TOPIC 2.7

Question 1

Explain the Hershey and Chase experiment

Answer 1

Investigated what the genetic material of cells using viruses. Viruses inject genetic material into cells (non genetic material stays outside of cell) and the infected produce large numbers of the virus by bursting and releasing copies. Hershey and Chase studied T2 bacteriophage, infects E .Coli. It has simple structure . They used radioactive isotopes to label the virus (S for protein coat and P for DNA). They combined it with E Coli and separated them with centrifuge. The small virus remained in the supernatant and the bacteria formed a pellet . Sulphur (outside of virus) remained in supernatant and P (DNA) found in pellet. Deduced that DNA was genetic material because it was being transferred into the bacteria, labeled by phosphorus.

Question 2

a. Explain what a supernatant is

Answer 2

A supernatant is the liquid that remains after centrifusion. In the case of this experiment, the supernatant contains small viruses without their genetic material, tagged by radioactive sulphur isotopes .

Question 3

b. Explain why genetic material should be found in the pellet and not supernatant

Answer 3

This is because the pellet is formed by the bacteria, and if the virus as infected the bacteria, its genetic information should be found in it, and therefore in the pellet. The supernatant consists of virus protein coats, without their genetic material, so it would not be found in the supernatant. .

Question 4

d. Discuss the evidence that DNA is the chemical which transforms the bacteria into infected cells

Answer 4

DNA is found in the pellet, and therefore is in the bacteria. This suggests that it is the genetic material, as a virus injects only its genetic material into the bacteria, and the rest of the virus stays outside .

Question 5

What are the different structures for DNA bases?

Answer 5

Purine - double ring

Pyramidine - singe ring

Purine base attached to pyramidine base

Question 6

Which carbon atoms are connected to which molecules in DNA

Answer 6

1 - nucleotide base
3 - OH group
5 - phosphate group

Question 7

What is helicase?

Answer 7

The enzyme which initiates the separation of DNA into two single strands. It moves one
complementary base pair at a time, breaking
the hydrogen bonds so the double-stranded
DNA molecule becomes two separate strands

Question 8

What kind of process is DNA replication?

Answer 8

Semi conservative - half of a pre-existing DNA molecule is always
conserve, however, they new double strand is still identical thanks to complementary base pairing.

Question 9

What does DNA control in a cell?

Answer 9

Proteins, through protein synthesis

Question 10

What are genes?

Answer 10

Sections of DNA that code for polypeptides

Question 11

What is RNA and what does it do?

Answer 11

RNA is an intermediary molecule that carries the message of the DNA (the code)
to the cytoplasm where the enzymes, ribosome, and amino acids are found, to allow protein synthesis to occur, since they are synthesized outside the nucleus

Question 12

Is RNA single stranded or double stranded?

Answer 12

Single stranded - free RNA nucleotides are found in the nucleoplasm

Question 13

Outline the process of transcription

Answer 13

Transcription is the process by which a DNA sequence (gene) is copied into a complementary RNA sequence by RNA polymerase. In initiation, the enzyme RNA polymerase binds to the promotor region. It then unwinds the DNA double helix by breaking hydrogen bonds. In elongation, it covalently bonds the free RNA nucleotides together opposite the antisense strand and through complementary base pairing. It does this with the help of energy from the release of two phosphates from the nucleotide triphosphates. It synthesises the growing mRNA strand in the 5’ - 3’ direction. In termination, the newly synthesised mRNA strand reaches a terminator, , and RNA polymerase detaches from the DNA strand and transcription stops.

Question 14

How does transcription help with making amino acids?

Answer 14

The message written into the mRNA molecule is the message that
determines the order of the amino acids.

Question 15

Explain how genetic code is written in triplets

Answer 15

Researchers found experimentally that the
genetic code is written in a language of three bases; a set of three bases contains enough information to code for one of the 20 amino acids. Any set of three
Gene expression

Question 16

What are the different types of RNA molecules?

Answer 16

• mRNA, messenger RNA, as described above, each mRNA is a complementary copy
  of a DNA gene and has enough genetic information to code for a single polypeptide
  • rRNA, ribosomal RNA, each ribosome is composed of rRNA and ribosomal protein
  • tRNA, transfer RNA, each type of tRNA transfers one of the 20 amino acids to the
  ribosome for polypeptide formation

Question 17

Outline the translation process

Answer 17

MRNA locates ribosome and aligns so that first two codon triplets are in its boundaries .

Question 18

What are the three major parts of all nucleotides?

Answer 18

nucleobase, a five-carbon sugar (ribose or deoxyribose), and a phosphate group consisting of one to three phosphates.

Question 19

In semi-conservative replication, the new molecule of DNA has one strand from the original
molecule and one new strand produced from nucleotides in the surrounding environment.
How does generation 1 support this model?

Answer 19

Generation 1 is 100% identical to generation 0, which supports the semi conservative theory because one strand of generation 0 would have been paired suing complementary base pairing with free nucleotides to produce a 100% identical strain .

Question 20

Why does generation 2 support the semi-conservative model?

Answer 20

Because half of the generation 1 strain is completely identical?

Question 21

ER QUESTION: Explain how DNA is replicated within the cells of living organisms

Answer 21

DNA replication is the process of splitting DNA into two strands from its double helix form. DNA is split into two single strands by the enzyme helicase, which breaks the hydrogen bonds between complementary base pairs in order to separate the strands. Then free nucleotides in the nucleoplasm are attached to the nitrogenous bases of the single strand of the separated DNA by complementary base pairing, in order to create an identical strand of DNA to the original. There are two types of bases; purine and pyramidine. Purine and pyramidine bases attach to each other; A (purine) attaches to T (pyramidine) and G (purine) attaches to C (pyramidine). The covalent bond created between the base pairs is catalysed by the polymerase enzyme. The process is started by one nucleotide attaching on an origin point on the open strand, then a second one joining the first (and so on). DNA replication is a semi-conservative process, meaning because half of the pre-exisiting DNA molecule is always present.

Question 22

What is the function of histone proteins?

Answer 22

To supercoil DNA. 8 histone proteins make a core and the DNA wraps around the core twice. A 9th histone protein completes the structure and holds the DNA in place

Question 23

How many histone proteins make up the core of the nucleosome?

Answer 23

8

Question 24

How many different types of histone proteins make up the core?

Answer 24

4 different types; H2A, H2B, H3, and H, two of each

Question 25

What direction does DNA replication happen in?

Answer 25

From 5’ to 3’

Question 26

What is a valid comparison between prokaryotic and eukaryotic DNA?

Answer 26

Plasmids persent in prokaryotic DNA, plasmids absent in eukaryotic DNA

Question 27

Outline the role of short tandem repeats in DNA profiling

Answer 27

STRs are repeating regions of non-coding DNA. Tandem repeats can be excised using restriction enzymes and then separated with gel electrophoresis for comparison.
As individuals will likely have different numbers of repeats at a given satellite DNA locus, they will generate unique DNA profiles.

Question 28

Which regions of DNA code for the production of specific proteins?

Answer 28

Exons

Question 29

What is the function of DNA polymerase 1?

Answer 29

Removes RNA primers from the lagging strand and replaces them with DNA nucleotides

Question 30

What is the function of DNA polymerase 1?

Answer 30

Removes RNA primers from the lagging strand and replaces them with DNA nucleotides

Question 31

What is the function of DNA polymerase 3?

Answer 31

It aligns free DNA nucleotides to single strand of DNA by complementary base pairing. It moves in the 5’-3’ direction, but in opposite directions on the two strands. On the leading strand, polymerase three moves towards the replication fork, and this synthesis is continuous. On the lagging strand, polymerase 3 moves away from the replication fork, and synthesises in fragments, called Okazaki fragments. This is a discontinuous process.

Question 32

What is the function of DNA primase?

Answer 32

It generates short RNA primers (10-15 nucleotides long) on the template strands of the DNA, to provide an initiation point for DNA polymerase 3 to begin synthesis.

Question 33

What is the function of Single Stranded Binding Proteins/

Answer 33

Bind to DNA strands after being separated helicase to keep them from re-annealing and allow synthesis to take place.

Question 34

What is the function of DNA gyrase?

Answer 34

Reduces torsional strain cerated by unwinding of DNA by helicase, by relaxing positive supercoils

Question 35

What is the function of DNA ligase?

Answer 35

It joins Okazaki fragments to form a continuous strand. It does this by attaching sugar phosphate backbones of lagging strand fragments to form a single DNA strand

Question 36

Explain DNA replication

Answer 36

Helicase unwinds the DNA double helix by breaking the hydrogen bonds between nucleotide bases. SSBP bind to the DNA strands to prevent them from re-annealing. DNA primase attaches small sections of RNA primers to provide an initiation point for DNA Polymerase 3 to begin synthesis. DNA polymerase 3 attaches free nucleotides to the cytoplasm to the nucleotides on the existing DNA strands through complementary base pairing. It synthesises the new DNA strand in a 5’ to 3’ direction. On the leading strand, this synthesis happens in the direction of the replication fork, so the process is continuous. On the lagging strand, it happens in the opposite direction of the replication fork, so the process is discontinuous and the synthesis happens in fragments called Okazaki fragments. DNA polymerase 1 replaces the RNA primase with nucleotides. DNA ligase joins the Okazaki fragments together to form a continuous DNA strand by attaching the sugar phosphate backbones. DNA gyrase moves in advance of helicase and reduces the torsional strain caused by the unwinding of helicase.

Question 37

In transcription, which strand is transcribed into RNA?

Answer 37

The antisense strand.

Question 38

What is non-coding DNA?

Answer 38

Genes that don’t code for polypeptides - introns.