Topic 7 The Discovery of DNA & Its Roles

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Transcript Topic 7 The Discovery of DNA & Its Roles

Topic 8
From Gene
…to Protein
Biology 1001
October 17, 2005
IIIA. Details of Transcription
DNA coding strand
DNA template strand
RNA synthesis
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
IIIB. Translation – the Basic Concept
 Translation is the synthesis of a
polypeptide under the direction of the
 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
 Transcription occurs in the
nucleus, translation in the
 Transcription initially results in
a pre-mRNA or primary
transcript which undergoes
RNA processing before being
 The ribosomal subunits (rRNA
& proteins) are assembled in the
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
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)