Nucleic Acids 1- Nucleic Acids and chromosomes

Question 1

What are chromosomes made of

Answer 1

DNA and histones- they segregate as cells divide.

Question 2

What is a nucleotide?

Answer 2

A polymer made up of nucleic acid monomers.

Question 3

Characteristics of RNA

Answer 3

Single stranded and it can assume a variety of shapes.

Question 4

What does a nucleotide consist of?

Answer 4

A pentose sugar, a nitrogenous base and one or more phosphate groups.

Question 5

What is the difference between deoxyribose and ribose?

Answer 5

no -OH group on 2’ C atom in deoxyribose- just a H group.

Question 6

What ‘ Carbons are bonded to what

Answer 6

1’ C- base
3’C- OH group
5’C- phosphate group.

Question 7

What are the differences between adenine and guanine

Answer 7

Adenine has an amine group attached to the purine ring- guanine has a carbonyl group and amine group attached to the purine ring.

Question 8

What is the difference between uracil, cytosine and purine

Answer 8

Cytosine has an amine group attached to the pyrimidine ring, thymine has a carbonyl group attached to the pyrimidine ring, uracil has a methyl group attached to the pyrimidine ring.

Question 9

What do nucleosides consist of

Answer 9

Pentose sugar and a base.

Question 10

What are the names of the nucleosides

Answer 10

adenine	  (deoxy)adenosine
guanine	  (deoxy)guanosine
cytosine  (deoxy)cytidine
uracil	  (deoxy)uridine
thymine  (deoxy)thymidine

Question 11

Describe the structure of a single chain of DNA

Answer 11

A long chain of deoxyribonucleotide monomers linked by phosphodiester bonds.

Question 12

Describe the bonding between adjacent nucleotides.

Answer 12

The 3’-OH of sugar of one nucleotide is linked to phosphate group, which in turn is joined to 5’-OH of adjacent sugar.

Question 13

What is the directionality of DNA

Answer 13

5’ end and 3’ end- DNA is asymmetrical.

Question 14

In what sequence are the nucleotides read

Answer 14

The first nucleotide in a sequence has a free 5’ phosphate, and the last one a free 3’ OH.

Question 15

How did Watson and Crick determine the secondary structure of DNA?

Answer 15

Analysing X-Ray diffraction patterns from DNA fibres.

Question 16

Where are the phosphate groups found in the double helix

Answer 16

on the outside-giving DNA a negative charge.

Question 17

Where are the bases found in the double helix.

Answer 17

They point inwards and the flat planes are perpendicular to the helix. They stack on top of each other- contributing to the stability of the double helix.

Question 18

How many bases are there per helical turn

Answer 18

10

Question 19

What is the width of DNA

Answer 19

2nm

Question 20

Why are the hydrogen bonds between C and G more stable

Answer 20

Because there are more of them

Question 21

Describe the melting and re-annealing of DNA.

Answer 21

Melting- heat to near boiling point or have a low salt conc- denatures DNA
Re-annealing- cool or high salt conc- hydrogen bonds reform- and due to CBP- original DNA strand reformed- hence it is a reversible process.

Question 22

What is the genome

Answer 22

The entire DNA coding for the organism

Question 23

Describe the E.coli genome

Answer 23

has 4.6 x 106 base pairs in a single circular double-stranded molecule. The length of E.coli DNA is 1.4 mm- longer than itself- has to be supercoiled. Less DNA- no chromosomes- DNA is circular.

Question 24

Describe the human genome

Answer 24

~ 3 x 109 base pairs of DNA divided into chromosomes that each contain a single, linear double-helical DNA molecule of ~ 200 x 106 base pairs.

Question 25

When are chromosomes visible

Answer 25

Just before cells begin mitosis- hence they are not visible in non-dividing cells.

Question 26

Describe the difference between mitotic and interphase chromosomes

Answer 26

Mitotic- highly condensed chromosomes
Interphase- more extended form
Chromosomes are not all the same size.

Question 27

What is the Karyotype

Answer 27

A karyotype is an organised profile of someone’s chromosomes.
A diploid human cell has 46 chromosomes
22 pairs of ‘normal’ chromosomes (autosomes) and
2 sex chromosomes (X and Y)

Question 28

What can the Karyotype be used for

Answer 28

Can be used to detect abnormalities such as genetic diseases and cancer. Each chromosome is stained with unique fluorescent dye. For example Abnormalities in breast tumour: 48 chromosomes instead of 46, multiple translocations, eg two pieces of chromosome 8 (green) plus piece of chromosome 17 (purple).

Question 29

Why is it important that the DNA is coiled.

Answer 29

The DNA in a diploid human cell is nearly 2m long- but nucleus is only 5 - 8 m in diameter.

Question 30

How is DNA packaged in eukaryotic cells

Answer 30

Eukaryotic DNA is tightly packaged, forming a complex with proteins. This complex is called chromatin. During interphase, when cells are not dividing, chromatin is more extended. Further condensation during mitosis produces chromosomes.

Question 31

What is the lowest level of packaging

Answer 31

A nucleosome- which consists of DNA wrapped around histones.

Question 32

Describe the structure of the nucleosome

Answer 32

8 histones; 2 each of 2A, 2B, 3 and 4
About 150 base pairs of DNA wrapped around the histones (core DNA)
Histone 1 between the nucleosomes- on the outside.
Linker DNA between each nucleosome.

Question 33

How do histones and DNA interact.

Answer 33

Histones: positively charged; interact with negatively charged sugar-phosphate backbone of DNA

Question 34

What fold condensation do nucleosomes give

Answer 34

around 7 fold- 200 bp is 68 nm when not wrapped- but wrapping around each histone octamer is around 10nm.

Question 35

How are the nucleosomes packed further to give chromatin

Answer 35

Nucleosomes are folded to give 30nm fibre- 40 fold condensation- 6 nucleosomes then coil together to fold into loops. Net result: each DNA molecule has been packaged into a mitotic chromosome that is 10 000 fold shorter than its fully extended length.
Interphase chromosomes will have a lower degree of folding.