ch 16: the molecular basis of inheritance

where are genes located?

on chromosomes

chromosomes

DNA + protein

S strain

pathogenic

R strain

harmless

heat-killed S + living R →

living pathogenic S cells

transformation

uptake of genetic material changes genotype and phenotype

(bacterio)phages

viruses widely used in molecular genetics research

virus

DNA (sometimes RNA) enclosed by a protective coat, often simply protein

what are the components of a bacterial cell?

from top to bottom:

phage head

DNA

tail sheath

tail fiber

genetic material

what was the conclusion of the hershey-chase experiment?

DNA, not protein, is the genetic material

what is DNA?

a polymer of nucleotides

components of a nucleotide

1. a nitrogenous base

2. a deoxyribose sugar

3. a phosphate group

what are the four nitrogenous bases?

1. adenine (A)

2. thymine (T)

3. guanine (G)

4. cytosine (C)

what type of bond holds the nitrogenous base pairs together?

hydrogen bonds

DNA replication

the copying of DNA:

1. the two parental DNA strands separate

2. each strand serves as a template

3. complementary nucleotides build new strands

one parental strand + one new strand =

each daughter DNA molecule

how does DNA replicate?

semiconservative model: one old strand + one new strand

where does replication begin?

at origins of replication:

• where the two DNA strands are separated

• and the DNA opens into replication bubbles

• two replication forks

replication fork

a y-shaped region where new DNA strands are elongating

helicases

enzymes that untwist the double helix at the replication fork; opens DNA

single-stranded binding proteins (SSB)

bind to and stabilize single-stranded DNA, and keep separated strands apart; keep it open

topoisomerase

relieves twisting ahead of the fork by breaking, swiveling, and rejoining DNA strands; releases tension ahead

primase

synthesizes a short RNA primer

primer

provides a free 3’ end

what does DNA polymerase do?

adds DNA nucleotides to the 3’ end

in what direction is DNA synthesized in?

5’ → 3’

which end does DNA polymerase add nucleotides to?

only to the 3’ end

leading strand

synthesized continuously toward the replication fork

lagging strand

synthesized discontinuously away from the fork; made as short okazaki fragments (each beginning w/ an RNA primer); DNA ligase joins the fragments

what proofreads newly synthesized DNA?

DNA polymerases

mismatch repair

corrects remaining replication errors

nucleotide excision repair

a nuclease removes damaged DNA

what happens when an error is not repaired?

mutation → genetic variation

telomeres

repetitive DNA at chromosome ends that protects genes from being lost during shortening

telomerase

can extend telomeres in certain cells

replication the ends of DNA molecules

linear chromosome end → last RNA primer removed → no 3’-OH available for DNA polymerase → chromosome becomes shorter

bacteria

usually one circular DNA chromosome that is supercoiled in the nucleoid

eukaryotes

linear DNA chromosomes that are associated w/ large amounts of protein

chromatin

DNA + proteins; undergoes multiple levels of packing → chromosome

histones

“beads on a string”; organize DNA into nucleosomes

histone tails

help regular gene expression; can be modified

DNA simplified

condensed chromosome → chromatin → nucleosome → DNA

ligase

joins fragments

big picture

DNA structure → accurate replication → repair & end protection → packaging into chromosomes

DNA pol I

removes RNA nucleotides of primer from 5’ end and replaces them w/ DNA nucleotides added to the 3’ end of adjacent fragment

DNA pol III

using parental DNA as a template, synthesizes new DNA strand by adding nucleotides to an RNA primer or a pre-existing DNA strand