis of
proteins
• Proteins are the links between genotype and
phenotype
• Gene expression, the process by which DNA
directs protein synthesis, includes two stages:
transcription and translation
The central dogma…
• DNA RNA Protein
• RNA is the intermediate between genes and the proteins
for which they code
• Transcription is the synthesis of RNA under the
direction of DNA
• Happens in the nucleus
• Transcription produces messenger RNA (mRNA),
carries DNA message to ribosome
• Translation is the synthesis of a polypeptide, which
occurs under the direction of mRNA
• Ribosomes are the sites of translation
Codons
• There are 20 amino acids, but there are only four
nucleotide bases in DNA
• The flow of information from gene to protein is based on
a triplet code: a series of non-overlapping, threenucleotide words
• A triplet=1 codon=1 amino acid
Overview
• During transcription, one of the two
DNA strands called the template
strand provides a template for
ordering the sequence of
nucleotides in an RNA transcript
• During translation, the mRNA base
triplets, called codons, are read in
the 5 to 3 direction
• Each codon specifies the order of
amino acids to make a protein
• RNA bases: A, G, C, and Uracil
instead of Thymine
Transcription
• The three stages of transcription:
• Initiation
• Promoters signal the initiation of RNA synthesis
and transcription factors mediate the binding of
RNA polymerase and the initiation of transcription
• Elongation
• RNA polymerase unwinds helix and adds RNA
nucleotides to the 3’ end. DNA helix reforms after
RNA Polymerase moves away
• Termination
• The polymerase falls off the DNA after the mRNA is
formed
mRNA processing
• mRNA is ‘processed’ after transcription to prepare it for
leaving the nucleus to travel to the ribosome.
• It receives a 5’ cap and poly A tail to the 3’ end
• Parts of the mRNA transcript are removed (spliced)
5 Exon Intron
Exon
Exon
Intron
3
Pre-mRNA 5 Cap
Poly-A tail
1
30
31
Coding
segment
mRNA 5 Cap
5
1
UTR
104
105
146
Introns cut out and
exons spliced together
Poly-A tail
146
3 UTR
Translation
• The three stages of translation:
• Initiation
• mRNA binds with the ribosome and moves along it
until it reaches a start codon (AUG)
• Elongation
• tRNA (transfer) brings amino acids to the ribosome
and adds them 1 by 1 based on the mRNA
sequence
• Termination
• Codons (3-bases) are read and amino acids are
added until the ribosome reaches a STOP codon
(UGA, UAA, UAG)
Fig. 17-18-4
Amino end
of polypeptide
E
3
mRNA
Ribosome ready for
next aminoacyl tRNA
P A
site site
5
GTP
GDP
E
E
P A
P A
GDP
GTP
E
P A
• http://www.youtub
e.com/watch?v=5
bLEDd-PSTQ
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Mutations
• Mutations are changes in the genetic material of a cell or
virus
• Point mutations are chemical changes in just one base
pair of a gene
Types of Point Mutation
• A base-pair substitution replaces one nucleotide with
another pair
• Silent mutations have no effect on the amino acid
produced by a codon (codes for the same amino acid)
• Missense mutations still code for an amino acid, but not
the right amino acid
• Nonsense mutations change an amino acid codon into
a stop codon, nearly always leading to a nonfunctional
protein
• Insertions and deletions are additions or losses of
nucleotide pairs in a gene (changes the reading frame)
Wild-type
DNA template strand 3
5
5
3
mRNA 5
3
Protein
Stop
Amino end
Carboxyl end
A instead of G
3
5
Extra A
5
3
3
5
3
5
U instead of C
5
5
3
Extra U
3
Stop
Stop
Silent (no effect on amino acid sequence)
Frameshift causing immediate nonsense (1 base-pair insertion)
T instead of C
missing
3
5
5
3
3
5
3
5
5
3
A instead of G
missing
5
3
Stop
Missense
Frameshift causing extensive missense (1 base-pair deletion)
missing
A instead of T
5
3
3
5
U instead of A
5
5
3
3
5
missing
3
5
Stop
Stop
Nonsense
(a) Base-pair substitution
3
No frameshift, but one amino acid missing (3 base-pair deletion)
(b) Base-pair insertion or deletion