1.2 Replication of DNA Flashcards
What is replication?
replication is the process by which DNA molecules can direct the synthesis of identical copies of themselves
When does DNA replication occur?
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
Why is DNA replication important?
CATCHPHRASE ANSWER
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
Describe phases of DNA replication?
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
describe semi conservative replication?
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
TOPTIP
semi - half
conserve - save
Explain the full process of DNA replication?
- DNA molecule unwinds
- Hydrogen bonds break unzipping the molecules and exposing the bases on both DNA strands to form a y shaped replication fork
- A primer attaches to one end of each exposed DNA template strand
- DNA polymerase can now add free complementary nucleotides to the 3’ end of the growing strand
- hydrogen bonds form between the bases
- strong chemical bonds form between the phosphate and deoxyribose sugar of adjacent nucleotides
- Ligase enzymes joins the fragments to form a complete lagging strand
- Each replicated DNA molecules is made of one original template strand and a newly synthesised strand
Why when the DNA in a chromosome is being replicated, many replication forks are formed at the same time?
it results in the dna of whole chromosomes being replicated quickly and precisely
What are primers?
short complementary sequences of nucleotides that allow DNA polymerase to bind
TOP TIP
dna polymerase can only add nucleotides to a primer, not directly to the beginning of the DNA strand
What is the leading strand?
replication of DNA from 3’ end is continuous moving towards the replication fork
What is the lagging strand?
replication from 5’ end is discontinuous (in fragments) moving away from the replication fork
TOP TIP
each strand can only be synthesised in a 5’ to 3’ direction (by DNA polymerase adding nucleotides to the 3’ end of the primer)
Function of the enzyme dna polymerase?
forms strong chemical bonds between nucleotides from the 3’ end of a primer
Function of the enzyme ligase?
forms strong chemical bonds between fragments in the lagging strand
What is DNA amplification?
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)
One cycle of PCR involves what?
three steps carried out at different temperatures - thermocycling
Primers in PCR?
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
TOP TIP
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
TOPTIP
heat tolerant DNA polymerase comes from extremophile bacteria that live in hot springs. Their enzymes are adapted to work at high temperatures.
Explain fully the process of PCR?
- DNA heated to 95 degrees celsius to separate the original DNA strands
- Sample cooled to 55 degrees celsius. complementary primers can now be added to anneal to the start of each strand to be copied
- Sample heated to 72 degrees celsius and heat tolerant DNA polymerase is added
- Complementary free DNA nucleotides are added to the 3’ end of the new strands
- the number of original molecules has now doubled - this is called amplification
- steps 1 - 5 are repeated for 25 - 35 more cycles amplifying the DNA to make many copies
TOP TIP
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
TOP TIP
PCR involves repeated cycles of heating and cooling to make copies of DNA
How many copies would you have of a single DNA fragment after 10 cycles through a PCR machine?
2 to the power 10 = 1024
Uses of PCR?
forensic science to test for genetic diseases early detection of infection phylogenetics Research archeobiology paternity cases
Use of PCR in forensic science?
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
Use of PCR to test for genetic diseases?
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
Use of PCR in early detection of infection?
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
Use of PCR in phylogenetics?
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.
Use of PCR in Research?
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.
Use of PCR in Archeobiology?
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
Use of PCR in Paternity Cases?
PCR can be used to amplify DNA fragments from two possible fathers and a child can be used to confirm paternity.
