1. Forensic DNA Analysis
Protein Synthesis

2. 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

3. 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

5. 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, three-nucleotide words
A triplet=1 codon=1 amino acid

6. 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

8. 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

10. 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
Intron
Exon 3
Pre-mRNA
5 Cap
Poly-A tail 1 30 31
104
105
146
Introns cut out and
exons spliced together
Coding
segment
mRNA
5 Cap
Poly-A tail 1
146
5 UTR
3 UTR

12. 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)

13. GDP GDP Amino end of polypeptide E 3 mRNA Ribosome ready for
Fig
Amino end
of polypeptide E 3
mRNA
Ribosome ready for
next aminoacyl tRNA P
site A
site 5
GTP
GDP E E P A P A
Figure The elongation cycle of translation
GDP
GTP E P A

15.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

17. 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

18. 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)

19. Silent (no effect on amino acid sequence)
Wild-type
DNA template strand 3 5 5 3
mRNA 5 3
Protein
Stop
Amino end
Carboxyl end
A instead of G
Extra A 3 5 3 5 5 3 5 3
U instead of C
Extra U 5 3 5 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 3 5 5 3 5 3
A instead of G
missing 5 3 5 3
Stop
Missense
Frameshift causing extensive missense (1 base-pair deletion)
A instead of T
missing 3 5 3 5 5 3 5 3
U instead of A
missing 5 3 5 3
Stop
Stop
Nonsense
No frameshift, but one amino acid missing (3 base-pair deletion)
(a) Base-pair substitution
(b) Base-pair insertion or deletion