Lecture 19: Organisation of DNA

Question 1

Howmanychromosomesdoyouhave?

Answer 1

46

Question 2

How manychromosomesdidyouinheritfromyourfather?Fromyourmother?

Answer 2

23 each

Question 3

How many different types of chromosomes do males/females have?

Answer 3

male: 24 (XY)
female: 23 (XX)

Question 4

Major landmarks on a chromosome?

Answer 4

Telomere and centromere
Telomere: located at the ends of chromosomes; protects it during replication so it doesn’t become shortened
Centromere: mitotic spindle attaches here

Question 5

What are the stages in the CELL CYCLE?

Answer 5

G1 G0 S G2 Mitosis

Question 6

What is interphase?

Answer 6

All the rest of the cell cycle except mitosis

Cells spend the most time here

Question 7

Howdoesthepackingofachromosomeschangeatdifferentstagesofthecellcycle?

Answer 7

It becomes more condensed as it goes into mitosis but loosens after the cell undergoes cytokinesis and re enters interphase

Question 8

WhatisaDNAsupercoil? Why is it important?

Answer 8

DNA supercoiling refers to the over- or under-winding of a DNA strand, and is an expression of the strain on that strand. Supercoiling is important in a number of biological processes, such as compacting DNA, and by regulating access to the genetic code, DNA supercoiling strongly affects DNA metabolism and possibly gene expression.

Question 9

Why do chromosomes need to be tightly packed during mitosis?

Answer 9

This makes the chromosomes compact and unlikely to tangle up or break while they are being organized and sorted.

Question 10

Is DNA generally underwound or overwound?

Answer 10

Overwound. DNA has the natural tendency to coil up (more twists)

Question 11

What enzyme is used to relieve tension in DNA?

Answer 11

DNA topoisomerase

Question 12

If you were to run a circular DNA on agarose gel, would the underwounded or overwounded DNA travel the furthest?

Answer 12

The overwound would travel further

Question 13

Is it easier to separate underwounded or overwounded strands?

Answer 13

Underwounded

Question 14

Describe how DNA Topoisomerase relieves tension

Answer 14

The Topoisomerase first takes a section of the DNA and binds it to its C terminus end. Then it takes another section from the SAME DNA strand, and binds that to its N-terminus end. The C-terminus strand gets cut in half, and the N terminus strand moves through this cut and is released through the C terminus end. The C terminus strand is ligated.

Question 15

Name the different type of histones. Where are they found?

Answer 15

H1, H2A, H2B, H3, H4
All BUT H1 are found wrapped by a DNA strand.
H1 is fond on the outside, “locking” everything in place

Question 16

WhatisimportantaboutthehistoneN‐terminaltails?

Answer 16

they are highly disordered, sticks out to the side, and can interact with tail of neighboring nucleosome

Question 17

What is a nucleosome? What is its overall structure like?

Answer 17

Is a small DNA bundle with all the components.

Histones (2 each of each type of histone), wrapped in DNA, held in place by H1

Question 18

How are chromosomes regulated and modified?

Answer 18

modification of HISTONES; it alters HOW the chromosome is assembled; how assesseble the bases are etc…

Question 19

Namethreetypesofhistonemodificationthatcanbeusedtoregulatechromatinpacking

Answer 19

Methylation, acetylation, phosphorylation…

Question 20

What is the “histone code”?

Answer 20

A hypothesis that the transcription of genetic information encoded in DNA is in part regulated by chemical modifications to histone proteins

Question 21

What is the 30nm fibre

Answer 21

The 30nm fiber is the DNA in its interphase state. The DNA is wrapped around the histone complex and then is twisted into a solenoid structure, the 30 nm fiber.
It is when H1 is added and DNA is not accessible

Question 22

What enzymes are involved in acetylation of chromatin?

Answer 22

Acetyltransferases and histone deacetylases

Question 23

WhatisthedifferenceinchromatinpackingbetweenapartoftheDNAthatisnotbeing
accessedbyproteins,andapartoftheDNAthatisbeingactivelyused

Answer 23

Being used: must become less tightly strained, so that DNA binding proteins can easily access it.

Question 24

Namethreeexamplesofchangestothehistonesthatbringaboutchangesinthechromatinpacking.

Answer 24

1. Change the positioning of the nucleosomes; how far apart they are
2. Precense of histone variants: can remove histones/replace existing ones
3. Covalent modification of nucleosomes; acetylation, phosphorylation, methylation…

Question 25

WhatDNAbaseischemicallymodifiedintheregulationofgeneexpression?Isgene
expressionincreasedordecreasedbecauseofit?

Answer 25

Cytosine becomes 5’-methylcytosine; METHYLATED

Gene is not expressed

Question 26

What is a CpG island?

Answer 26

CpG islands typically occur at or near the transcription start site of genes. a cytosine base followed immediately by a G (guanine) base (a CpG) is rare in vertebrate DNA because the cytosines in such an arrangement tend to be methylated.

Question 27

What is 5-methycytosine and where does it occur?

Answer 27

it is a modification of cytosine; occur on CpG islands

Question 28

Where are CpG typically found?

Answer 28

Clusters of CpG typically found together. Usually found on promotor regions

Question 29

When one side is methylated, so is the other. What enzyme does this?

Answer 29

DNA methyltransferase

Question 30

What is the outcome of CpG methylation?

Answer 30

Represses the promotor; gene is regulated (not expressed)