Chapter 15: DNA and the Gene Flashcards
How many origins of replication do bacteria have? Eukaryotes?
Bacteria only have 1 origin of replication on their circular chromosome while eukaryotes have multiple along their chromosomes
Synthesized away from the replication fork
Lagging strand
Relieves strain/stress ahead of replication fork caused by unwinding by breaking, swiveling, and rejoining DNA strands
Topoisomerase
What did the Meselson-Stahl experiment prove?
DNA is replicated semiconservatively
What allows cells to continue to divide without shortening their chromosomes?
Telomerase
What are the parts of DNA structure?
5’ and 3’ ends, phosphate group, sugar, and nitrogenous bases
Bind to separated DNA strands to prevent repairing (keeps bases from snapping back together)
Single-strand DNA-binding proteins (SSBPs)
What is telomerase made of?
Protein and RNA
Who found that genes are located on chromosomes?
Morgan with his fruit flies and X linked traits
Enzymes (not DNA polymerases) that remove and replace incorrectly paired nucleotides
Mismatch repair (MMR)
Creates Okazaki fragments
Lagging strand
Why do eukaryotes have multiple origins of replication?
It speeds up the process of DNA replication
When is telomerase active?
During embryonic development but shut off in most somatic cells at later stage. Also in male germ cells, activated lymphocytes, some stem cells, and 90% of tumors
What are Okazaki fragments?
Chunks of the replicated DNA strand that consist of a primer and DNA. There are many created on the lagging strand
What is the problem that occurs at the end of DNA replication?
The end fragment of the chromosome is not replicated bc the lagging strand is too short. This causes the chromosome to shorten
How are DNA strands oriented?
Antiparallel
How are errors between incoming bases and the template strand corrected?
DNA polymerases proofread each base as soon as it is added and if it is incorrectly paired, it is removed and synthesis resumes
What is the location where DNA is unwound at called (the place where open DNA meets double stranded)?
Replication fork
When are replication errors most common?
Between incoming bases and the template strand
What does lagging strand synthesis require?
Multiple primers. One for each fragment
What is the protein component fo telomerase?
Telomerase reverse transcriptase (TERT)
How is DNA constantly monitored and repaired?
DNA polymerases, mismatch repair (MMR), and nucleotide excision repair (NER)
Replaces the RNA primer with DNA nucleotides
DNA polymerase I
This cuts out a damaged DNA segment using a nuclease and the gap is filled by DNA polymerase and ligase using the undamaged strand as a template
Nucleotide excision repair (NER)
Occurs continuously and only requires one primer
Leading strand
What does primate provide?
The initial chain, in the form of an RNA primer, so replication can proceed (new nucleotides can be added to it)
How does topoisomerase relieve tension?
It cuts DNA, lets it unwind a little, then seals it back up ahead of helicase
Where does DNA replication begin?
Specific sites called origins of replication
What do enzymes which synthesize DNA need to start making a strand and why?
They need an -OH group bc they cant start a new chain on their own and can only add to an existing strand
What are dNTPs?
Nucleotide triphosphate added by DNA polymerases to create a replicated strand of DNA
What is xeroderma pigmentosum?
Disorder caused be inherited defect in NER that causes a hypersensitivity to light
When else can errors arise other than replication?
Environment (sun, smoke, carcinogens, etc)
What is mismatch repair similar to?
Tumor suppressors. They stop the cell from leaving incorrect pairs
Untwists the double helix and separates the strands of DNA
Helicase
Discontinuous. A new fragment cannot be started until the fork moves forward and exposes template
Lagging strand
Joins all of the fragments (Okazaki fragments) of the lagging strand into a continuous strand
DNA ligase
Consists of a short nucleotide sequence (TTAGGG) in humans that is repeated 100-1000x times
Telomeres
Which way does replication proceed from the origin of replication?
It proceeds in both directions
Lays down an RNA primer which is complementary to the template strand
Primase
Synthesized toward the replication fork
Leading strand
How long does it take to copy DNA and how often do errors occur?
Only takes a few hours to copy with about 1 error in every 1 billion nucleotides
The ends of chromosomes that don’t encode any genes
Telomeres
What is the RNA component of telomerase?
Telomerase RNA (TERC)
What happens if DNA polymerases miss a mistake?
Mismatch repair (MMR) kicks in
What are defects in mismatch repair enzymes linked to?
Cancer
Is the leading strand on top or bottom? What about the lagging strand?
They are both on top and bottom bc the DNA strands run antiparallel
What direction must DNA polymerase add dNTPs in?
5’ to 3’ (this is the direction of the strand being made)
What happens when telomeres reach a critical limit? How long does it take to get there?
The cell enters senescence (no longer divides). Cells can usually divide about 50 times before they hit this point
Who found that DNA was the genetic material and not protein?
Hershey and Chase (1952). Phage transferred DNA to bacteria that was dyed and showed up in bacteria
A large macromolecule machine formed by the many enzymes involved in DNA replication
Replisome
What links the sugar-phosphate groups of the DNA backbone?
Phosphodiester bonds
Adds a nucleotide to the RNA primer and then keeps adding complementary nucleotides to the growing strand
DNA polymerase III
Where must DNA polymerases add nucleotides?
The free 3’ end (they need the free OH to attach the next base)
Enzymes which catalyze the synthesis of DNA by adding nucleotides to an existing chain
DNA polymerases
