Transmission and Expression of the Genome

Question 1

what is semi- conservative replication?

Answer 1

double stranded DNA separates into 2 strands- one of which is the template strand and the other forms a strand complimentary to the template (i.e one new and one old)

Question 2

what does DNA polymerase require for a new strand of DNA to be formed?

Answer 2

• template strand
• requires dNTPs
• nucleotides are added at the 3’ end
• RNA primer

Question 3

what are dNTPs?

Answer 3

deoxynucleoside triphosphates (dNTPs)
NB: The N is substituted for the base e.g. dATP or dCTP etc.
Recruitment of dNTPs and consequent binding via a phosphodiester bond to form sugar-phosphate chain by DNA polymerase causes loss of 2 phosphates

Question 4

why do we need an RNA primer to replicate DNA?

Answer 4

DNA polymerase can only extend from a double stranded nucleotide. (can’t start from nothing)
So the first enzyme that comes in is Primase to produce a primer which is recognised by DNA polymerase and adds bases in a 5’ to 3’ direction.
Primer is then removed.

Question 5

what is DNA polymerases role in DNA replication?

Answer 5

Adds 1000 bases per sec to the chain
has proof reading activity- its adds a base, moves back to check it (proof reading), removes it if wrong (by detecting size of double stranded base pair), and then moves on forward e.g C=C bond swapped for C=G bond

Question 6

explain the mechanism of replication

Answer 6

1) DNA helix is unwound by helicases (energy is required to break H bonds)- leaves two exposed strands
2) DNA is produced in a 5’ to 3’ direction
Because of this, both strands cannot replicate fully. The strand that is replicating away from the fork develops through Okazaki fragments.
3) The leading strand- synthesised continuously as one piece by DNA polymerase towards replication fork- from 5’ to 3’
4) The lagging strand- is synthesised discontinuously in the form of Okazaki fragments which are later joined together by DNA ligase enzyme- from 3’ to 5’

Question 7

where does DNA replication occur (on the chromosome)?

Answer 7

DNA is too large to have replication occurring at one site
There are multiple origins of replication present along length of chromosome – these are called REPLICATION BUBBLES
Replication bubbles form at multiple origins
Spread out in both directions until united with one another- efficient & faster replication

Question 8

how are Okazaki fragments made and joined together?

Answer 8

multiple short fragments are being made in the lagging strand therefore multiple RNA primers need to be used
1) the Okazaki fragment has been synthesised with an old RNA primer on it
2) a new RNA primer anneals to the template strand
3) DNA polymerase adds to new RNA primer to start a new Okazaki fragment
4) once it reaches the old RNA primer from the previous Okazaki fragment, it gets removed and replaced by DNA
5) ligase joins the two fragments together.

Question 9

what methods are used to repair mutations in DNA?

Answer 9

DNA polymerase has proof reading ability
stability of base pairing gives high fidelity
Mismatch repair systems- recognises and repairs insertion, deletion, and mis-incorporation of bases (corrects 99% of errors)

Question 10

how do mismatch repair mechanisms work?

Answer 10

idk if I need to know this

Question 11

what chemical reactions can damage DNA?

Answer 11

depurination
deamination

Question 12

what is depurination?

Answer 12

when a nucleotide loses a purine base (could be pyramiding but less common)

Question 13

what is deamination?

Answer 13

where cytosine can become uracil
problem arises when pairing because C-G and U-A

Question 14

how does UV affect DNA?

Answer 14

UV can cause thymidine dimers:
Ultraviolet light is absorbed by a double bond in thymine (and cytosine) bases in DNA. This added energy opens up the bond and allows it to react with a neighbouring thymine and form a dimer

Question 15

what is mitosis?

Answer 15

cell division of a diploid cell to produce two diploid daughter cells
repair

Question 16

what is the purpose of mitosis?

Answer 16

cell division/ organismal growth
repair

Question 17

what are kinetichores?

Answer 17

where spindle microtubules attach to
there are two kinetochores (one for each sister chromatid) at the centromere

Question 18

what are the different spindle microtubules?

Answer 18

aster microtubules- extend from the spindle pole to the cell membrane
kinetochore microtubules- attach the chromosomes to the spindle pole
interpolar microtubules- extend from the spindle pole to the opposite spindle pole

Question 19

what are the phases of mitosis?

Answer 19

Prophase
Prometaphase
Metaphase
Anaphase
Telophase
Cytokinesis

Question 20

what happens in prophase?

Answer 20

chromosomes condense
organelles called centrosomes (positioned and orientated by astral microtubules) produce kinetochore microtubules and interpolar microtubules (spindle)
they also move apart

Question 21

what happens in prometaphase?

Answer 21

nuclear envelope breaks down
kinetochore microtubules bind to the kinetochore of the chromosome
chromosomes begin to migrate to the metaphase plate. Interpolar microtubules from different centrosomes, bind together.

Question 22

what happens in metaphase?

Answer 22

chromosomes fully condensed & located at metaphase plate
(M-phase checkpoint – checks all centromeres are connected to microtubules).

Question 23

what happens in anaphase?

Answer 23

each centromere splits
sister chromatids of each chromosome pulled to opposite poles due to the kinetochore microtubule (attached to the kinetochore) shortening
The interpolar microtubules extend further aiding in the movement of chromosomes to the poles.
happens fast

Question 24

what happens in telophase?

Answer 24

chromosomes reach poles & decondense and start to become transcriptionally active again
nuclear membrane reforms
cytoplasm starts to divide.

Question 25

what happens in cytokinesis?

Answer 25

contractile ring of actin and myosin filament’s forms a cleavage furrow- divides the cytoplasm
two genetically identical daughter cells produced

Question 26

what is meiosis?

Answer 26

produces 4 genetically different haploid gametes (via reduction division- 23 chromosomes per gamete)

Question 27

what is the function of meiosis and how is it achieved?

Answer 27

Induce genetic variation in offspring
This is achieved via independent, random segregation & crossing-over.

Question 28

how is re-assortment by crossing over achieved?

Answer 28

Metaphase I: Homologous chromosomes pair up, forming bivalents. Moving them through the cytoplasm causes them to become entangled. This is called crossing over. Sections of DNA on ‘non-sister’ chromatids, which became entangled now break off (chiasmata) and there is an exchange of genetic material between the non-sister chromatids of the homologous chromosomes.This forms recombinant chromatids.

Question 29

what is a chiasmata?

Answer 29

The points at which the chromatids break and re-join

Question 30

how is independent random segregation achieved?

Answer 30

Metaphase I: The orientation of homologous pairs on the metaphase plate is random and independent of any each. The maternal or paternal chromosomes can end up facing either pole. It is therefore completely random which chromosome from each pair (maternal or paternal) end up in which daughter cell.
Metaphase II: Due to crossing over, the chromatids are no longer identical so there is independent assortment again when they line up at the metaphase plate and the non-identical sister chromatids separate.

Question 31

how many cell divisions are required to produce sperm?

Answer 31

No. of cell divisions to produce a sperm is much greater than to produce egg

Question 32

where do sperm cells come from?

Answer 32

Sperm come from a parent cell called spermatogonium.
Each spermatogonium cell in the testis at age 15 (puberty) is the result of 30 previous cell divisions

Question 33

how many times does Spermatogonium divide?

Answer 33

Spermatogonium divide every 16 days from puberty to produce a stockpile of sperm

Question 34

what process does Spermatogonium undergo to make sperm?

Answer 34

Spermatogonium first undergo mitosis to produce two daughter cells
One daughter cell remains as a spermatogonium to maintain supply
Other daughter cell makes 4 spermatozoa by meiosis.

Question 35

how many cell divisions are required to produce egg cells and what is the process known as?

Answer 35

22 cell divisions occur by 5 months gestation period (before birth) to make stock of 2.6 million oocytes
This process is known as oogenesis and ceases at birth.

Question 36

what happens to the oocyte in meiosis 1?

Answer 36

Each oocyte remains at the crossing over stage of meiosis I.
Each month one oocyte is ovulated:
Meiosis one is completed at ovulation producing two diploid cells. One cell however takes almost all the cytoplasm and the other one which has little is known as a polar body.

Question 37

what happens to the oocyte in meiosis 2?

Answer 37

Meiosis two occurs during fertilisation. Once the sperm has entered the oocyte the oocyte undergoes meiosis II halving its DNA to become haploid and the extra DNA is removed via another polar body.
The sperm nucleus then combines with the ovum nucleus to produce a zygote.