Fourth Exam

Question 1

Copying of DNA is called

Answer 1

DNA replication

Question 2

Experiments by Meselson and Stahl

Answer 2

Support for the semiconservative model

Question 3

DNA Replication:

Answer 3

Semiconservative Model

Question 4

DNA Replication:

Answer 4

DNA strands separate, since the two strands of DNA are complementary, each strand acts as a template for building a new strand in replication. This yields two exact replicas of the “parental” molecule.

Question 5

How Replication Starts:

Answer 5

Origin of Replication- replication bubble- replication fork

Question 6

Helicase: Start DNA Replication

Answer 6

Unwinds and separates the parental DNA strands

Question 7

Topoisomerase: Start DNA Replication

Answer 7

Breaks, swivels, and rejoins DNA ahead of replication fork.

Question 8

Single-Stranded binding proteins: Start DNA Replication

Answer 8

Stabilize unwound DNA

Question 9

Process of Copying DNA

Answer 9

Enzyme that copies DNA is DNA Polymerase; but it can’t just START, can only add to a chain

Question 10

Primase

Answer 10

Makes a short RNA ‘primer’

Question 11

Primer

Answer 11

Chain of RNA nucleotides (has about 5-10 nucleotides) Now, the new DNA strand will start from 3’ end of the RNA primer

Question 12

DNA Polymerase III

Answer 12

Enzyme that copies the DNA, adds new nucleotides to a pre-existing piece of RNA

Question 13

Adding Nucleotides: DNA Pol III

Answer 13

Adds Nucleoside triphosphate to the 3’ end of new strand. Synthesis of new strand happens in the 5’-3’ direction of the new strand

Question 14

New nucleotides can be added only to

Answer 14

3’ ends

Question 15

Antiparallel:

Answer 15

Each strand’s copy has to made differently

Question 16

Leading Strand

Answer 16

One strand is being made continuously. As proteins open the DNA more, replication keeps going. Nucleotides added on to the 3’ end. DNA Polymerase III moves from 5’ end toward 3’ end of the strand being made.

Question 17

Lagging Strand:

Answer 17

The DNA strand elongating away from the replication fork

Question 18

Lagging Strand Step One:

Answer 18

Primase uses RNA nucleotide to make a primer

Question 19

Lagging Strand Step 2:

Answer 19

DNA Pol III adds DNA nucleotides to the primer
Creates: Okazaki fragment

Question 20

Lagging Strand Step 3:

Answer 20

Reaches the next RNA primer – DNA Pol III detaches

Question 21

Lagging Strand Step 4:

Answer 21

Then moves over to the next RNA primer (closer to the replication fork) and adds DNA nucleotides to 3’ end

Question 22

Lagging Strand Step 5:

Answer 22

DNA Polymerase I – removes RNA and replaces with DNA
- Adds on to 3’ end of Okazaki fragment

Question 23

Lagging Strand Step 6:

Answer 23

DNA Ligase forms a covalent bond in backbone of DNA fragments

Question 24

Lagging Stand Step 7:

Answer 24

Lagging strand is now complete in this region!!

Question 25

Lagging Strand Terms: DNA Pol III

Answer 25

only add nucleotides to 3’ ends, copies the lagging strand in fragments, with lots of RNA primers

Question 26

Lagging Strand Terms Okazaki Fragments:

Answer 26

segments of DNA that are synthesized discontinuously in the lagging strand

Question 27

Lagging Stand DNA Pol I

Answer 27

removes the RNA primer and fills in with DNA nucleotides

Question 28

DNA Ligase Lagging Strand

Answer 28

‘glues’ together the okazaki fragments; forms the covalent bonds in sugar-phosphate backbone.

Question 29

As DNA is replicating:

Answer 29

DNA polymerases check every nucleotide they add against the template (the parental strand)
Not a correct match? Remove it and add a new one
So most errors are fixed right away

Question 30

Repairing Mismatches

Answer 30

Some errors escape repair by DNA Pol
Nucleotide excision repair – an example of a mismatch repair system
Other enzymes can help remove and replace incorrectly paired or altered nucleotides

If repair isn’t done  Mutation!

Question 31

What can alter nucleotides?

Answer 31

Toxic chemicals, UV light, Cigarette smoke

Question 32

What does UV light do to nucleotides?

Answer 32

Thymine dimers – two thymine bases next to each other form a covalent bond between the bases

Question 33

Nucleotide excision repair

Answer 33

Specific enzymes detect damaged DNA (different ones for different types of damage)
Nuclease enzyme cuts out damaged section
DNA Pol replaces missing nucleotides – based on sequence of other strand.
DNA Ligase - ‘glues’ new piece

Question 34

Linear DNA replication:

Answer 34

Every time it’s copied – lagging strand is shortened!
RNA primer is removed, but no DNA can be added because no 3’ end exists!

Question 35

Problem: DNA keeps getting lost at ends

Answer 35

Solution: Have DNA at the ends that you don’t need!
Telomeres – special repetitive nucleotide sequences at ends of chromosomes
TTAGGG – 100-1,000 times!
No genes!

Question 36

Limit the number of cell divisions?

Answer 36

Shortening of telomeres may be connected to aging of tissues or even whole organism
Normal shortening may help protect against cancer by preventing excess cell division

Question 37

Are the shortening telomers a problem in an embryo?

Answer 37

Can’t have embryos start with length of telomere that mother has at that point in her life!
Solution… re-synthesize the telomeres in germ cells

Question 38

Telomerase =

Answer 38

Enzyme re-lengthens telomeres in germ cells
Embryos will have full-length telomeres!

Question 39

The Dark Side of Telomerase

Answer 39

Cancer cells divide uncontrolled
In theory, shortened telomeres should stop their growth
BUT researchers have found activated telomerase in somatic cancer cells!!
Keeping the telomeres from getting too short