DNA Replication Flashcards
In which phase of the cell cycle does DNA replication occur?
Usually S-phase
DNA replication occurs during the S phase of the cell cycle. This phase occurs between G1 and G2 phases and involves the synthesis of a second copy of DNA.
Key Takeaway:
DNA replication takes place in the S phase of the cell cycle.
Which of the following are the correct directions that DNA is read and synthesized?
C; Synthesized 5’ to 3’ and read 3’ to 5’
DNA is read in the 3’-5’ direction and is synthesized in the 5’-3’ direction. When DNA synthesis occurs, nucleic acids are added to the OH group of the existing DNA strand.
Mnemonic: The order of DNA reading and synthesis can be remembered with the phrase “Read Up, Write Down”; DNA is read up 3’-5’ and written down (synthesized) 5’-3’.
Key Takeaway:
DNA strands are read in the 3’-5’ direction and synthesized in the 5’-3’ direction.
What did the Meselson-Stahl experiment reveal about DNA replication?
DNA is replicated semi-conservatively
The Meselson-Stahl experiment demonstrated that DNA replication is semiconservative. In semiconservative replication, each DNA molecule is composed of one original strand and one new strand.
Which of the following is true of DNA topoisomerase II?
It is ATP-dependent
Topoisomerase I and II are enzymes that unwind supercoiled DNA into a relaxed form that allows replication to occur. They accomplish this through single- and double-stranded DNA breaks, respectively.
Topoisomerase II is ATP-dependent and causes double-stranded DNA breaks by cleaving the covalent bonds in the backbone of both strands of DNA.
Key Takeaway:
DNA topoisomerases I and II unwind supercoiled DNA into a more relaxed shape.
DNA topoisomerase I is ATP-independent and causes single-stranded DNA breaks.
DNA topoisomerase II is ATP-dependent and causes double-stranded DNA breaks.
Which of the following enzymes links Okazaki Fragments together?
DNA Ligase
DNA ligase links Okazaki fragments together. These fragments are the result of DNA synthesis occurring in the 5’-3’ direction on the lagging strand, leaving gaps between newly-synthesized DNA.
Which of the following enzymes has proofreading activity?
3’-5’ exonuclease activity of DNA polymerase I
DNA polymerase I is a prokaryotic DNA polymerase that has exonuclease activity that works in both the 3’-5’ and 5’-3’ directions.
Its 3’-5’ exonuclease activity involves proofreading of the DNA strand while its 5’-3’ exonuclease activity involves the removal of RNA primers.
Overactivity of which of the following enzymes allows cancer cells to be “immortal?”
DNA telomerase
DNA telomerase adds TTAGGG repeats to the ends of DNA strands. This sequence serves as a buffer to prevent degradation of the coding sequences during replication.
Some cancerous cells have overactive telomerase activity. This results in the generation of large telomeres at the ends of chromosomes that prevent degradation of these mutated chromosomes, thus allowing these cells to be “immortal.”
Key Takeaway:
Telomerase adds TTAGGG repeat sequences to the ends of DNA strands to prevent the ends of chromosomes from DNA damage or fusion with other chromosomes
Some cancer cells have overactive telomerase activity that prevents their mutated DNA from degrading.
are the main enzymes that separate paired strands of DNA and help them to unwind.
Helicases
are required for both the leading and lagging strands of the DNA molecule being transcribed.
RNA primers
To facilitate the replication of their long DNA molecules, eukaryotic cells demonstrate multiple
origins of replication
DNA is the enzyme that joins the Okazaki fragments together once they are synthesized.
ligase
binding proteins attach to the separated parental DNA strands and prevent them from reattaching during replication or transcription.
Single-stranded Binding Proteins
DNA replication is often called replication because a new DNA molecule consists of one new and one old nucleotide strand.
Semi-conservative
DNA polymerases can only add nucleotides to the daughter strand in the direction.
5’ to 3’ direction
is the enzyme that breaks the phosphodiester bond of one DNA strand backbone to facilitate unwinding of the helical strands for DNA replication or transcription.
Topoisomerase I
