Ch 21 Epigenetics

Question 1

what is epigenetics?

Answer 1

“above” genetics; studies phenotypes and processes transmitted to cells that are not a result of differences in DNA base sequence

Question 2

describe how DNA methylation affects gene expression

Answer 2

the addition of methyl groups to nucleotide bases represses transcription

Question 3

what is the most common base that is methylated?

Answer 3

cytosine

Question 4

what are CpG islands and where are they most often located?

Answer 4

DNA regions with many CpG nucleotides, and they are most often located at or near promoters

Question 5

enzymes that add methyl groups

Answer 5

DNA methyltransferases

Question 6

enzymes that remove methyl groups

Answer 6

demethylases

Question 7

if genes are actively being transcribed, how would you expect the CpG islands to be?

Answer 7

the CpG islands would not be methylated if the genes are actively being transcribed

Question 8

explain how DNA methylation is maintained through the replication process

Answer 8

Original parental strand is methylated, and new DNA strands are synthesized unmethylated
Methyltransferase enzymes recognize the DNA is hemimethylated, and add methyl groups to the new strand
Now DNA is fully methylated

Question 9

how does DNA methylation repress transcription?

Answer 9

the methyl group sits in the major groove of DNA and prevents binding of transcription factors and proteins

Question 10

explain how restriction enzymes are used to detect DNA methylation?

Answer 10

enzymes that are not sensitive to methylation and enzymes that are sensitive methylation cut the same fragments of DNA, and areas that are differently cut represent methylated regions

Question 11

explain how bisulfite sequencing is used for analyzing DNA methylation

Answer 11

DNA is treated with sodium bisulfite, where unmethylated cytosine is converted into uracil and detected as thymine, whereas methylated cytosine is not converted. analysis of the DNA shows cytosine as methylated

Question 12

explain how histone acetylation alters chromatin structure and affects gene expression?

Answer 12

acetyl groups are added to amino acids in histone tails, which destabilizes chromatin, making it open and accessible and increase transcription

Question 13

explain how histone methylation alters chromatin structure and affects gene expression

Answer 13

methyl groups are added to amino acids of histones, and effects of transcription vary based on which amino acid is methylated

Question 14

explain how miRNA plays a role in gene expression

Answer 14

miRNA combines with RISC, which imperfectly pairs with complementary mRNA sequence and slows down its translation

Question 15

explain how siRNA plays a role in gene expression? (two ways)

Answer 15

siRNA can combine with RISC, which perfectly pairs with complementary mRNA sequence and degrades the mRNA, preventing translation
siRNA can combine with RITS, which attracts methylase enzymes that methylate DNA and producing a heterochromatin structure, preventing transcription

Question 16

explain how epigenetic changes are responsible for cell differentiation

Answer 16

DNA methylation and changes in chromatin structure silence and activate certain genes to allow specialization of cells

Question 17

what is pluripotency

Answer 17

the ability to form every type of cell in an organism

Question 18

an interaction between two alleles that leads to a heritable change in expression of one allele

Answer 18

paramutation

Question 19

what are three important features of paramutations?

Answer 19

1. expression pattern of converted allele passed to future generations
2. altered allele can convert other alleles
3. no associated DNA sequence changes

Question 20

genetically identical alleles that produce heritable differences in phenotypes through epigenetics

Answer 20

epialleles

Question 21

describe the process of X-inactivation

Answer 21

both X chromosomes have the Xist gene
Xist gene is transcribed on one X chromosome into a lncRNA which coats that X chromosome
that produce epigenetic marks and the chromosome is methylated to repress transcription and permanent silencing of the X chromosome

Question 22

describe genomic imprinting

Answer 22

sex of a parent that contributes an allele influences how the allele is expressed, most likely due to different degrees of expression (methylation) of the alleles inherited from the parents