Topic 6 - Nucleic Acids and DNA Replication Flashcards
Nucleic acids
Biological macromolecules that carry the cell’s genetic blueprint and carry instructions for the cell’s functioning
What are nucleic acids composed of?
Nucleotides
Nucleotides
- Monomer of nucleic acids
- Contains a nitrogenous base, a pentose sugar, and one or more phosphate groups
Pyrimidines
- Type of nitrogenous base in DNA and RNA
- 6-carbon ring
- Cytosine, thymine (in DNA), and uracil (RNA)
True or false: thymine is only present in DNA.
True
True or false: uracil is only present in RNA.
True
Purines
- Type of nitrogenous base in DNA and RNA
- 6-carbon ring and 5-carbon ring
- Adenine and guanine
How do nucleotides pair in DNA?
- Cytosine-guanine and adenine-thymine
How do nucleotides pair in RNA?
- Cytosine-guanine and adenine-uracil
What sugar is found in DNA?
Deoxyribose
- Lacks a hydroxyl group
What sugar is found in RNA?
Ribose
Phosphate groups
- Monophosphate (e.g. AMP, RNA, DNA)
- Diphosphate (e.g. ADP)
- Triphosphate (e.g. ATP)
What are the functions of nucleic acids?
- Energy storage and transfer (ATP, GTP, NADH)
- Information transfer (RNA)
- Information storage (DNA)
Messenger RNA (mRNA)
RNA that carries information from DNA to ribosomes during protein synthesis
- Copied from DNA
Transfer RNA (tRNA)
RNA that carries activated amino acids to the site of protein synthesis on the ribosome
- Translates RNA to protein
Ribosomal RNA (rRNA)
RNA that ensures the proper alignment of the mRNA and the ribosomes during protein synthesis and catalyzes forming the peptide linkage
- Facilitates translation process
True or false: DNA remains in the nucleus.
True
What is the structure of DNA?
Double-helix; sugar-phosphate backbone with nitrogenous bases in the middle
The two strands in a DNA molecule run _____ to each other.
Antiparallel (one strand runs 5’ to 3’ and the other 3’ to 5’)
What do 3’ and 5’ refer to in DNA?
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.
What kind of bonds form base pairs?
Hydrogen
How does the structure of DNA reveal its replication process?
- 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
Semi-conservative model
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
How did Meselson and Stahl demonstrate semiconservative replication?
- 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
During what phase of mitosis does DNA replication occur?
S (“synthesis”) phase of interphase
Summarize the process of DNA replication.
- Helicase opens up (“unzips”) the DNA at the replication fork.
- Single-strand binding proteins (SSB) coat the DNA around the replication fork to prevent the rewinding of DNA.
- Topoisomerase works at the region ahead of the replication fork to prevent supercoiling.
- Primase synthesizes RNA primers complementary to the DNA strand.
- 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 - RNA primers are removed and replaced with DNA by DNA polymerase I.
- The gaps between DNA fragments are sealed by DNA ligase.
DNA polymerase I
Removes RNA primer and replaces it with newly synthesized DNA
DNA polymerase III
Main enzyme that adds nucleotides in the 5’-3’ direction
Helicase
Opens the DNA helix by breaking hydrogen bonds between the nitrogenous bases
Ligase
Seals the gaps between the Okazaki fragments to create one continuous DNA strand
Primase
Synthesizes RNA primers needed to start replication
Sliding Clamp
Helps to hold the DNA polymerase in place when nucleotides are being added
Topoisomerase
Helps relieve the strain on DNA when unwinding by causing breaks, and then resealing DNA
Single-strand binding proteins (SSB)
Binds to single-stranded DNA to prevent DNA from rewinding back
Primer
RNA segment complementary to the template DNA
- About five to ten nucleotides long
- Synthesized by primase
Okazaki fragment
DNA fragment that is synthesized in short stretches on the lagging strand
Leading strand
Strand that is synthesized continuously in the 5’-3’ direction towards the replication fork
Lagging strand
Strand that is replicated in short fragments away from the replication fork
- 3’-5’
How are eukaryotic chromosomes different from prokaryotic chromosomes?
Eukaryotic chromosomes are linear.
Why do linear chromosomes shorten during each round of DNA replication?
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.
Telomere
DNA at the end of linear chromosomes
Telomerase
Enzyme that contains a catalytic part and an inbuilt RNA template; it functions to maintain telomeres at chromosome ends
How does telomerase work?
- Attaches to end of chromosome and DNA nucleotides complementary to the RNA template are added on to the 3’ end of the DNA strand
- Once the 3’ end of the lagging strand is sufficiently elongated, DNA polymerase can add nucleotides complementary to the ends of the chromosomes
- Thus, the ends of chromosomes are replicated.
In what kind of cells is telomerase found?
Gametes, stem cells, and cancer cells
Mismatch repair
Type of repair mechanism in which mismatched bases are removed after replication
- Completed by DNA polymerase
Excision repair
Type of repair mechanism in which the wrong base, along with a few nucleotides upstream or downstream, are removed
- Completed by nuclease
