Chapter 14: Gene Expression
Feb 25, 2016
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.
+
Fig. 14.13, p. 231
platform for chain assembly
Ribosome Structure (rRNA)
Fig. 14.12, p. 231
codon in mRNA
anticodon
amino acid OH
amino acidattachment site
anticodon
tRNA MOLECULE
amino acid attachment site
Transfer RNA Structure
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.
Fig. 14.8, p. 228-229
3’5’
growing RNA transcript
5’
3’ 5’
3’
direction of transcription
RNA polymerase
sugar-phosphate backbone of one strand of nucleotides in a DNA double helix
sugar-phosphate backbone ofthe 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
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
Fig. 14.9, p. 229
unit of transcription in a DNA strand
exon intron
mature mRNA transcript
poly-Atail
5’
5’ 3’
3’
(snipped out) (snipped out)
exon exonintron
cap
transcription into pre-mRNA
3’ 5’
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. MutationsB. 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
Translate3’ 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’