S7- Intro to DNA

Question 1

a) What is the length of DNA
b) What is the width of DNA?
c) what is the significance of the differences between the two?

Answer 1

a) 2 metres per cell
b) 2nm
c) It means that there has to be ALOT of packaging in the nucleus!

Question 2

What is are the steps of DNA to protein?

Answer 2

• Genes in the nucleus (code for proteins)

- Proteins in cytoplasm

Question 3

Outline the process of DNA packaging

Answer 3

1. Double stranded DNA alpha helix (lots of negative charged phosphates)
2. Beads on a string: Positively charged histones bind DNA (interactions with negative charge) wrap around DNA to form a nucleosome
3. Forms a solenoid: FIBRE Called chromatin- lots of packed nucleosomes
4. Also forms loose chromatin
5. Packaged into a chromosome

Question 4

Difference between euchromatin and heterochromatin?

Answer 4

• Heterochromatin is firmly packed on chromsomes (genetically inactive) - solenoid 30nm fibre
• Euchromatin uncoiled/loosely packed (genetically active)- beads on a string

Question 5

a) What are genes and where are they located?
b) What is a genome?
c) what is the human genome?

Answer 5

a) A gene is a sequence of nucleotides of DNA that code for the production of one polypeptide (carry the code for proteins) located in a fixed manner on chromosome
b) The entire DNA sequence
c) 24 chromosomes (22 autosomes plus 2 sex chromosomes X and Y)

Question 6

a) What sort of molecule is dna and rna
b) what do they stand for?
c) What is the difference

Answer 6

a) Nucleic acids
b) Deoxyribonucleic acid and ribonucleic acid

c) They have different Pentose sugars:
RNA: contains RIBOSE (oxy on C 2)
DNA: contains 2-deoxyribose (no oxy on C2)

Question 7

a) What is the difference between a nucleoside and nucleotide?
b) What is a nucleic acid?

Answer 7

a) Nucleoside: base and sugar
Nucleotide: base and sugar and Phosphate

b) Linear polymers of nucleotides (Polynucleotides)

Question 8

Structure of a nucleotide

Answer 8

• Phosphate group
• Pentose sugar
• Nitrogenous base

Question 9

What are the two types of nitrogenous bases and their differences?

Answer 9

1. Purines (Adenine and guanine) - 2 tings

2. Pyrimidines (thymine, cytosine and uracil) - 1 ring

Question 10

What are the base pairing rules?

Answer 10

Complementary base pairing

• Purine to Pyrimidine:
• -> Adenine to Thymine (A–T) Form 2 h bonds or uracil replaces thymine
• -> Guanine to Cytosine (G—C) Form 3 H bonds

Question 11

How do nucleotides join?

Answer 11

• via phosphodiester bonds
• chain has polarity 5’ to 3’
• Phosphate group at 5’ forms a covalent bond with the and OH at 3’

Question 12

How does DNA form?

Answer 12

forms when 2 deoxyribose polynucleotides chain join by H bonds to form a double stranded molecule

• DNA strands are complimentary and anti-parallel
• right handed double helix

Question 13

How do you normally write the DNA sequence?

Answer 13

From 5’ to 3’

Question 14

a) What is the cell cycle?

b) why is it important?

Answer 14

a) The cell cycle, or cell-division cycle, is the series of events that take place in a cell leading to duplication of its DNA and division of cytoplasm and organelles to produce two daughter cells

b) Has check points
important for cancer cells to die

Question 15

How is dna replicated?

Answer 15

Semi-conservative DNA replication

- results in one old strand and one new strand present in each daughter molecule

Question 16

Steps of DNA replication?

Answer 16

1. INITIATION:
   - DNA helicase unzips DNA helix
   Reaction catalysed by DNA polymerase, binds to one end and breaks bonds
2. ELONGATION:
   - DNA strands exposed: both act as template strand
   - Polymerase only works in one direction (3- to 5’) so one strand (lagging strand) has okazagi fragments
   - Picks up complementary free nucleotides and makes complementary strand (adjancent nucelotides form phosphodiester bonds and bases form h bonds)
3. TERMINATION: When two facing replication forks meet and DNA ligase joins final fragments