Transcript Topic 7 The Discovery of DNA & Its Roles

Topic 8
From Gene
…to Protein
Biology 1001
October 17, 2005
IIIA. Details of Transcription
5’-GCCAGGCTAAACTA-3’
DNA coding strand
3’-CGGTCCGATTTGAT-5’
DNA template strand
5’-GCCAGG…-3’
RNA synthesis
3’-CGGTCCGATTTGAT-5’
5’-GCCAGGCUAAACUA-3’
messenger RNA (mRNA)

RNA is similar to DNA except that it is always single-stranded, the sugar has a
2’-OH group, and the nitrogenous bases are A, C, G, & Uracil

Messenger RNA is transcribed or “copied” from a DNA template according to
the base pairing rules, except that U pairs with A

The mRNA sequence is the same as the DNA coding strand sequence, except
that there are Us instead of As
Figure 17.7 -Transcription
has three stages
initiation, elongation and
termination
IIIB. Translation – the Basic Concept
 Translation is the synthesis of a
polypeptide under the direction of the
mRNA
 The nucleotide sequence of the mRNA
is converted to the amino acid sequence
of a polypeptide
 The site at which this occurs is in the
cell is the ribosome
 Amino acids are carried to the growing
polypeptide chain by tRNA molecules
Figure 17.13
Figure 17.14 –
The structure of transfer RNA
Figure 17.16 –
The anatomy of a ribosome
IIIC. Features of Gene Expression in
Eukaryotes
 Transcription occurs in the
nucleus, translation in the
cytoplasm
 Transcription initially results in
a pre-mRNA or primary
transcript which undergoes
RNA processing before being
translated
 The ribosomal subunits (rRNA
& proteins) are assembled in the
nucleolus
Figure 17.26
IV. The Genetic Code
 Four nucleotides need to specify 20 amino acids, so the
genetic code is a triplet code  41 = 4, 42 = 16, 43 = 64
 A codon is a sequence of three nucleotides that specifies a
particular amino acid; 3 of the 64 possible codons function as
stop signals and one (AUG) codes for the amino acid
methionine & functions as a start signal
 The code is read from the mRNA which is transcribed from
the template strand of DNA
 The code is degenerate – it is redundant but not ambiguous
 The sequence needs to be read in non-overlapping tandem
groups of three and in the correct reading frame
 The code is nearly universal
IV. The Genetic Code
Figure 17.4
Figure 17.5
V. The Effect of Mutation on Protein Function





Mutation – a change in the genetic material
Point mutation – a change in just one nucleotide (base) pair
A substitution of one nucleotide pair for another produces
silent, missense, or nonsense mutations
An indel (insertion or deletion of a base pair) produces a
frameshift mutation
Mutations are either spontaneous errors during DNA
replication or caused by mutagens – physical or chemical
agents that interact with and damage DNA


Egs. Physical – UV, X-rays
Egs. Chemical – base analogs, intercalating agents
Figure 17.24 Base-pair substitution
Silent
Review Videos of Transcription and Translation
(These animations are intended to aid your conceptual understanding. I won’t test you on
the content unless I mentioned it elsewhere in Topic 8)