Lecture 9

Question 1

Chromosone are composed of ________

Answer 1

DNA

Question 2

Describe the experiment conducted by Fred Griffith in the 1920’s.

Answer 2

1. Injected two strains of S. pneumonaie into mice (one lethal (S-strain) and one harmless (R-strain))
2. heat killed the S-strain and injected it into mice –> mice survived
3. when cells of the heat killed the S-strain mixed with the R-strain and injected it into the mice –> mice died
   - this represented the phenomenon “transformation” which is the ability of the unknown factor to transform a harmless strain into a harmful one

Question 3

What ideas did Avery, MacLeod and McCarthy add to Griffiths experiments in 1944?

Answer 3

• they isolated individual components from S-strain cells and tested their ability to transform the R-strain
• conclusion: the molecule that contains the heritable information is DNA

Question 4

What did Hershey and Chase conclude about DNA from their experiments in 1952?

Answer 4

• they concluded that DNA enters bacteria and carries genetic information

Question 5

What are the primary, secondary, and tertiary structures of DNA?

Answer 5

• Primary structure: the nucleotide sequence
• Secondary structure: An regular, stable structure taken up by some or all of the nucleotides
• Tertiary structure: Complex 3D folding of nucleic acid (e.g bacterial nucleoids)

Question 6

What is the difference between a nucleoside and a nucleotide?

Answer 6

• nucleoside: base + sugar (adenosine)
• nucleotide: base + sugar + phosphate (adenosine monophosphate AMP)

Question 7

How do ribose and deoxyribose sugars differ from one another?

Answer 7

• In RNA the 2’ carbon has a hydroxyl group where as in DNA it only as a hydrogen attached. The hydrogen is less reactive than the hydroxyl which is important in RNA splicing reactions

Question 8

phosphodiester linkages

Answer 8

• forms a repeating backbone
• the 5’ phosphate of one nucleotide unit is joined to the 3’ hydroxyl group of the next nucleotide creating the phophodiester linkage

Question 9

What is the difference in structure between a purine and a pyrimidine?

Answer 9

• Purines have 2 rings (Guanine, Adenine)
• Pyrimidines have 1 ring (Cytosine, Uracil (RNA), Thymine (DNA))

Question 10

What is a B-Glysosidic linkage

Answer 10

• The pentose forms a covalent bond with the base called a B-Glycosidic linkage (a bond joining a sugar to another group)

Question 11

What is the secondary structure of DNA?

Answer 11

The double helix
- two strands that interact via base pairing (hydrogen bonding), and wound around the same axis
- strands run in opposite direction
- parallel: 5’ to 3’
- antiparallel: 3’ to 5’

Question 12

What are the complementary base pairs in DNA?

Answer 12

• base pairing occurs through hydrogen bonding between complementary bases
• Adenine forms 2 H-bonds with thymine or uracil (A=T, A=U)
• Guanine forms 3 H-bonds with cytosine

Question 13

What is base stacking

Answer 13

• base stacking between hydrophobic bases minimizes contact with water and is very important for stabilizing the double helix
• base stacking gives arise to van Der wall interactions and dipole-dipole interactions
• bases are not directly on top of one another

Question 14

How many base pairs are there per turn in a DNA double helix?

Answer 14

10.5 base pairs/turn

Question 15

Who determined the double helix structure of DNA and defined base pairing specificity

Answer 15

• James Watson and Francis Crick

Question 16

What is Chargaff’s rule?

Answer 16

A=T G=C
A+G=T+C

Question 17

The Watson and Crick model can explain _________

Answer 17

DNA replication

Question 18

What are DNA grooves

Answer 18

• DNA contains two grooves where the bases are exposed known as major groove and minor groove
• common location for protein-DNA interaction
• a helix fits perfectly in major groove

Question 19

What is chromatin

Answer 19

• a complex of protein and DNA

Question 20

What is a chromosome

Answer 20

• chromatin in a compacted form

Question 21

Human chromosome

Answer 21

• humans have 46 chromosomes, most of which come in homologous chromosome (one maternal, one parental)
• this excludes sex chromosomes x and y

Question 22

What is karyotype

Answer 22

• an individuals complete set of chromosomes

Question 23

How can specific chromosomes be visualized

Answer 23

• by using specific DNA molecules coupled to a fluorochrome
• can be used detect genetic defects

Question 24

What is a genome

Answer 24

• the total genetic information carried by all chromosomes in the cell
• Most DNA in our genome is called “junk DNA” because its function is unknown (doesn’t mean its useless

Question 25

Compare interphase and mitosis.

Answer 25

1. interphase: when chromosomes are duplicated - interphase chromosomes are extended and hard to see with a light microscope 2. mitosis: when chromosomes are segregated to two daughter molecules - mitosis chromosomes are extremely compact and easy to visualize (compaction of DNA has an extreme affect on gene expression)

Question 26

What are telomeres

Answer 26

- repeated sequence at the end of a chromosome - protect against degradation

Question 27

What is replication origin

Answer 27

- sequence where replication beings

Question 28

What is the centromere

Answer 28

- sequence that allows DNA to be separated during M phase - region of attachment to mitotic spindle

Question 29

Interphase chromosomes are much less _______ than mitotic chromosomes.

Answer 29

compact