EPIGENETICS

Question 1

LEARNING OBJECTIVES

Answer 1

1. explain significance of epigenetic marks in context of gene expression and phenotypes

• compare and contrast different types of epigenetic marks
• discuss mitotic and meiotic mechanisms of epigenetic inheritance

Question 2

definition

Answer 2

Epigenetics: The study of heritable changes in gene expression and gene function that are not explained by changes in DNA sequence.

Labels DNA and proteins with chemical markers, influencing whether specific genes are read or not.

Question 3

epigenetic marks and cell function

Answer 3

Each cell has a unique epigenome, allowing for differentiation into different types of cells and forming organs.

Epigenetic marks provide an additional layer of information that influences how identical genomes are expressed differently in various cell types.

Question 4

DNA methylation

Answer 4

Occurs in both prokaryotes and eukaryotes.

Methyl group is transferred to cytosine nucleotides (often CG sequences) by specific writer enzymes and removed by eraser proteins.

Methylation in promoter regions inhibits transcription factor binding, silencing genes. by blocking TF from binding or attracting repressors to DNA

Leads to differences in gene expression between cells.

Question 5

example of epigenetics in twins

Answer 5

Despite having identical DNA, twins have unique epigenomes influenced by their environment and lifestyle choices.

As a result, twins can develop different phenotypes and even live different lives.

Question 6

maintenance during DNA replication

Answer 6

Methylation needs to be re-established on newly synthesized strands.

Hemi-methylated DNA is recognized by specific writer/reader proteins, which methylate the new DNA strand.

the parental strand already has methlyation, new daughter strand does not - DNA methyltransferases fix this

Question 7

mitotic inheritance

Answer 7

epigenetic marks are maintained during cell division

daughter cells retain same epigenetic patterns as the parent cell, ensuring cell identity and gene expression patterns are preserved

important as if replacing a certain cell, must need to express same things

Question 8

histone code

Answer 8

Histones are proteins that DNA wraps around, forming nucleosomes.

Four main types of histones: H2A, H2B, H3, H4.

Histones can be modified by acetylation, methylation, phosphorylation, and ubiquitylation, impacting DNA accessibility.

Histone modifications create a unique “code” that influences gene expression.

Question 9

Histone Acetylation (H3K9ac)

Answer 9

H3K9ac: Histone 3, lysine residue at position 9, acetylated.

Acetylation neutralizes the positive charge on histones, loosening the DNA and making it more accessible to transcription machinery.

Increases transcription by promoting gene expression.

Question 10

Histone Methylation (HEK27me3)

Answer 10

HEK27me3: Histone 3, lysine residue at position 27, tri-methylated.

Causes gene silencing by affecting nucleosome positioning and condensation.

Methylation at other histone positions may promote transcription, depending on the context.

Question 11

Maintenance During DNA Replication (Nucleosomes)

Answer 11

Nucleosomes disassemble ahead of replication, and histones are re-deposited after replication.

Parental histones are shared between daughter strands, and new histones are added from the cytosol.

Marks on parental histones recruit writers to deposit similar marks on newly formed histones.

Question 12

Maintenance During Meiosis

Answer 12

For epigenetic marks to be inherited across generations, they must be maintained in the germline.

In animal gametes, epigenetic marks are often removed by eraser proteins.

If erasure is incomplete or marks are re-established, this can lead to transgenerational inheritance.

This phenomenon is more commonly recognized in plants and remains contentious in animals.

Question 13

Regulation of Flowering Time in Plants (Autumn)

Answer 13

Autumn: Germination occurs followed by vegetative growth.

Flowering is repressed by the expression of Flowering Locus C (FLC), which encodes a transcription factor repressing floral transition.

Question 14

example of epigenetic influence in plants - flowering

Answer 14

time of flowering controlled by flowering regulatory genes - FT FLC

FLC - major repressor of flowering so must be silenced for flowering to occur

Question 15

Regulation of Flowering Time in Plants (Winter)

Answer 15

Vernalization (cold exposure) leads to histone modification.
FLC is epigenetically silenced due to histone modifications - methylation
after long term silencing, maintains the suppression of FLC for flowering to start in spring

This causes repression of FLC transcription and induces stable epigenetic silencing.

Question 16

Regulation of Flowering Time in Plants (Spring)

Answer 16

Spring: Warmer temperatures maintain the epigenetic silencing of FLC.

The plant enters the flowering stage due to this “memory of the cold.”

Question 17

plant epigenetic inheritance

Answer 17

unlike in animal , plant epigenome is not reset

epigenetic memory helps plant to better cope with stress

for transgenerational inheritance, observe F2 gene since germ-line cells are not set aside early unlike in animals

Question 18

Stress Memory Mechanism 1 (Stress-Adjusted)

Answer 18

Stress-adjusted memory: Constant altered expression of stress response genes, even in the absence of stress.

example - drought adjusted plants - always expressing tricky waxy cuticle to reduce water loss, even if water is present sometimes.

Question 19

Stress Memory Mechanism 2 (Stress-Primed)

Answer 19

Stress-primed memory: Genes have an altered transcriptional response to stress and can respond more quickly to environmental triggers.

Question 20

Role of Diet in Queen and Worker Bee Development

Answer 20

Queen Bee: Develops from larvae fed royal jelly, leading to faster development and fertility.

Worker Bee: Develops from larvae fed worker jelly, leading to slower development and infertility.

Royal Jelly: Produced by worker bees and accelerates development into queens.

Worker Jelly: Made from worker bee secretions mixed with honey and pollen.

Question 21

DNA Methylation in Bees

Answer 21

Workers: Show higher genome methylation, leading to skipping of exons and production of longer proteins.

Queens: Have lower methylation, leading to exon retention and production of shorter proteins.