Gene Protein

Chapter 17

Protein Synthesis / Gene Expression

Gene expression: The translation of information encoded in a gene into protein or RNA. Expressed genes include genes that are transcribed into messenger RNA (mRNA) and then translated into protein, as well as genes that are transcribed into types of RNA such as transfer RNA (tRNA) and ribosomal RNA (rRNA) that are not translated into protein.

Gerrod - 1909

genes dictate phenotype through enzymes that catalyze specific chemical reactions in the cell

symptoms of an inherited disease reflect a person’s inability to synthesize a particular enzyme

one gene - one enzyme hypothesis

Development of the Theory

one gene - one proteinnot all proteins are enzymes

one gene - one polypeptidemany proteins are composed of several

polypeptides, each of which has its own gene

RNA

contains ribose as its sugar

substitutes the base uracil for thymine

consists of a single strand

transcription and translation

Transcription DNA strand provides a template for the synthesis of a complementary RNA strand

Translation the information contained in the order of nucleotides in mRNA is used to determine the amino acid sequence of a polypeptide

Transcription

template strand of DNA provides a template for the sequence of nucleotides in RNA

The complementary RNA molecule is synthesized according to base-pairing rulesexcept that uracil is the complementary base

to adenine

RNA polymerase

separates the DNA strands at the appropriate point

bonds the RNA nucleotides as they base-pair along the DNA template

http://www-class.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a2.html

At the 5’ end of the pre-mRNA molecule, a modified form of guanine is added, the 5’ cap

At the 3’ end, an enzyme adds 50 to 250 adenine nucleotides, the poly(A) tail

RNA splicing Most eukaryotic genes and their RNA transcripts have

long noncoding stretches of nucleotides Spliceosome removes introns and joins exons to

create an mRNA molecule with a continuous coding sequence

What’s the point of splicing?

at least some introns contain sequences that control gene activity in some way

splicing itself may regulate the passage of mRNA from the nucleus to the cytoplasm

one clear benefit of split genes is to enable a one gene to encode for more than one polypeptide.

Fig. 17-11-3RNA transcript (pre-mRNA)

Exon 1 Exon 2Intron

ProteinsnRNA

snRNPs

Otherproteins

5

5

Spliceosome

Spliceosomecomponents

Cut-outintronmRNA

Exon 1 Exon 25

Translation

blocks of three nucleotides, codons, are decoded into a sequence of amino acids

codons are read in the 5’->3’ direction along the mRNA

ribosome adds each amino acid carried by tRNA to the growing end of the polypeptide chain

tRNA

carries a specific amino acid at one end and has a specific nucleotide triplet, an anticodon, at the other

The anticodon base-pairs with a complementary codon on mRNA

http://www-class.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a3.html

tRNA molecule

binding site for mRNA

The P site holds the tRNA carrying the polypeptide chainThe A site carries the tRNA with the next amino acidDischarged tRNAs leave the ribosome at the E site

Initiation

brings together mRNA, a tRNA with the first amino acid, and the two ribosomal subunits

Codons

Elongation

each amino acid is added to the proceeding one

codon recognition peptide bond formation translocation

Elongation

Termination

Stop codon Releasing factor

polyribosomes

Fig. 17-23Wild-type

3DNA template strand

5

5

53

3

Stop

Carboxyl endAmino end

Protein

mRNA

33

3

55

5

A instead of G

U instead of C

Silent (no effect on amino acid sequence)

Stop

T instead of C

33

3

55

5

A instead of G

Stop

Missense

A instead of T

U instead of A

33

3

5

5

5

Stop

Nonsense No frameshift, but one amino acid missing (3 base-pair deletion)

Frameshift causing extensive missense (1 base-pair deletion)

Frameshift causing immediate nonsense (1 base-pair insertion)

5

5

533

3

Stop

missing

missing

3

3

3

5

55

missing

missing

Stop

5

5533

3

Extra U

Extra A

(a) Base-pair substitution (b) Base-pair insertion or deletion

Producing a protein