1.2 Replication of DNA

Question 1

What is replication?

Answer 1

replication is the process by which DNA molecules can direct the synthesis of identical copies of themselves

Question 2

When does DNA replication occur?

Answer 2

the DNA in the nucleus of a cell must make an exact copy of all genetic info (replicate) before a cell can divide by mitosis

Question 3

Why is DNA replication important?

CATCHPHRASE ANSWER

Answer 3

ensures that an exact copy of species genetic info is passed on from cell to cell during growth and from generation to generation during reproduction

Question 4

Describe phases of DNA replication?

Answer 4

46 chromosomes - 23 from each parent
INTERPHASE - dna replication, 92 sister chromatids generated
PROPHASE - Nuclear membrane dissasembles
METAPHASE - chromosomes align at the equator of the cell attached to spindle fibres at centromere (kinetochore)
ANAPHASE - sister chromatids are pulled apart
TELOPHASE - cytoplasm divides (cytokenesis)
two daughter cells formed each with 46 chromosomes

Question 5

describe semi conservative replication?

Answer 5

Each strand of DNA acts as a template for a new complementary strand so each DNA molecule formed during replication would be identical, each containing one ‘parental’ strand and one newly synthesised strand

Question 6

TOPTIP

Answer 6

semi - half

conserve - save

Question 7

Explain the full process of DNA replication?

Answer 7

1. DNA molecule unwinds
2. Hydrogen bonds break unzipping the molecules and exposing the bases on both DNA strands to form a y shaped replication fork
3. A primer attaches to one end of each exposed DNA template strand
4. DNA polymerase can now add free complementary nucleotides to the 3’ end of the growing strand
5. hydrogen bonds form between the bases
6. strong chemical bonds form between the phosphate and deoxyribose sugar of adjacent nucleotides
7. Ligase enzymes joins the fragments to form a complete lagging strand
8. Each replicated DNA molecules is made of one original template strand and a newly synthesised strand

Question 8

Why when the DNA in a chromosome is being replicated, many replication forks are formed at the same time?

Answer 8

it results in the dna of whole chromosomes being replicated quickly and precisely

Question 9

What are primers?

Answer 9

short complementary sequences of nucleotides that allow DNA polymerase to bind

Question 10

TOP TIP

Answer 10

dna polymerase can only add nucleotides to a primer, not directly to the beginning of the DNA strand

Question 11

What is the leading strand?

Answer 11

replication of DNA from 3’ end is continuous moving towards the replication fork

Question 12

What is the lagging strand?

Answer 12

replication from 5’ end is discontinuous (in fragments) moving away from the replication fork

Question 13

TOP TIP

Answer 13

each strand can only be synthesised in a 5’ to 3’ direction (by DNA polymerase adding nucleotides to the 3’ end of the primer)

Question 14

Function of the enzyme dna polymerase?

Answer 14

forms strong chemical bonds between nucleotides from the 3’ end of a primer

Question 15

Function of the enzyme ligase?

Answer 15

forms strong chemical bonds between fragments in the lagging strand

Question 16

What is DNA amplification?

Answer 16

tiny fragments of DNA may be copied to provide enough for analysis. This is called amplification of DNA and is done using a process called polymerase chain reaction (PCR)

Question 17

One cycle of PCR involves what?

Answer 17

three steps carried out at different temperatures - thermocycling

Question 18

Primers in PCR?

Answer 18

in the lab, a single strand of DNA is made with a complementary base sequence to the beginning of the dna fragment to be amplified
this artificial strand of DNA is called a primer and is used to locate the specific DNA target sequence to be copied

Question 19

TOP TIP

Answer 19

In vitro means outside the body of an organism - the opposite of in vivo which means inside the body
DNA replication is in vivo
PCR is in vitro

Question 20

TOPTIP

Answer 20

heat tolerant DNA polymerase comes from extremophile bacteria that live in hot springs. Their enzymes are adapted to work at high temperatures.

Question 21

Explain fully the process of PCR?

Answer 21

1. DNA heated to 95 degrees celsius to separate the original DNA strands
2. Sample cooled to 55 degrees celsius. complementary primers can now be added to anneal to the start of each strand to be copied
3. Sample heated to 72 degrees celsius and heat tolerant DNA polymerase is added
4. Complementary free DNA nucleotides are added to the 3’ end of the new strands
5. the number of original molecules has now doubled - this is called amplification
6. steps 1 - 5 are repeated for 25 - 35 more cycles amplifying the DNA to make many copies

Question 22

TOP TIP

Answer 22

you should be able to calculate the no. of DNA molecules present after a number of cycles in a PCR machine. Remember after one cycle there are two molecules and the number doubles after every further cycle

Question 23

TOP TIP

Answer 23

PCR involves repeated cycles of heating and cooling to make copies of DNA

Question 24

How many copies would you have of a single DNA fragment after 10 cycles through a PCR machine?

Answer 24

2 to the power 10 = 1024

Question 25

Uses of PCR?

Answer 25

forensic science
to test for genetic diseases 
early detection of infection
phylogenetics
Research
archeobiology 
paternity cases

Question 26

Use of PCR in forensic science?

Answer 26

Production of DNA fingerprints from a tiny quantity of genetic material (blood or tissue) found at a crime scene requires PCR to provide enough genetic material for testing
If the genes of a suspect and the genetic material match a positive identification may be made

Question 27

Use of PCR to test for genetic diseases?

Answer 27

to amplify genetic material in embryonic cells to provide enough to test for genetic diseases such as

• fragile x syndrome
• cystic fibrosis
• Duchenne muscular dystrophy
• Huntingtons disease
• neurofibromatosis

Question 28

Use of PCR in early detection of infection?

Answer 28

PCR can be used to amplify viral DNA when only a few cells out of a million are infected e.g. HIV
Otherwise a worried individual may have to wait several months before sufficient cells show infection and a diagnosis can be positively detected

Question 29

Use of PCR in phylogenetics?

Answer 29

looks at evolution of species and common ancestors. small samples from mummified or fossil evidence can be amplified for testing
Using chloroplasts DNA can provide info about the evolutionary relationships between plants and allows plants to be classified according to similarities and differences in their DNA.

Question 30

Use of PCR in Research?

Answer 30

PCR can be used to build up a large bank of stock material from a small initial source e.g. A mutated gene sequence
The stock can then be subjected to several types of analyses and can be divided so that several research teams can work on it.

Question 31

Use of PCR in Archeobiology?

Answer 31

The DNA found in mummified bodies, skeletal remains or insects trapped in amber is usually highly degraded.
However any small samples may be amplified and sequenced to look for connections with other mummified remains, find living relations or build a picture of life at that time

Question 32

Use of PCR in Paternity Cases?

Answer 32

PCR can be used to amplify DNA fragments from two possible fathers and a child can be used to confirm paternity.