DNA Replication Flashcards
Step 1 of DNA Replication
DNA must unwind to expose a single stranded template
What does helicase do?
- Unzips and unwinds the double helix
- Breaks hydrogen bonds
What enzymes are used in step 1 of DNA replication?
- Helicase
- Toposiomerase (gyrase)
What does topoisomerase (gyrase) do?
- Detangler
- Prevents supercoils by nicking and resealing DNA
Step 2 of DNA Replication
DNA must be held open, not to reform a coil
What do Single Stranded Binding Proteins do?
- Bind to the uncoiled DNA and keep it open
Step 3 of DNA Replication
A primer must be created before DNA synthesis can occur
What is a primer?
- Short sequence of RNA, complementary to template
- Provides 3’ end where DNA synthesis can begin
What is primase?
Enzyme that synthesizes RNA primer
What is RNA polymerase?
- Synthesizes primers on leading strand
- Type of RNA polymerase
Leading strand
- New strand that is synthesized continuously
- Synthesized towards replication fork
Lagging Strand
- Synthesized in short fragments called Okazaki fragments which are later joined together
- Discontinuously
- Synthesized away from replication fork
Step 4 of DNA Replication
DNA synthesis begins
What is DNA polymerase 3 (builder)
- Enzyme that catalyzes the addition of nucleotides complementary to the template strand
What direction does DNA synthesis occur in?
5’ to 3’ direction
DNA polymerase 1, first function
- Edits mistakes
- Proofreads chain
- Exonuclease activity
Step 5 of DNA Replication
Synthesis of DNA occurs on both template strands
Step 6 of DNA Replication
RNA primers are removed
DNA polymerase 1, second function
- chews up RNA primers and replaces them with DNA nucleotides
- All primers are removed
Step 7 of DNA Replication
All nicks are repaired
What is DNA Ligase
- Acts like glue to make phosphodiester linkages
- Seals the nicks between Okazaki fragments
Proofreading and repairing DNA
- DNA polymerase I proofread newly made DNA, replacing any incorrect nucleotides
What happens in mismatch repair of DNA?
repair enzymes correct errors in base pairing
Some reasons DNA can be damaged
- exposure to harmful chemical or physical agents
- can undergo spontaneous changes
What does a nuclease do in DNA repair?
Cuts out and replaces damaged stretches of DNA
What are telomeres?
Repeating DNA sequences at the ends of chromosomes that protect coding regions during replication
Do telomeres prevent DNA shortening?
No, but they help delay the loss of important genes near the ends of chromosomes.
What are shortened telomeres linked to?
Aging, including wrinkles and grey hair
What is telomerase?
An enzyme that catalyzes the lengthening of telomeres in germ cells
How is telomerase linked to cancer?
- Cancer cells produce telomerase in high quantities
Prokaryotic DNA Organization
- Double stranded circular DNA molecules associated with small amounts of protein
- DNA is supercoiled and found in the nucleoid
Eukaryotic DNA Organization
- Linear DNA molecules associated with large amounts of protein
What is a nucleosome?
A unit of DNA storage made up of 8 histones
What is a histone?
Structural Protein
