6.4.5 DNA Replication: A Summary Flashcards
review/note
- Review: DNA consists of a double helix of nucleotide strands. The nucleotide subunit is composed of a sugar, a phosphate, and either a purine or pyrimidine base.
· Within a strand, phosphodiester bonds join the nucleotides.
· Between strands, specific base pairing occurs.
• Review: The initiation of replication occurs when bonds between the strands break. New strands are synthesized in opposite directions of each other and are called leading and lagging strands. - A DNA strand is made up of nucleotide subunits. The subunit is composed of a sugar, a phosphate, and either a purine or pyrimidine base.
- Within a strand, phosphodiester bonds join the nucleotides.The sugars and phosphates form the backbone of the strand.
- Between DNA strands, specific base pairing occurs.
- Adenine and guanine are classified as purines. Thymine and cytosine are classified as pyrimidines. Purines bond with pyrimidines.
- Adenine bonds only with thymine, and guanine bonds only with cytosine through hydrogen bonding. The specificity in base pairing means that the strands are complementary.
- Initiation of DNA replication begins when the hydrogen bonds between the two strands break. The molecule replicates semi-conservatively with each strand acting as a template for the synthesis of a new complementary strand.
- (a) Replication bubbles appear at multiple points along the DNA strand.
- Replication forks occur on both sides of the replication
bubble. DNA polymerase synthesizes new strands, requiring a 3’ OH group to build the strand. The enzyme reads a parental strand in the 3’ to 5’ direction and builds the new strands in the 5’ to 3’ direction. - At a replication fork, the overall direction of replication
occurs in one direction. Because DNA polymerase can
synthesize only in the 5’ to 3’ direction, portions of strands are synthesized in opposite directions. - (b) Leading strands are synthesized are synthesized
continuously. Lagging strands are synthesized
discontinuously. - Illustration (a) shows that RNA primer provides a temporary 3’ end for DNA poly- merase to attach to. RNA polymerase synthesizes RNA primer.
- Ligase bonds nucleotides of a strand together. Another form of DNA polymerase replaces any incorrect bases.
- The ends of the DNA molecule are flanked with telomeres, repeating nonsensical units, to protect coding information.
True or false?
The leading strand of DNA is synthesized smoothly, whereas the lagging strand of DNA is synthesized discontinuously.
- true
During DNA replication, a new strand is made
- in the 5′ to 3′ direction only
Which of the following nucleotides are purines?
- guanine and adenine
What happens to a DNA molecule that enables the strands to separate?
- Hydrogen bonds between the complementary bases are broken
- The enzyme helicase attaches to the molecule and splits the strands
_________________ removes RNA primers from the new DNA strand.
- DNA polymerase I
True or false?
Okazaki fragments occur on the leading strand of DNA. Each fragment has its own RNA primer.
- false
__________________ is two nucleotide strands twisted together and __________________ is when hereditary information is copied.
- A DNA helix; replication
Many enzymes are involved in DNA synthesis. ______________ separates the coils of DNA; ______________ polymerizes DNA; and __________________ connects Okazaki fragments.
- Helicase; DNA polymerase; ligase
Telomeres
- are repeating units at the ends of chromosomes that protect against the loss of essential information
Replication bubbles
- speed up DNA replication so that it only takes a few minutes in eukaryotes
- provide multiple sites for replication along a DNA strand
- are regions where the DNA has uncoiled and is being replicated
______________________ is the overall direction of two ____________________ strands of DNA relative to one another so that the _________________ run opposite of each other.
- Antiparallel; complementary; 5′ to 3′ orientations
