biolA 190 - ch 9

9.1

9.1

1. information

must contain information necessary to construct entire organism

2. replication

must be accurately copied

3. transmission

must be passed from parents to offspring and from cell to cell during cell division

4. variation

be able to account for differences between individuals and between species

a biochemical basis of heredity was postulated in the late

1800s

Frederick Griffith (heat)

smooth strains (S)secrete capsules; are typically deadly

rough strains (R) do not secrete capsules; are typically survivable

a mix of live type R and heat-killed type S bacteria injected

mouse died

living type S bacteria were isolated from the blood

Griffith postulated that

a substance (genetic material) from the dead type S cells had transformed the type R cells into type S

1940s Avery, MacLeod, and McCarty used Griffith’s observations as part of

an experimental strategy to biochemically identify genetic material

enzymes that break down DNA, RHNA, or protein were used to

degrade potential contaminants

9.2

9.2

DNA and RNA are nucleic acids, polymers of nucleotides that are responsible for the

storage, expression, and transmission of genetic information

a nucleotide has 3 components

pentose sugar

phosphate group

nitrogen-containing base

a strand is formed when nucleotides are

covalently attached

phosphodiester bonds

link nucleotides together

a sugar in one nucleotide is linked to a phosphate group in the next nucleotide forming a

sugar-phosphate backbone

strands have directionality

based on orientation of the sugar molecules

the 5’ end has a free phosphate group

and the 3’ end has a free hydroxyl group

double-stranded helix

with outer backbone and bases on the inside

stabilized by H-bonds between base pairs (AT/CG)

complimentary

antiparallel

9.3

9.3

X-ray diffraction

key experimental tool that led to the discovery of the DNA double helix structure

1950s Rosalind Franklin

helical structure

uniform diameter

diameter too big to be a single-strand

1950 - Erwin Chargaff

analyzed the base composition of DNA that was isolated from many different species and found a pattern

A was similar to T

C was similar to G

Watson and Crick

synthesized works of others to discover the structure of DNA

9.4

9.4

in the late 1950s 3 different models for DNA replication had been proposed

semiconservative mechanism

conservative mechanism

dispersive mechanism

original strands are

parent strands

new strands are

daughter strands

during replication, the 2 parental strands are separated and serve as

template stands for the synthesis of daughter strands

9.5

9.5

origin of replication

a site within a chromosome that serves as a starting point for DNA replication

eukaryotic chromosomes are larger and have a

linear structure with multiple origins of replication

DNA helicase, DNA topoisomerase, and single-strand binding proteins are responsible for

fork formation and movement

2 enzymes are needed to synthesize DNA strands during replication:

DNA polymerase and DNA primase

DNA polymerase covalently

links nucleotides together

DNA polymerase has 2 important functional constraints

1. DNA polymerase cannot begin synthesis on a bare template strand; it can only extend a pre-existing strand

2. DNA polymerase synthesizes DNA in a 5’ to 3’ direction

DNA primase makes a complimentary primer of RNA

(10-12 nucleotides in length) that can be extended by DNA polymerase

daughter strands are synthesized

differently at the replication fork

permanent mistakes in DNA replication are

extraordinarily RARE

9.6

9.6

a typical eukaryotic chromosome can be hundreds of millions of base pairs in length and must fit inside the nucleus meaning

chromosomes must be folded and compacted

eukaryotic DNA is first compacted by wrapping around a group of proteins called

histones, which form structures called nucleosomes

nucleosomes are organized into a more

compact structure that is 30 nm in diameter

a third level of compaction involves interactions between the

30-nm fibers and proteins to form loop domains

the level of compaction of chromosomes is not

uniform

heterochromatin is highly compacted whereas

euchromatin is less condensed

When a cell prepares to divide

each chromosome becomes entirely condensed