Chapter 19

Question 1

Expression of genes must be regulated with regard to

Answer 1

Place- which cell expels a gene (neurotransmitter in neurons). Time- when during development or when product is needed (growth factor signaling turns on gene needed for cell division). Amount- how much of a gene is needed (muscle cells need lots of actin)

Question 2

How is genetic and epigentic regulation controlled

Answer 2

Chromatin structures and gene expression in eukaryotes. Many steps from gene to function- all can be regulated. Gene expression in prokaryotes- lac operon

Question 3

Epigentic regulation

Answer 3

Changes to DNA that do not change the DNA sequence, but alter gene expression.

Question 4

Epigentic regulation: DNA methylation

Answer 4

Methylation (-CH3) of a cytosine (C) residue inhibits transcription. Occurs at a C residue of a CpG dinucleotide. Reversible (can change over time), used in normal situation to regulate gene expression. In cancer cells, genes that block cell division are often methylated to turn them off.

Question 5

Methylation

Answer 5

Usually prevents transcription activators from binding to promoter and enhancers-> no transcription

Question 6

CpG island

Answer 6

Cluster of CpG pairs

Question 7

Chromatin remodeling-open

Answer 7

DNA is packaged by wrapping around histones- prevents transcription, chromatin structure must open to allow transcription. Chemical modification of histones occurs on lysine residues.

Question 8

Chromatin remodeling-open: acetylation

Answer 8

Addition of an acetyl group. Loosens chromatin-> more transcription. Also 1 methylation -> open-> more transcription.

Question 9

Chromatin remodeling -closed

Answer 9

Methylation x3 -> closed -> less transcription. Reversible. Chemical modification of specific lysine amino acids in the tails of histones protruding from nucleosomes affects the transcription of the gene

Question 10

X-inactivation- expression regulation at the level of the entire chromosome

Answer 10

Different # of X in males and females. Problem: would lead to 2x the expression level of x-linked genes in females. Solution: dosage compensation; even out expression of X-linked genes in the sexes. Different mechanisms in different animals.

Question 11

X-inactivation: mammals

Answer 11

One X chromosome is randomly inactivated in females. Ex: calico cats- always female, results from random inactivation of one X chromosome

Question 12

Females protected against x-linked diseases because …

Answer 12

Half of their cells, in female carriers, express genes from good X chromosome. Inactive X chromosome is not completely inactive (low level of transcription

Question 13

How X chromosome inactivation occurs at a molecular level

Answer 13

Initiated by the x-inactivation center (region on X chromosome). XIC includes the Xist gene- not translated, functions as RNA. Inactive X chromosome forms a dense chromosome called a Barr body

Question 14

Transcriptional regulation in eukaryotes- regulatory transcription factors

Answer 14

Many exist. Bind to DNA enhancer sequences (specific for individual factors). Each gene has a combination of enhancers. Recruit general transcription factors. General transcription factors- always the same.

Question 15

Gene expression regulation at transcript level- splicing

Answer 15

Splicing produces different proteins in a tissue specific manner. Alternative splicing- use a different combo of exons, multiple proteins from a single gene, can be cell specific. 70,000 proteins from 21,000 genes in humans

Question 16

Sequence of mRNA can be edited- rna editing

Answer 16

Chemical modification of bases of mRNA allows pairing with different bases. Some transcripts can be edited differently even in the same cell or can be tissue specific. Different function with the change of a single base.

Question 17

RNA interference (RNAi)- small regulatory RNAs

Answer 17

MicroRNA (miRNA), small interfering RNA (siRNA), dsRNA- about 20-25 base pairs, RISC- RNA induced silencing complex

Question 18

Differences between miRNAs and siRNAs

Answer 18

miRNAs- not perfect pairing with target, inhibit translation, known to regulate many genes, often over or under expressed in cancer. siRNAs- perfect complement to target, cause mRNA degradation, may fight RNA viruses, often used in labs to decrease expression of gene if interest.

Question 19

Translation can also be regulated

Answer 19

Initiation requires initiation factor binding to 5’ cap-scan for AUG. UTR- untranslated region; 5’UTR has secondary structure(folding)- can affect initiation of translation, 3’UTR can fold back to interact with 5’UTR and also affect initiation

Question 20

RNA binding proteins- translation regulation

Answer 20

Often binds to sequences in 5’UTR and 3’UTR. Can repress translation. Can target mRNA to a specific location in cell. 3’UTR is often targeted by miRNAs

Question 21

Posttranslational regulation

Answer 21

Modifications can affect protein function- phosphorylation -> signal transduction, glycosylation- occurs in golgi. Modifications can also signal for degradation of protein- tag added to protein targets for degradation.

Question 22

Proteases- posttranslational regulation

Answer 22

Enzymes that degrade proteins. Dangerous to the cell. Made as inactive form- activated when needed. Activation often is done by cleaving off an inhibitory portion.