Chapter 11 - DNA STRUCTURE & REPLICATION

What is DNA made of?

DNA is made of nucleotides, which consist of a phosphate group, a sugar molecule (deoxyribose), and a nitrogenous base (adenine, thymine, cytosine, or guanine). These components create the double helix structure of DNA.

Components of one nucleotide?

A phosphate group, deoxyribose sugar, and a nitrogenous base.

DNA sugar name?

Deoxyribose sugar

DNA bases?

Adenine, thymine, cytosine, and guanine. (A,T,G,C)

Purines?

A type of nitrogenous base that includes adenine and guanine, characterized by a two-ring structure. (A and G)

Pyrimidines?

A type of nitrogenous base that includes cytosine, thymine, and uracil, characterized by a single-ring structure. (C and T)

Chargaff’s rule?

A principle stating that in any given DNA molecule, the amount of adenine equals thymine and the amount of cytosine equals guanine, reflecting base pairing. (A=T, G=C)

DNA strands are antiparallel—meaning?

they run in opposite directions, with one strand oriented 5' to 3' and the other 3' to 5'. This arrangement is crucial for replication and function.

Hydrogen bonds: A–T vs G–C

Hydrogen bonds between adenine and thymine consist of two bonds, while guanine and cytosine form three bonds. This difference in bonding contributes to the stability of DNA.

DNA backbone is made of what?

sugar and phosphate groups

DNA replication model supported by evidence?

The semiconservative model, where each new DNA strand consists of one original and one new strand, supported by experiments like those of Meselson and Stahl.

Semiconservative means?

DNA replication in which each strand serves as a template for a new complementary strand, resulting in two DNA molecules with one original and one newly synthesized strand.

Direction DNA polymerase works?

DNA polymerase synthesizes DNA in the 5' to 3' direction, adding nucleotides to the growing strand. This means it can only add nucleotides to the 3' end of the growing DNA strand.

Template strand is read in what direction?

The template strand is read in the 3' to 5' direction, allowing for the synthesis of the new strand in the 5' to 3' direction.

Why can DNA polymerase only add to 3′ end?

DNA polymerase requires a free 3′ hydroxyl group to attach incoming nucleotides, which is essential for the formation of phosphodiester bonds during DNA synthesis.

What enzyme unwinds DNA?

Helicase is the enzyme that unwinds DNA by breaking the hydrogen bonds between base pairs, allowing replication to occur.

What enzyme prevents supercoiling?

Topoisomerase is the enzyme that prevents supercoiling by introducing cuts in the DNA strand, relieving the tension during replication.

What stabilizes single-stranded DNA?

Single-strand binding proteins (SSBPs) stabilize single-stranded DNA by binding to it and preventing re-annealing or degradation during replication.

What enzyme makes RNA primers?

Primase is the enzyme that synthesizes RNA primers, providing a starting point for DNA synthesis during replication.

Why are RNA primers needed?

RNA primers are needed to provide a starting point for DNA polymerase, as DNA synthesis cannot initiate without a primer.

Leading strand synthesis is…

the continuous synthesis of DNA in the same direction as the replication fork, allowing DNA polymerase to add nucleotides without interruption.

Lagging strand synthesis is…

the discontinuous synthesis of DNA in the opposite direction of the replication fork, resulting in segments of DNA called Okazaki fragments that must be joined together.

What are Okazaki fragments?

short DNA segments synthesized during lagging strand replication that are later linked together.

Which enzyme replaces RNA primers with DNA?

DNA polymerase I

Which enzyme joins Okazaki fragments?

DNA ligase

Replication begins at what site in bacteria?

The origin of replication, known as oriC, where the DNA unwinds and replication machinery assembles.

What is a replication fork?

A structure that forms during DNA replication where the DNA double helix is unwound to allow for the synthesis of new strands.

What happens to DNA at the fork?

The DNA strands are separated, with each strand serving as a template for the synthesis of a new complementary strand, leading to the formation of two daughter DNA molecules.

What problem do linear chromosomes face?

Linear chromosomes face the issue of incomplete replication at their ends, leading to potential loss of genetic information.

What are telomeres?

Repeated sequences at chromosome ends that protect against degradation and prevent loss of genetic information during replication.

What does telomerase do?

Telomerase is an enzyme that adds repetitive nucleotide sequences to the ends of telomeres, extending their length and preventing chromosome shortening during DNA replication. (Extends 3’ end of the chromosome)

Telomerase active in which cells?

Telomerase is active in germ cells, stem cells, and certain cancer cells, allowing them to maintain telomere length and avoid senescence.

What is proofreading?

The process by which DNA polymerase checks and corrects errors during DNA replication, ensuring high fidelity in genetic material.

What is mismatch repair?

A cellular mechanism that corrects errors that escape proofreading during DNA replication, specifically correcting mispaired nucleotides.

UV light commonly causes what damage?

DNA damage, often leading to mutations such as thymine dimers.