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Chapter 18

BIO SCI 93 Chapter Notes - Chapter 18: Start Codon, Intron, Polyadenylation


Department
Biological Sciences
Course Code
BIO SCI 93
Professor
Justin Shaffer
Chapter
18

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Chapter 18 pages 365 - 373
Differential gene expression is the expression of different genes by cells with the same
genome. Function of any cell depends on the appropriate set of genes being expressed.
Cell must locate right genes at the right time. When gene expression proceeds
abnormally, serious imbalances and diseases can arise
Histone acetylation promotes transcription by opening up the chromatin structure
N-terminus or histone tails protrude outwards from the nucleosome, accessible to
various modifying enzymes that catalyze the addition or removal of specific
chemical groups.
DNA methylation is the process in which enzymes methylate certain bases in the DNA,
usually cytosine
Individual genes usually more heavily methylated in cells are not expressed.
Removal of extra methyl groups can turn on some of these genes. Once
methylated, genes usually stay that way through successive cell divisions in a
given individual
Inheritance of traits transmitted by mechanisms not involving the nucleotide sequence
itself is called epigenetic inheritance.
DNA mutations are permanent, while modifications to chromatin can be reversed.
DNA methylation patterns are largely erased and reestablished during gamete
formation
Enhancer: groupings of distal control elements that may be thousands of nucleotide
upstream or downstream of a gene or even within an intron. Each enhancer generally
associated with only that gene and no other.
Promoter: a DNA sequence where RNA polymerase binds and starts transcription
Terminator: where transcription ends
Proximal control elements: control elements located close to the promoter
Distal control elements: more distant control elements
Exon: part of the gene that is transcribed
Intron: part of the gene that is ignored/cut out before the transcription of mRNA.
Primary RNA transcript: pre-mRNA that hasn’t been spliced the introns from the exons
yet.
5’ cap: Phosphate groups attached to 5’ end of the mRNA; works to protect mRNA after
it leaves the nucleus
Poly-A tail: tail of adenines added to the 3’ end; works to protect mRNA after it leaves
the nucleus.
Summary of how a eukaryotic gene is transcribed and primary transcript is processed
RNA polymerase binds to the promoter where transcription begins. From there,
the pre-mRNA transcribed until it reaches the transcription termination region.
The transcribed pre-mRNA undergoes splicing, in which the introns are cut out
and the exons are glued together, and also RNA processing in which the 5’ cap
and poly-A tail is added. The primary transcript will be processed once the mRNA
leaves the nucleus to go through a ribosome in the cytoplasm. The ribosome will
read the mRNA sequence in codons until it reaches a start codon to begin
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