November 10 (continued from Nov.3 handout

2. Kinds of RNA
a. Messenger RNA (mRNA): serves as the template for proteins
i. Can be seen as a set of codons (3 base long set of RNA bases that codes fora particular amino acid
ii. Short lived; dissolves in ~30 seconds if not actively being translated
iii. Come from DNA genes from all the previous steps in transcription but never get translated into proteins
b. Transfer RNA (tRNA): clover-leaf shaped w/ anticodon loop that is complementary to mRNA codon at the top
i. Charged + RNA has correct amino acid for mRNA complementary codon attached to the opposite side of the anticodon loop
ii. Come from other genes but do not get translated into proteins
iii. Long lived
c. Ribosomal RNA (rRNA): main component of ribosomes (80% of ribosomes by weight)
i. Ribosomes translate mRNA to protein
ii. Long lived
3. Overview of Translation
a. DNA genemRNA transcriptionamino acid chain
i. mRNA has 3-base long codons that are complementary to anticodons on tRNAs
ii. 20 amino acids and 3 bases/codon and 4 bases= 43= 64 possible codons
b. Codon: 3 mRNA nucleotide sequence that codes for specific amino acids
i. Table of codon uses the 4 letters of RNA sequence to determine the amino acid
c. Have mRNA (copy of DNA gene)
i. Small unit of ribosome (rRNA) attaches to shine-dalgarno sequence on mRNA, which is upstream of where codons for amino acids to make protein beginlarge subunit of ribosome attaches to small sumbunit/mRNA
d. At start codon (AUG or methionine), translation begins
e. tRNAs bearing amino acids brough in by ribosome, matched to codons
i. Ribosomes will bring in anticodons that are complementary to mRNA codons
f. Amino acids on backs of tRNA will be linked into a chain of a.a.s
g. At stop codon (UAA, UAF, or UGA) ribosome releases mRNA and amino acid chin
i. Don’t have complementary tRNA instead, stop codon binds with protein, RF1 (release factor 1) or RF2 (release factor 2)
1. Enzymes that detach last tRNA from its amino acid
h. Polyribosomes: lots of ribosomes translating an mRNA at the same time
4. Kinds of mutations (mutations occur in DNA sequence)
a. Point mutations(Examples are on the handout)
i. Missense: just one base in DNA sequence is changed; leads to a change to amino acid sequence
ii. Nonsense: just one base in DNA sequence is changed; leads to an early stop codon in mRNA, thus short protein
iii. Silent: change to DNA, and thus to mRNA, but no change in amino acid sequence
b. Frameshift mutations: change in the DNA sequence so that in mRNA there is either -2 bases added/deleted from “reading frame” of the mRNA; leads to a different amino acid sequence

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