lecture 10

Question 1

Are mitotic or interphase chromosomes more compacted?

Answer 1

mitotic are much more compact than interphase chromosomes

Question 2

The DNA in chromatin is tightly associated with proteins called

Answer 2

histones

Question 3

histones package and order the DNA into structured units called

Answer 3

nucleosomes
(a fundamental unit of chromatin “beads-on-a-string”)

Question 4

histone

Answer 4

small proteins associated with DNA

Question 5

which amino acids would you expect histones to be rich in?

Answer 5

basic amino acids

Question 6

how is the nucleosome formed

Answer 6

DNA is wrapped around the histone core to form the nucleosome

histone H1 clamps around the DNA

Question 7

describe histone interaction with DNA

Answer 7

not specific

Question 8

Histone N-terminal tails (4 characteristics)

Answer 8

-protrude from the octameric disk

-highly-conserved between histones

-positively charged w many Lys and Arg residues: basic amino acids (good bc DNA is negative)

-highly flexible

Question 9

Histones (4 characteristics)

Answer 9

-determine the structure of the nucleosome
-treat DNA w nuclease
-DNA bound to the histone is protected from degredation
-the core can be dissociated by high [salt] concentration

Question 10

When you add high concentration of salt to histone

Answer 10

DNA dissociates from histone

Question 11

30nm chromatin fiber of packed nucleosomes

Answer 11

the beads-on-string coil to form 30nm chromatin fibers

also known as 30nm chromatin fibers or “solenoid”

Question 12

H1 holds the DNA on the

Answer 12

histone core

Question 13

H1 is necessary for the formation of

Answer 13

the 30nm fiber

-contains a long pair of flexible tails
-H1 is located in the interior of the 30nm fiber

Question 14

Regions of the DNA that are transcriptionally active are

Answer 14

less well ordered and contain very little histone H1

Question 15

does transcription occur in the 30nm fiber?

Answer 15

no

compaction makes the DNA inaccessible to transcription and replication factors

Question 16

N-terminals and the 30nm fiber: interactions

Answer 16

Histone tails may help pack nucleosomes into the 30nm fiber

+ charged N-termini bind (-) charged sugar-phosphate backbone on DNA of neighboring nucleosomes

Question 17

The 30nm fiber is held together by (2)

Answer 17

1. histone H1 proteins which “pull” the nucleosomes together
2. N-terminal tails

Question 18

High order of chromatin organization

Answer 18

not well understood!!

-further looping/coiling of solenoid DNA into highly compacted, transcriptionally silent, form of chromatin occurs during mitosis

-certain regions of the DNA associate with a nuclear or chromosome “scaffold”

Question 19

Chromatin remodelling complexes

Answer 19

protein machines that use ATP to change the position of DNA wrapped around nucleosomes

-makes the DNA more OR less accessible!!!

Question 20

How does the cell remodel to change the compaction and gene expression

Answer 20

modification of histones
-histone code

Question 21

histone code

Answer 21

hypothesis that specific combinations of modifications dictate gene expression or silencing

-works with chromatin-remodelling complexes
-covalent modifications may act as a docking site for regulatory proteins

Question 22

Histone tails are often —- that results in changes in chromatin structure (and example)

Answer 22

covalent modifications

-ex. addition of acetyl phosphate or methyl groups

Question 23

Interphase chromatin is

Answer 23

not uniformly packed, contains regions that are expressed more or less often

Question 24

2 forms of interphase chromatin

Answer 24

Heterochromatin= most highly condensed form
Euchromatin= the rest of the interphase chromatin

Question 25

Heterochromatin can

Answer 25

spread

Question 26

heterochromatin-specific modifications can

Answer 26

recruit proteins that will make the same modifications

will continue making these modifications until it is blocked by a barrier DNA sequence

Question 27

Heterochromatin and disease (and example)

Answer 27

if an important gene is appropriately packaged into heterochromatin, there would be limited gene expression!!!!

ex. deficiency of beta-globin causing anemia (affects level of DNA compaction)

Question 28

Female mammal X chromosomes

Answer 28

they have two X chromosomes:

-only one is active
-the other is inactivated (at random) by condensing it into heterochromatin
-All daughter cells inherit the same histone modification

Question 29

example of epigenetic inheritance

Answer 29

transmission of a pattern of gene expression from one cell to the cells it creates that does not involve altering the nucleotide sequence of the DNA

Question 30

Mutations in a gene on the X chromosome result in a particular form of colour blindness. If a woman if colourblind, what can we conclude about her father?

Answer 30

If the woman is colourblind, she has a mutation in this gene on both X chromosomes meaning that the father must be colourblind.

Question 31

Would you expect her mother to be colourblind too? If no, would she have normal colour vision?

Answer 31

The mother could be colourblind and be carrying two defective copies. She might only have one defective copy. It’s also possible that she has one defective copy and may see coloured objects with reduced resolution.

Question 32

less compact chromatin means

Answer 32

more gene activity