Topic 9

Question 1

What are the two types of chromatin?

Answer 1

euchromatin
light
genes are expressed
beads on a string

heterochromatin
dark
genes are not expressed
solenoid 30nm fibre

Question 2

How, in general terms, are proteins made?

Answer 2

DNA makes RNA which makes proteins

Question 3

How is DNA packaged?

Answer 3

DNA is complexed with histones to form nucleosomes
each nucleosomes consists of 8 DNA proteins around which the DNA wraps
nucleosomes form “beads” on a DNA string
the nucleosomes fold up to get a solenoid

Question 4

What are genes and where are they found?

Answer 4

carry the code for proteins

found in a chromosomal location

Question 5

What is the human genome?

Answer 5

the entire DNA sequence of a species
human genome is 24 chromosomes
22 autosomes plus 2 sex chromosomes X and Y

Question 6

What is the difference between nucleosides and nucleotides?

Answer 6

nucleosides
base and sugar

nucleotides
base and sugar and phosphate

Question 7

What is the main structural difference between ribose and deoxyribose?

Answer 7

deoxyribose has H on second carbon

ribose has OH on second carbon

Question 8

What are the two types of nitrogenous bases?

Answer 8

purine
guanine and adenine

pyrimidine
cytosine, thymine, uracil

A G C found in DNA and RNA
T only found in DNA
U only found in RNA

Question 9

How are nucleotides formed?

Answer 9

nucleosides can be phosphorylated into nucleotides

base connected to carbon 1, phosphate group connects to carbon 5 on deoxyribose

Question 10

How can nucleotides monomers be joined together?

Answer 10

connected by phosphodiester bond (connected by phosphate group) to form polynucleotides
this is the primary structure of DNA
chain has polarity 5’ to 3’
chain has distinct ends: 5’P and 3’OH

Question 11

Name the three types of base pairings and their number of hydrogen bonds present

Answer 11

G-C = 3 hydrogen bonds
A-T = 2 hydrogen bonds
A-U = 2 hydrogen bonds

Question 12

How are secondary structures of DNA formed?

Answer 12

base pairings allows DNA to form duplex structures
bases are complimentary
strands run antiparallel
each single strand held together by covalent bonds between the sugars- phosphates
the double strand is held together by hydrogen bonds formed by the complementary base pairs (these are weaker than the covalent bonds)

Question 13

What is the tertiary structure and quaternary structure of DNA that is formed?

Answer 13

tertiary:
right handed double helix with anti parallel strands
held together by stacking of base pairings, Van Der Waals forces hold this together

quaternary:
the formation of the nucleosomes

Question 14

Where does DNA replication take place?

Answer 14

during the S phase

Question 15

How is DNA replication semi- conservative?

Answer 15

because each old DNA strand acts as a template for a new DNA strand

Question 16

Describe the reaction of DNA replication

Answer 16

catalysed by DNA polymerase
hydrolysis of PPi by pyrophosphatase drives the reaction
dNTPs needed as building blocks

separation of the double stranded DNA template
stepwise reaction
synthesis from 5’ to 3’

Question 17

Describe initiation in DNA replication for prokaryotes

Answer 17

DNA is grouped in a circle in bacteria
starts at origin of replication
helicase unwinds the DNA
DNA primase creates RNA primers for the DNA polymerase to build the new strand from
one origin of replication results in two replication forks

Question 18

Describe elongation in DNA replication for prokaryotes

Answer 18

helicase continues to unwind the DNA
DNA polymerase begins to build the new strand by extending the 3’ ends only, creating the new strand 5’ to 3’
this leads to a lagging strand and a leading strand being created
the leading strand is where DNA polymerase continuously extends and builds the new strand 5’ to 3’ on the new strand and from the 3’ end on the template strand
the lagging strand is formed due to the fact that as the DNA unwinds, the DNA polymerase has to keep going back to where unwinding is happening as on the lagging strand it is building 5’ to 3’ on the template strand and 3’ to 5’ on the new strand
primers have to keep being built so DNA polymerase can continue building the strand
Okazaki fragments are created as a result and are joined together by DNA ligase

Question 19

Describe termination in DNA replication for prokaryotes

Answer 19

when the two facing replication forks meet
DNA ligase joins the final fragments
results in two identical DNA molecules

Question 20

Describe the structure of chromosomes

Answer 20

1 chromosome = 1 DNA molecule

replicated chromosome is two chromatids connected by a centromere so that you get one chromosome = two DNA molecules