Unit 10 part 2

Question 1

Synthesizing a new DNA Strand

Answer 1

• The initial nucleotide strand is a short RNA primer
• The RNA primer is synthesized by the enzyme
  primase
• Primase can start an RNA chain from scratch:
• Adds RNA nucleotides one at a time
• Uses parental DNA as a template
• The primer is short (5–10 nucleotides long) and the 3′ end of primer serves as the starting point for the new DNA strand

Question 2

primase

Answer 2

• start RNA chain from scratch
• adds the rna nucleotides one at a time
• 3’ end of primer serves as the starting point for the new dna strand

Question 3

Building nucleic acid polymers

Answer 3

• Each nucleotide that is added to a growing DNA strand is a nucleoside triphosphate (dNTP)
• Each monomer joins the DNA strand, viaa dehydration reaction
• two phosphate groups are lost as a molecule of pyrophosphate

Question 4

dATP

Answer 4

supplies adenine to dna and is similar to the ATP of energy metabolism
- deoxyribose… atp has reibose as sugar

Question 5

Antiparallel Elongation

Answer 5

• The antiparallel structure of the double helix affects replication
• DNA polymerases add nucleotides only to the free 3′ end of a growing strand
• Therefore, a new DNA strand can elongate only in the 5′ -> 3′ direction

Question 6

When the replication bubble opens, replication occurs

Answer 6

in two directions
* Replication bubbles use leading and lagging strand synthesis

Question 7

Leading Strand Synthesis

Answer 7

• Along one template strand of DNA, the DNA polymerase synthesizes a leading strand continuously, moving toward the replication fork (5’ –> 3’)
• Requires:
• RNA primer, primase, DNA Pol III, template DNA, helicase, ssBPs

Question 8

leading strand requires

Answer 8

rna primer, primase, DNA pol III, template DNA, helicase, ssBPs

Question 9

Lagging Strand Synthesis

Answer 9

• To elongate the other new strand (the lagging strand):
• DNA polymerase works in the direction away from the replication fork
• The lagging strand is synthesized as a series of segments called Okazaki fragments, which are joined together by DNA ligase

Question 10

DNA POL III

Answer 10

• using parental DNA as a template, synthesis new DNA strand by adding nucleotides to an RNA primer or a pre-exsisting DNA strand

Question 11

DNA pol I

Answer 11

removes RNA nucleotides of primer from 5’ end and replaces them with DNA nucleotides added to 3’ end of adjacent fragment

Question 12

ligase

Answer 12

joins okazaki fragments of lagging strand
- joins the 3’ end of dna

Question 12

Proofreading and Repairing DNA

Answer 12

• DNA polymerases proofread newly made DNA, replacing any incorrect nucleotides

Question 13

• DNA can be damaged by:

Answer 13

• exposure to harmful chemicals or physcial agents such as cigs, xray, UV rays
• spontaneous chnages

Question 14

mismatch repair

Answer 14

repair enzymes correct errors in base pairing

Question 15

nucleotide excision repair

Answer 15

nuclease cuts out adn replaces damaged stretches of DNA

Question 15

If errors are not repaired

Answer 15

• The error rate after proofreading and repair is low but not zero
• Sequence changes may become permanent and can be passed on to the next generation

Question 16

mutations are

Answer 16

0 source of genetic variation upon which natural selection operates
- ultimately responsible for the appearance of new species
- some mutations can be deleterious (disease causing)

Question 17

Limitations of DNA polymerase create

Answer 17

problems for the linear DNA of eukaryotic chromosomes
* Only adds nucleotides at 3’ end of polymer
* Not an issue for prokaryotes: circular
chromosomes
Replication machinery provides no way to complete the 5′ ends
- repeated rounds of replication produce shorter DNA molecules with uneven ends

Question 18

Telomeres

Answer 18

Eukaryotic chromosomal DNA molecules have special nucleotide sequences at their ends
* Telomeres do not prevent the shortening of DNA molecules
* Telomeres postpone the erosion of genes near the ends of DNA molecules
- repeates of sequence at end of chromosomes
- similar to shoelace caps

• shortening of telomers is linked to aging

Question 19

telomerase

Answer 19

catalyzes the lengthening of telomeres in germ cells
- if not, essential genes would be mussing in the gametes they produce (germ cells)

There is evidence of telomerase activity in cancer cells, which may allow cancer cells to persist

Question 20

Chromosomal Packaging

Answer 20

• DNA is combined with proteins in a complex known as chromatin
• Double helix
• DNA sequence
• Histones:
• Proteins that help with first level of DNA packing
• Histone tails involved in gene expression regulation
• Nucleosome:
• DNA wrapped around histones
• Beads on a string, 10 nm fiber

Question 21

• Metaphase chromosome:

Answer 21

• Compacted chromatin we see as metaphase chromosome

Question 21

Chromosome location in cells

Answer 21

• Interphase chromosomes occupy specific restricted regions in the nucleus
• The fibers of different chromosomes do not become entangled
• Homologous chromosomes are not next to each other

Question 22

Euchromatin

Answer 22

• loosely packed chromatin, more accessible for gene expression

Question 23

heterochromatin

Answer 23

• highly condense chromatin, less accessible for gene expression