Topic 6 - Nucleic Acids and DNA Replication

Question 1

Nucleic acids

Answer 1

Biological macromolecules that carry the cell’s genetic blueprint and carry instructions for the cell’s functioning

Question 2

What are nucleic acids composed of?

Answer 2

Nucleotides

Question 3

Nucleotides

Answer 3

• Monomer of nucleic acids
• Contains a nitrogenous base, a pentose sugar, and one or more phosphate groups

Question 4

Pyrimidines

Answer 4

• Type of nitrogenous base in DNA and RNA
• 6-carbon ring
• Cytosine, thymine (in DNA), and uracil (RNA)

Question 5

True or false: thymine is only present in DNA.

Answer 5

True

Question 6

True or false: uracil is only present in RNA.

Answer 6

True

Question 7

Purines

Answer 7

• Type of nitrogenous base in DNA and RNA
• 6-carbon ring and 5-carbon ring
• Adenine and guanine

Question 8

How do nucleotides pair in DNA?

Answer 8

• Cytosine-guanine and adenine-thymine

Question 9

How do nucleotides pair in RNA?

Answer 9

• Cytosine-guanine and adenine-uracil

Question 10

What sugar is found in DNA?

Answer 10

Deoxyribose
- Lacks a hydroxyl group

Question 11

What sugar is found in RNA?

Answer 11

Ribose

Question 12

Phosphate groups

Answer 12

1. Monophosphate (e.g. AMP, RNA, DNA)
2. Diphosphate (e.g. ADP)
3. Triphosphate (e.g. ATP)

Question 13

What are the functions of nucleic acids?

Answer 13

• Energy storage and transfer (ATP, GTP, NADH)
• Information transfer (RNA)
• Information storage (DNA)

Question 14

Messenger RNA (mRNA)

Answer 14

RNA that carries information from DNA to ribosomes during protein synthesis
- Copied from DNA

Question 15

Transfer RNA (tRNA)

Answer 15

RNA that carries activated amino acids to the site of protein synthesis on the ribosome
- Translates RNA to protein

Question 16

Ribosomal RNA (rRNA)

Answer 16

RNA that ensures the proper alignment of the mRNA and the ribosomes during protein synthesis and catalyzes forming the peptide linkage
- Facilitates translation process

Question 17

True or false: DNA remains in the nucleus.

Answer 17

True

Question 18

What is the structure of DNA?

Answer 18

Double-helix; sugar-phosphate backbone with nitrogenous bases in the middle

Question 19

The two strands in a DNA molecule run _____ to each other.

Answer 19

Antiparallel (one strand runs 5’ to 3’ and the other 3’ to 5’)

Question 20

What do 3’ and 5’ refer to in DNA?

Answer 20

The number of carbon atoms in a deoxyribose sugar molecule that a phosphate group binds to.
- The carbons in the pentose sugar are numbered clockwise.

Question 21

What kind of bonds form base pairs?

Answer 21

Hydrogen

Question 22

How does the structure of DNA reveal its replication process?

Answer 22

• The double-helix model suggests that two strands of the double helix separate during replication, and each strand serves as a template from which the new complementary strand is copied.
• With specific base-pairs the sequence of one DNA strand can be predicted from its complement

Question 23

Semi-conservative model

Answer 23

Two parental strands of DNA act as a template for new DNA to be synthesized; after replication, each double-stranded DNA includes one parental or “old” strand and one “new” strand

Question 24

How did Meselson and Stahl demonstrate semiconservative replication?

Answer 24

• E.coli DNA with an N15 nitrogen isotope (heavier than common nitrogen) was placed in N14 media, then separated based on densities relative to a cesium chloride solution in an ultracentrifuge.
• After one round of cell division the DNA sedimented halfway between the N15 and N14 levels, indicating that it contained 50% 14N (“new” DNA.)
• In subsequent divisions, an increasing amount of DNA contained 14N only, meaning the process was semi-conservative

Question 25

During what phase of mitosis does DNA replication occur?

Answer 25

S ("synthesis") phase of interphase

Question 26

Summarize the process of DNA replication.

Answer 26

1. Helicase opens up ("unzips") the DNA at the replication fork. 2. Single-strand binding proteins (SSB) coat the DNA around the replication fork to prevent the rewinding of DNA. 3. Topoisomerase works at the region ahead of the replication fork to prevent supercoiling. 4. Primase synthesizes RNA primers complementary to the DNA strand. 5. DNA polymerase III extends the primers, adding nucleotides to the 3'-OH end of the primer. - Leading strand is continuous - Okazaki fragments formed in lagging strand 6. RNA primers are removed and replaced with DNA by DNA polymerase I. 7. The gaps between DNA fragments are sealed by DNA ligase.

Question 27

DNA polymerase I

Answer 27

Removes RNA primer and replaces it with newly synthesized DNA

Question 28

DNA polymerase III

Answer 28

Main enzyme that adds nucleotides in the 5'-3' direction

Question 29

Helicase

Answer 29

Opens the DNA helix by breaking hydrogen bonds between the nitrogenous bases

Question 30

Ligase

Answer 30

Seals the gaps between the Okazaki fragments to create one continuous DNA strand

Question 31

Primase

Answer 31

Synthesizes RNA primers needed to start replication

Question 32

Sliding Clamp

Answer 32

Helps to hold the DNA polymerase in place when nucleotides are being added

Question 33

Topoisomerase

Answer 33

Helps relieve the strain on DNA when unwinding by causing breaks, and then resealing DNA

Question 34

Single-strand binding proteins (SSB)

Answer 34

Binds to single-stranded DNA to prevent DNA from rewinding back

Question 35

Primer

Answer 35

RNA segment complementary to the template DNA - About five to ten nucleotides long - Synthesized by primase

Question 36

Okazaki fragment

Answer 36

DNA fragment that is synthesized in short stretches on the lagging strand

Question 37

Leading strand

Answer 37

Strand that is synthesized continuously in the 5'-3' direction towards the replication fork

Question 38

Lagging strand

Answer 38

Strand that is replicated in short fragments away from the replication fork - 3'-5'

Question 39

How are eukaryotic chromosomes different from prokaryotic chromosomes?

Answer 39

Eukaryotic chromosomes are linear.

Question 40

Why do linear chromosomes shorten during each round of DNA replication?

Answer 40

Because DNA polymerase III can only add nucleotides in the 5'-3' direction, there is no way to replace the primer on the 5' end of the lagging strand when the replication fork reaches the end of the linear chromosome. The DNA remains unpaired, and over time these ends (telomeres) become shorter as cells continue to divide.

Question 41

Telomere

Answer 41

DNA at the end of linear chromosomes

Question 42

Telomerase

Answer 42

Enzyme that contains a catalytic part and an inbuilt RNA template; it functions to maintain telomeres at chromosome ends

Question 43

How does telomerase work?

Answer 43

1. Attaches to end of chromosome and DNA nucleotides complementary to the RNA template are added on to the 3' end of the DNA strand 2. Once the 3' end of the lagging strand is sufficiently elongated, DNA polymerase can add nucleotides complementary to the ends of the chromosomes 3. Thus, the ends of chromosomes are replicated.

Question 44

In what kind of cells is telomerase found?

Answer 44

Gametes, stem cells, and cancer cells

Question 45

Mismatch repair

Answer 45

Type of repair mechanism in which mismatched bases are removed after replication - Completed by DNA polymerase

Question 46

Excision repair

Answer 46

Type of repair mechanism in which the wrong base, along with a few nucleotides upstream or downstream, are removed - Completed by nuclease