DNA, Chromosomes & Genomes II

Question 1

What are two views that challenge the theory that histones are more than just structural proteins?

Answer 1

Mammalian chromatin contains equal mass of histone and non-histone proteins.

Histones are highly conserved.

Question 2

How does heterochromatin affect DNA?

Answer 2

It silences the genes it packages without regard to sequence and is directly inherited by daughter cells.

Question 3

You are studying genetics in a laboratory. You examine a condensed chromatin that is highly concentrated around the centromeres and telomeres. Further testing shows that the chromatin contains few genes. What type of chromatin are you studying?

Answer 3

Heterochromatin

Question 4

What type of chromatin is less condensed?

Answer 4

Euchromatin

Question 5

What is position effect variegation?

Answer 5

Breakage events in the genes that bring heterochromatin near active genes tends to silence the genes.

Question 6

What are three histone modifications?

Answer 6

Acetylation of lysines

Mono, di and tri-methylation of lysines

Phosphorylation of serines.

Question 7

When are major histones synthesized?

Answer 7

During S-phase. They are assembled into nucleosomes on daughter DNA helices just behind replication fork.

Question 8

When are variant histones synthesized?

Answer 8

During interphase. They are inserted into already-formed chromatin.

Question 9

What is the purpose of the histone code?

Answer 9

It determines how/when DNA is packaged in a nucleosome. The code is also read by the code reader complex.

Question 10

Reading a histone code invovles joint recognition of marks. Where are these marks found?

Answer 10

On the nucleosome and the histone tail. The meanings of the codes are largely unknown.

Question 11

What is the purpose of code reader-writer enzymes?

Answer 11

They spread the marks (signals) over chromosomes.

Question 12

What is the purpose of chromatin remodeling proteins?

Answer 12

They either condense or decondense long stretches of chromatin as the reader moves along.

Question 13

How is remodeling contained to one area of the chromosome?

Answer 13

Physical barriers

Enzymatic barriers

HS4 region (protects the beta globin locus from silencing and contains a cluster of histone acetylase binding sites).

Question 14

What is the purpose of centromeric heterochromatin?

Answer 14

It contains H3 histone CENP-A and other proteins that pack the nucleosomes into dense arrangements to form the kinetochore, a structure required for attachment of the mitotic spindle.

Question 15

What is satellite DNA?

Answer 15

Centromere sequences in humans that consist of short repetitive DNA sequences.

Question 16

What are neocentromeres?

Answer 16

Centromeres that form spontaneously on fragmented chromosomes, some of which lack alspha satellite DNA.

Question 17

What does de novo centromere formation require?

Answer 17

A seeding event on allpha satellte DNA.

Question 18

How are histones inherited?

Answer 18

H3 - 4 tetramers are directly inherited by the daughter strands at the replication fork.

Question 19

What are the larges chromosomes known?

Answer 19

Lampbrush chromosomes.

It is a series of large chromatin loops emanating from a linear chromosome axis.

Question 20

How are different genetic activities localized in the nucleus?

Answer 20

Different “neighborhoods” have effects on gene expression.

Distinct biochemical environments have high local concentrations of enzymes and molecules for specific processes.

Question 21

What is the final step in chromosome packaging?

Answer 21

Condensing the mitotic chromosomes.

Question 22

How are sister chromatids formed and how are they held together?

Answer 22

Two daughter DNA molecules replicated in interphase are separately folded to produce two sister chromatids. They are held together at their centromeres.

Question 23

What are the two purposes of condensation?

Answer 23

Disentanglement of sister chromatids to allow separation for cell division.

Protection of fragile DNA molecule as separation occurs.

Question 24

Compaction of DNA is aided by what proteins?

Answer 24

Condensins.

ATP hydrolysis coiles the DNA molecules into chromatids.

They are the major structural component of the core of every metaphase chromosome.

Question 25

What are homologues?

Answer 25

Genes that are similar in both sequence and function due to common ancestry.

Question 26

How do genomic changes occur?

Answer 26

1/1000 nucleotide pairs is randomly changed in the germ line every million years.

Question 27

How are sequence comparisons helpful?

Answer 27

They can provide insight into purpose or function.

Question 28

What are pseudogenes?

Answer 28

A duplicated gene that has become irreversibly inactivated by multiple mutations.

Question 29

What is divergence?

Answer 29

Both copies remain functional while dverging in sequence and pattern of expression.

Question 30

Human genes vary ___% from each other.

Answer 30

0.1%

Question 31

What are SNPs (single nucleotide polymorphisms)?

Answer 31

Points in the genome where one group has one nucleotide and another group has another.

Question 32

What are CNVs (copy number variants)?

Answer 32

Presence of many duplications and deletions of large blocks of DNA. Some blocks are common and others rare; the significance of most is unknown.

Question 33

What is epigenetics?

Answer 33

A form of inheritance that is superimposed on the genetic inheritance based on DNA. It is the study of trait variations that are caused by external or environmental factors that switch genes on and off and affects how cells read genes.

It includes DNA methylation, chromatin structure and histone modification.