Transcription, translation

part of Campbell and Reece, Chapter 17

Question: What is a gene?
One theoretical answer: The DNA sequence that codes for one protein.
But: In eukaryotes there is way too much DNA.
Explanations:
(1) There are extra stretches of DNA interspersed in the coding sequence.
(That will be one topic we cover here.)
(2) There are places between "genes," some of which regulate the genes because:

In multicellular eukaryotic organism,
(1) ALL CELLS HAVE SAME GENES
(2) CELLS ARE DIFFERENT BECAUSE OF WHICH GENES ARE TURNED EXPRESSED
(but this can be fairly permanent, development gene regulation)
This will come up again in the Chapter 19 coverage.

TRANSPARENCY (Fig. 17.9) RNA polymerase II makes "pre-mRNA"
methylated G nucleotide - at 5' cap
extra copied after end of gene is not capped, degraded
poly-A tail 100-200 residues of adenylic acid
site shows where end of gene transcription should be.
primary transcript
Exons are spliced together and form the coding sequence, and introns are spliced out.

Question: Is this splicing useful in any way? (other than to get rid of junk DNA)
TRANSPARENCY (Fig. 19.11 [2 chapters ahead])
Answer: Different exons can be spliced together (for instance in different tissues) to make several different proteins from the same gene.

In general, there are 3 RNA's, t (transfer), m (messenger), and r (ribosomal).
TRANSPARENCY (Fig. 17.12) here's how they work in the cytoplasm (endoplasmic reticulum and polysomes):
(1) Ribosome is the machinery, and it is big.
(2) mRNA codes for the protein.
(3) Many different tRNA's read (by base pairing) each codon and carry one amino acide to the growing peptide chain.

TRANSPARENCY (Fig. 17.18) step by step, peptide is elongated, aminoacyl tRNA (tRNA with amino acid hooked to it) brings in amino acid.

TRANSPARENCY (Fig. 17.19) eventually, when one of the 3 stop codons is encountered, the protein is released.

TRANSPARENCY (Fig. 17.20) In polysome, you can see that the ribosome reads along the mRNA just as a tape recorder head passes along the tape.

TRANSPARENCY (Fig. 17.7) This will be the first mention (it will come up again in the next outline, and a lot in the outline after that) of how genes are regulated. Notice that there are places "upstream" of the "gene" (coding sequence), the promoter, and places like the TATA "box" where transcription factors (proteins) bind to notify RNA polymerase to do its job. Since the 1980's, there has been a lot of interest in "promoter bashing," determining properties of the transcription factors and the DNA sequences they interact with.

This page was last updated 7/24/02

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