Chapter 14: Gene Expression

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Chapter 14: Gene Expression
I. RNA
A. Structure of RNA (Ribose Nucleic Acid) – A
genetic molecule similar to DNA, with three
major differences:
1. RNA is made with Uracil (U), not Thymine (T)
2. RNA is single-stranded, like half of a ladder.
3. Ribose is the sugar making up the backbone
of the molecule, not deoxyribose (thus RNA,
not DNA).
I. RNA
B. Types of RNA
rRNA – ribosomal; makes up ribosomes.
mRNA – messenger; copied from DNA,
takes ‘message’ to ribosomes.
tRNA – transfer; transfers amino acids to
ribosomes.
Ribosome Structure (rRNA)
platform
for chain
assembly
+
Fig. 14.13, p. 231
Transfer RNA Structure
anticodon
codon in mRNA
anticodon
amino acid
attachment site
tRNA MOLECULE
amino
acid
OH
amino acid attachment site
Fig. 14.12, p. 231
II. Transcription (DNA to mRNA)
A. Steps of Transcription:
1.Initiated at promoter region of DNA (a specific
base sequence). Occurs after DNA unzips.
2. RNA Polymerase joins free nucleotide
together that compliment the DNA code.
3. The single-stranded mRNA molecule moves
away from the DNA and is modified.
sugar-phosphate backbone of one strand
of nucleotides in a DNA double helix
sugar-phosphate
backbone of
the other strand
of nucleotides
part of the
sequence of
base pairs in DNA
transcribed DNA
winds up again
DNA to be
transcribed
unwinds
Newly forming
RNA transcript
The DNA
template at the
assembly site
growing RNA transcript
5’
3’
5’
direction of transcription
3’
5’
3’
RNA polymerase
Fig. 14.8, p. 228-229
II. Transcription
B. Modification of Transcript
1. Stretches of the mRNA molecule will be
removed, known as introns (they
remain IN the nucleus).
2. Remaining segments are exons
(these EXIT the nucleus).
3. The various exon segments may
be rearranged by splicesomes
unit of transcription in a DNA strand
exon
intron
exon
intron
exon
3’
5’
transcription into pre-mRNA
poly-A
tail
cap
5’
3’
(snipped out)
(snipped out)
5’
3’
mature mRNA transcript
Fig. 14.9, p. 229
III. Translation (RNA to AA)
A. Steps of Translation
1. The mRNA transcript will move out to
the ribosome, which will assemble the
amino acids.
2. Three consecutitive nucleotides on the
mRNA transcript make one command,
called a codon.
3. Each codon will match with a
corresponding tRNA, which has three
nucleotides, which are called anticodons.
Binding site for mRNA
P
(first
binding
site for
tRNA)
A
(second
binding
site for
tRNA)
Fig. 14.14a, p. 232
Fig. 14.14b, p. 233
IV. Mutations
A. Space holder DNA: About 97% of your
DNA is noncoding, but it is hardly useless.
If a mutation (error in replication or
transcription) occurs, it usually occurs in
this noncoding region (thus, there is no
effect on phenotype.
IV. Mutations
B. Types of Mutation
1. Substitution: One nucleotide
incorrectly replaced with another.
(One for One Switch)
2. Wobble Effect: If you total up the number
of nucleotides (A, U, C, G) and the
possible arrangements of 3 letter
combinations, you get a total of 64. Yet
these 64 codes translate into only 20
amino acids. So small errors often don’t
alter the protein/amino acid chain.
Consider the codons "AUC" and "AUA."
IV. Mutations
B. Frameshift Mutation
If reading by threes, the addition or
deletion of a nucleotide will throw off all
subsequent codons.
The Cat Ate All The BBQ
Ath Eca Tat Eal Lth Ebb Q (addition)
Hec Ata Tea Llt Heb Bq (deletion)
Fig. 14.11, p. 230
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3’ A A T T A C A C C A T G C C A A G G C G A T G C T T A C G G A C A A T 5’

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