Chapter 14: Gene Expression

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’