Chapter 17

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Genetic Code

DNA molecule is the code for all proteins in cell

Gene is coded DNA instructions that control production of specific proteins

TWO stages of gene expression: Transcription and translation

DNA -> RNA -> Protein

CAA -> GUU -> Valine (example)

Code:

1. Amino Acids

2. 20 diff amino acids/ 4 diff nucleotides

3. 3 nucleotides specify amino acid (64 possible combinations)

4. Triplet=Codon

Universal: Few exceptions all living organism use the same codons to amino acid code

Degenerate: More than one codon codes for the same amino acid (CCU, CCC, CCG = proline)

Unambiguous: One codon always equals one kind of amino acid

Start Codon: TAC = beginning of gene (AUG). Every protein starts with methionine

Stop Codons: Mark end of gene; transcription stops; UAA, UGA, UAG

RNA- Ribonucleic Acid

RNA=single strand; DNA=double strand

RNA sugar=ribose; DNA sugar=deoxyribose

RNA has uracil paired with adenine; DNA has thymine paired with adenine

RNA form complex shapes

mRNA: transcription

Messenger RNA- takes message from DNA to ribosomes

Nucleus to cytoplasm

Instructions in codons (start codon first & mRNA attaches to ribosome)

tRNA: translations

Transfer RNA - transfers amino acids to ribosomes

Gathers right amino acids for making protein

cytoplasmic pool to ribosome

All diff ( particular mRNA codon with particular amino acid)

Carries specific amino acid at one end and anticodon (nucleotide triplet) on the other.

End of tRNA attaches to ONE amino acid and carries to ribosomes

rRNA: protein synthesis

Ribosomal RNA - component of ribosomes

Bind mRNA and tRNA to ribosome.

Gene Expression:

DNA -> RNA -> Protein

Two phases: Transcription (DNA ->RNA)

Translation (tRNA -> Protein)

Transcription: DNA triplets into mRNA codons

DNA template: Promoter - sequence of DNA indicating beginning/starting point

Transcription Unit - stretch of DNA transcribed

Terminator - DNA sequence signaling end of transcription

RNA polymerase: Initiation, elongation, termination

Translation: mRNA-tRNA complex translates codon into amino acid

RNA Processing: Splicing out introns (noncoding regions)

Eukaryotic cells modify mRNA after transcription

Modifying before RNA is sent out - small section (mRNA) containing INTRONS and EXONS

Introns - sequences of nitrogen bases not involved in protein making (CUT out of RNA )

EXONS = good (used in protein making) INTRONS = bad (not involved in protein making)

Exons spliced back together

Mature Transcript: Final processed mRNA translated at ribosomes

Modification of pre-mRNA Ends

5' cap - aids in binging mRNA to ribosome

Poly-A Tail - protects 3' end of mRNA from RNAse

Protein Synthesis: (Translation)

Cell translate the base sequence of mRNA molecule into amino acid

Occurs in ribosome

1. DNA transcribes RNA

2. RNA sent to cytoplasm as mRNA

3. mRNA attaches to ribosome

4. Amino acid brought into ribosome by tRNA

5. Ribosome hitches amino acid with peptide bonds and proteins are made!

Ribosome:

Large Subunit

E - Exit site

P- P site (peptidyl tRNA binding)

A - A site (aminoacyl-tRNA binding)

Small Subunit - mRNA binding sight

Mutations

Gene mutations or chromosome mutations

Gene - produce change within single gene

Chromosome - mutation produce changes in whole chromosome

no mutation: ACA -> UGU -> Cysteine

Silent mutation: ACG ->UGC->Cysteine

Missense: TCA -> AGU -> Serine

Nonsense: ACT -> UGA -> STOP

Frameshift mutations - due to insertion or deletion of a nucleotide