TH- DNA and inheritance Flashcards
What is transformation according to Griffith?
a change in genotype and phenotype due to the assimilation of external DNA by a cell
What experiment lead Griffith to observe transformation?
he killed the pathogenic bacteria with heat and then mixed the cell remains with living bacteria of the nonpathogenic strain, some of the living cells become pathogenic
What are bacteriophages?
viruses that infect bacteria
Describe Hershey and Chase’s experiment that proved DNA is a genetic material (8)
- 2 samples of phages were grown, one was grown in radioactive isotope of Phosphorus that tagged the DNA and another was grown in radioactive isotope of Sulfur which tagged the protein
- both samples were mixed with bacteria allowing them to infect the bacterial cells
- The samples were then mixed in a blender to free the of the phage parts outside the cell
- then the mixture was centrifuged to separate the bacteria into a pellet at the bottom and the free phages and phage parts, which are lighter, remained suspended in the liquid above.
- both the samples radioactivity was measured
- the protein was found in the liquid and the DNA in the pellet
- this suggests that the DNA entered the bacterial cell
- hence DNA was carrying the genetic material as it was responsible in entering the cell and reprogramming it to produce viruses
What are Chargaff’s rules? (2)
(1) DNA base composition varies between species
(2) for each species, the percentages of A = T bases and G = C
Describe the structure of an individual DNA molecule
a deoxyribose sugar bonds to a phosphate group on its carbon 5 and a nitrohenous base on its carbon 1
Describe the structure of DNA strands
- via dehydration reaction phosphate on carbon 5 covalently bonds with Carbon 3 sugar
- that forms a phosphodiester bond between them and this is repeated many times to form a sugar-phosphate backbone
- hydrogen bonds form between the nitrogenous bases of the antiparallel strands keeping them together
- the hydrogen bonding between the bases also twists the strands into its double helix shape
What is double helix?
presence of 2 strands interwined
What is meant by antiparallel strands?
they run in opposite directions
Which bases are purines and how many rings do they have?
- Adenine and Guanine
- 2 rings
which bases are pyrimidines and how many rings do they have?
- Thymine, Cytosine and Uracil- (UCT)
- 1 ring
How many H bonds does A form with T and C form with G?
- A with T: 2 bonds
- C with G: 3 bonds
State the 3 alternative models for DNA replication
- semi-conservative
- conservative
- dispersive
Explain the semi-conservative model
two strands of the parental molecule separate, and each functions as a template for synthesis of a new, complementary strand.
explain the conservative model
two parental strands somehow come back together after acting as templates for new strands.
explain the dispersive model
a mixture of old and new DNA
What are origins of replication?
short stretches of DNA that have a specific sequence of nucleotides. and is where the replication process begins
what is a replication fork?
Y-shaped region where the parental strands of DNA are being unwound.
What does helicase do?
enzymes that untwist the double helix at the replication forks, separating the two parental strands
what does single-strand binding (SSB) protein do?
Binds and stabalizes single-stranded DNA until it is used as a template
what does Topoisomerase do?
it is an enzyme that relieves overwinding strain ahead of replication forks by breaking swiveling and rejoining DNA strands
what does primase do?
synthesizes RNA primers using DNA template strand
what does DNA pol III do?
Synthesizes a new DNA strand by adding nucleotides on the 3’ end
what does DNA pol I do?
Removes RNA nucleotides and adds DNA nucleotides
what does DNA ligase do?
Joins 3’ end of DNA to rest of the leading strand and joins the Okazaki fragments of lagging strand
What is a primer?
it is a short RNA strand and without it DNA polymerase wont know where to start
How does the replication process start
- proteins recognize the specific sequences of nucleotides at the origins of replication
- they attach to the DNA and separate the strands opening up a replication bubble
Describe the process of DNA replication (9)
1- Helicase separates the DNA strands by breaking the hydrogen bonds
2- topoisomerase is added to reduce strain and prevents supercoils ahead of the replication fork
3- SSB proteins binds to single DNA strands to stabilize the strands and prevent them from snapping back together
4- Primase comes in and makes RNA primer on the 3’ end of the template strands
5- DNA pol. III elongates the strand in 5’ to 3’ direction
6- it does that by moving from the 3’ to 5’ on the template strand adding nucleotides to elongate the new DNA strand
7- on the leading strand the DNA polymerase III elongates the strand continuously whereas on the lagging strands it elongates discontinuously forming okazaki fragments
8- DNA pol. I will replace primer with DNA
9- DNA ligase will join the DNA at the 3’ end to the rest of the leading strand and will seal the gaps between Okazaki fragments
What is the leading strand?
it moves in the direction of the replication fork
what is the lagging strand?
it moved opposite to the replication fork
why do okazaki fragments form?
The template of the lagging strand goes from 5’ to 3’ and DNA polymerases can add nucleotides only to
the 3′ end
-as it replicates in the opposite way of the replication fork it forms segments due to the gaps in between
How does proofreading take place during DNA replication?
If DNA polymerase makes a mistake it can stop remove the wrong nucleotide and replace it with the right one
What is mismatch repair?
enzymes remove and replace incorrectly paired nucleotides
Explain the nucleotide excision repair of DNA damage
1-enzymes detect and repair damaged DNA which distorts the DNA molecule.
2- nuclease enzyme cuts the damaged DNA strand at two points, and the damaged section is removed.
3- DNA polymerase adds the missing nucleotides
4- DNA ligase seals the free ends
What is nuclease?
DNA cutting enzyme
What are telomeres?
they are short repeated sequences
Provide 2 functions of telomeres
1- prevent the staggered ends of the daughter molecule from activating the cell’s systems for monitoring DNA damage as that can lead to cell cycle arrest or death.
2- they postpone the erosion of genes near the ends of DNA molecules
what are germ cells?
egg and sperm cells
What prevents telomeres of germ cells from getting shorter?
telomerase
What does telomerase do?
catalyzes the lengthening of telomeres in eukaryotic germ cell
why do prokaryotes not need telomeres?
Most prokaryotes have a circular chromosome, with no ends, so the shortening of DNA does not occur
How does normal shortening of telomeres prevent cancer?
by limiting the number of divisions that somatic cells can undergo
What is chromatin?
complex of DNA and protein found in eukaryotic cells
What proteins are responsible for the first level of DNA packing in chromatin and how do the DNA and this protein bind?
- Histones
- DNA backbone has a -ve charge and histones have a +ve charge allowing them to bind tightly together
What is a nucleosome?
consisting of a length of DNA coiled around a core of histones.
How is chromatin organised into fibres to form a metaphase chromosome? mention the final width of the chromatid
- DNA winds around histones to form nucleosome
- Interactions between nucleosomes cause the thin fiber to coil or fold into this thicker fiber
- fiber forms looped domains that attach to proteins
- looped domains coil further forming the metaphase chromosome
- width of a chromatid is 700 nm
What is heterochromatin?
tightly packed form of DNA
what is euchromatin?
loosely packed form of DNA
why is it difficult for a cell to express genetic information coded in heterochromatin?
DNA is largely inaccessible to the machinery in the cell responsible for transcribing the genetic information coded in the DNA
