F19 Flashcards
Essential concepts
- Before a cell divides, it must accurately replicate the vast quantity of genetic information carried in its DNA.
- Because the two strands of a DNA double helix are complementary, each strand can act as a template for the synthesis of the other. Thus DNA replication produces two identical, double-helical DNA molecules, enabling genetic information to be copied and passed on from a cell to its daughter cells and from a parent to its offspring.
- During replication, the two strands of a DNA double helix are pulled apart at a replication origin to form two Y-shaped replication forks. DNA polymerases at each fork produce a new, complementary DNA strand on each parental strand.
- DNA polymerase replicates a DNA template with remarkable fidelity, making only about one error in every 107 nucleotides copied. This accuracy is made possible, in part, by a proofreading process in which the enzyme corrects its own mistakes as it moves along the DNA.
- Because DNA polymerase synthesizes new DNA in the 5ʹ-to-3ʹ direction, only the leading strand at the replication fork can be synthesized in a continuous fashion. On the lagging strand, DNA is synthesized in a discontinuous backstitching process, producing short fragments of DNA that are later joined together by DNA ligase.
- DNA polymerase is incapable of starting a new DNA strand from scratch. Instead, DNA synthesis is primed by an RNA polymerase called primase, which makes short lengths of RNA primers that are then elongated by DNA polymerase. These primers are subsequently removed and replaced with DNA.
- DNA replication requires the cooperation of many proteins that form a multienzyme replication machine that pries open the double helix and copies the information contained in both DNA strands.
- In eukaryotes, a special enzyme called telomerase replicates the DNA at the ends of the chromosomes, particularly in rapidly dividing cells.
- The rare copying mistakes that escape proofreading are dealt with by mismatch repair proteins, which increase the accuracy of DNA replication to one mistake per 109 nucleotides copied.
- Damage to one of the two DNA strands, caused by unavoidable chemical reactions, is repaired by a variety of DNA repair enzymes that recognize damaged DNA and excise a short stretch of the damaged strand. The missing DNA is then resynthesized by a repair DNA polymerase, using the undamaged strand as a template.
- If both DNA strands are broken, the double-strand break can be rapidly repaired by nonhomologous end joining. Nucleotides are often lost in the process, altering the DNA sequence at the repair site.
- Homologous recombination can flawlessly repair double-strand breaks (and many other types of DNA damage) using an undamaged homologous double helix as a template.
- Highly accurate DNA replication and DNA repair processes play a key role in protecting us from the uncontrolled growth of somatic cells known as cancer.
Show how A new DNA strand is synthesized in the 5’ to 3’ direction
Show how At a replication fork, the two newly synthesized DNA strands are of opposite polarities
Show how At each replication fork, the lagging DNA strarnd is synthesized in pieces
Show how Cells can repair double-strand breaks in one of two ways
Show how Centrifugation in a cesium chloride gradient allows the separation of heavy and light DNA
Show how Chemical modifications of nucleotides, if left unrepaired, produce mutations
Show how Depurination and deamination are the most frequent chemical reactions known to create serious DNA damage in cells
Show how DNA acts as a template for its own replication
Show how DNA double helix is opened at replication origins
Show how DNA ligase joins together Okazaki fragments on the lagging strand during DNA synthesis
Show how DNA polymerase adds a deoxyribonucleotide to the 3’ and of a growing DNA strand
Show how DNA polymerase contains separate sites for DNA synthesis and proofreading
Show how DNA synthesis is carried out by a group of proteins that act together as a replication machine
Show how DNA synthesis occurs at Y-shaped junctions called replication forks.
Show how DNA topoisomerases relieve the tension that builds up in front of a replication fork
Show how During DNA synthesis, DNA polymerase proofreads its own work
Show how Errors made during DNA replication must be corrected to avoid mutations
Show how For proofreading to take place, DNA polymerization must proceed in the 5’ to 3’ direction
Show how Homologous recombination flawlessly repairs DNA double-strand breaks
Show how In each round of DNA replication, each of the two strands of DNA is used as a template for the formation of a new complementary strand
Show how Mismatch repair eliminates replication errors and resotres the original DNA sequence
