chapter 14 - DNA replication

hershel and chase

investigated bacteriophages to determine whether DNA or protein is the genetic material injected into bacteria

blender experiment

bacteria were infected with phage labeled 35S (protein) or 32P (DNA); phage were knocked off with a blender; bacteria were centrifuged to see what entered the cell

hershel and chase’s findings

only 32P entered the bacterial cells; DNA is the genetic material

DNA structure

a nucleic acid composed of nucleotides

nucleotides

made of sugar (deoxyribose), a phosphate group, and a nitrogenous base; has 1 free hydroxyl group (—OH)

phosphodiester bonds

covalent bonds formed b/w the phosphate group of one nucleotide and the 3’ —OH of the next nucleotide

orientation of nucleotide chain

5’-3’

phosphodiester backbone

repeating sugar and phosphate units joined by phosphodiester bonds

chargaff’s rule

there is always an equal proportion of purines (A and G) and pyrimidines (C and T)

bonds that hold the helix together

hydrogen

semiconservative replication

complementarity of DNA strands allows each strand to serve as a template for synthesis of the other strand

stages of DNA replication

initiation, elongation, termination

initiation stage

replication begins at the origin

helicase

breaks hydrogen bond b/w nucleotides and opens the bubble to begin unwinding DNA

single-stranded-binding proteins (SSBPs)

stabilizes single-stranded regions; keeps the replication bubble open to prevent the DNA from coiling back

gyrase

makes cuts upstream of bubble to avoid supercoiling

elongation stage

new strands of DNA are synthesized by DNA polymerase

leading strand

is synthesized continuously from an initial primer

lagging strand

is synthesized discontinuously with multiple priming events

primase

synthesizes RNA primers

DNA polymerase III

adds nucleotides to the free 3’ end of the new strand; requires an RNA primer

DNA polymerase I

removes RNA primers and replaces with DNA

DNA polymerase II

analyzes and repairs any errors

okazaki fragments

chunks of lagging strand that are each primed by RNA primer and DNA polymerase II

ligase

joins the final phosphodiester bonds together

termination stage

replication is terminated

eukaryotic replication

multiple origins along chromatin copy DNA bidirectionally and terminate when bubble points meet

telomeres

specialized structures found on the ends of eukaryotic chromosomes composed of short repeated sequences of DNA

telomere function

protect ends of chromosomes from nucleases and maintain integrity of linear chromosomes

telomerase

enzyme that makes telomere section of lagging strand using an internal RNA template

DNA damage

errors in proofreading and repair systems

mutagens

any agent that increases the number of mutations above background level