DNA Structure & Replication Quiz

DNA Structure & Replication

• James Watson and Francis Crick introduced the double-helical model of DNA in 1953
• DNA is the hereditary material, the most important molecule for inheritance
• Hereditary information is encoded in DNA and replicated in all body cells
• This DNA program directs the development of biochemical, anatomical, physiological, and behavioral traits

Nucleic Acid Structure

• There are two types of nucleic acids: Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA)
• DNA provides instructions for its own replication. As a cell divides, its genetic instructions are passed to each daughter cell
• DNA directs the synthesis of messenger RNA (mRNA) and, through mRNA, controls protein synthesis
• Protein synthesis occurs in ribosomes
• Nucleic acids are polymers called polynucleotides
• Each polynucleotide is made of monomers called nucleotides
• Each nucleotide consists of a nitrogenous base, a pentose sugar, and a phosphate group
• The portion of a nucleotide without the phosphate group is called a nucleoside

Nucleotide Monomers

• Two families of nitrogenous bases:
  • Pyrimidines (cytosine, thymine, and uracil) have a single six-membered ring
  • Purines (adenine and guanine) have a six-membered ring fused to a five-membered ring
• In DNA, the sugar is deoxyribose; in RNA, the sugar is ribose

DNA Double Helix

• A DNA molecule has two polynucleotides spiraling around an imaginary axis, forming a double helix
• The two backbones run in opposite 5' to 3' directions, forming an antiparallel arrangement
• DNA molecules include many genes
• Nitrogenous bases pair up and form hydrogen bonds: adenine (A) always with thymine (T), and guanine (G) always with cytosine (C)

DNA Replication

• DNA replication is remarkable for its speed and accuracy. Many proteins and enzymes are involved
• Replication begins at specific sites called origins of replication, creating a replication "bubble"
• Replication proceeds in both directions from each origin, until the entire molecule is copied
• Helicases untwist the double helix at replication forks.
• Single-strand binding proteins bind to and stabilize single-stranded DNA
• Topoisomerase corrects overwinding ahead of replication forks by breaking, swiveling, and rejoining DNA strands
• DNA polymerases cannot initiate synthesis; they can only add nucleotides to the 3' end
• The initial nucleotide strand is a short RNA primer
• An enzyme called primase can synthesize an RNA chain from scratch using a DNA template as a guide. The primer is short (~5-10 nucleotides long), with a free 3' end
• Each nucleotide is a nucleoside triphosphate (e.g., dATP). As each monomer joins the DNA strand it loses two phosphate groups to provide energy
• Elongation occurs in the 5' to 3' direction on the leading strand
• The lagging strand is synthesized as a series of segments called Okazaki fragments, which are joined together by DNA ligase
• Many proteins work together in a "DNA replication machine," which may be stationary during replication
• DNA polymerases proofread newly made DNA; replacing any incorrect nucleotides
• Enzymes correct errors in base pairing (mismatch repair)
• DNA can be damaged by harmful chemicals or physical agents. Damage repair mechanisms (e.g., nucleotide excision repair) exist
• Errors occur and can cause changes in the following generation. This provides the raw material for evolution

Replicating Ends of DNA Molecules

• Limitations of DNA polymerase create problems for the linear DNA of eukaryotic chromosomes.
• Telomeres, special nucleotide sequences at the ends of eukaryotic chromosomes, postpone the erosion of genes at the ends of DNA molecules.
• The enzyme telomerase catalyzes the lengthening of telomeres in germ cells. The shortening of telomeres may protect cells from cancerous growth by limiting the number of cell divisions.

Chromosome Structure

• The bacterial chromosome is a double-stranded, circular DNA molecule associated with a small amount of protein.
• Eukaryotic chromosomes have linear DNA molecules associated with a large amount of protein
• In a bacterium, the DNA is "supercoiled" and found in a region of the cell called the nucleoid
• Chromatin, a complex of DNA and protein, is found in the nucleus of eukaryotic cells.
• Chromosomes fit into the nucleus via a multilevel system of packing, using proteins like histones.