Chapter 13 - The Molecular Basis of Inheritance

The Molecular Basis of Inheritance

what were chromosomes believed to be made up of up until the 1940s?

proteins

Transformation

alteration of genotype and phenotype due to the assimilation of external DNA

Bacteriophage

virus which infects bacteria

Virus

DNA (sometimes RNA) surrounded by a protein coat that is a parasite and needs a host in order to survive

what enters the cell when a bacteriophage injects into the cell

DNA (not proteins)

what did the data collected by Rosalind Franklin in Maurice Wilkins lab show?

• that DNA was a helical molecule

• DNA is composed of two strands

• position of phosphate groups

• width and spacing of the bases

what did James Watson and Francis Crick contribute to the findings of the structure of DNA

• observed DNA’s basic components

• determined that the two strands of DNA are antiparallel

• determined base pairing and hydrogen bonding in base pairing

• applied Chagraff’s rules

• observed Franklin’s data on the width of the helix and spacing of bases

• applied Franklin’s conclusions about the position of phosphate sugars

what accounts for the large amount of diversity in organisms

sequence of bases in a DNA molecules

DNA packaging

plays an important role in both organization and regulation of expression of genetic material

what is chromatin made up of

made up of DNA and proteins

heterochromatin

highly condensed and mostly inaccessible to transcription and translation proteins

euchromatin

more dispersed and contains accessible regions which are actively transcribed

semi-conservative model of replication

• each new molecule of DNA contains 1 parental srand and 1 newly formed strand

3 theories of replication and explain each

• conservative mode - the two parental strands reassociae after acting as templates for new strands which restores the parental double helix

• semiconservative model - the two strands of the parental molecule separate and each functions as a template for synthesis of a new, complementary strand

• dispersive model - each strand of both daughter molecules contains a mixture of old and newly synthesized DNA

what would determine if conservative or dispersive replication was correct

if conservative replication was correct, DNA of two densities would have ben found after the first replication

if dispersive replication was correct, DNA of one density would be found after two replications

origins of replication

sites containing specific sequences where DNA replication begins

• specific proteins bind these sequences and open a replication bubble

origin of replication in prokaryotes vs eukaryotes

prokaryotes - 1 origin of replication

eukaryotes - 0 up to 1000 origins of replication

• more linear so it has multiple origins of replication

replication fork

at each end of the replication bubble where DNA unwinds

DNA Helicase

• enzyme that unwinds DNA strands at the replication fork so that the parent strands can act as replication templates

single-strand binding proteins

help to keep the single strands of DNA from rebinding to one another

topoisomerase

• enzyme that helps relieve strain ahead of the replication fork by repeatedly binding, swiveling, breaking, and then rebinding

primase

• enzyme which creates a short RNA primer (5-10 nucleotides) which complements the template strand of DNA

• new DNA strand is then synthesized by DNA polymerase starting from the end of the RNA primer

DNA polymerase

• synthesizes new strands of DNA

• builds from 5’ to 3’ direction starting from 3’ end towards 5’ end

• several different DNA polymerases exist

  • eukaryotes have 11+ different types of DNA polymerases

dNTP

• added by DNA polymerase onto DNA nucleotides to mark that its a deoxyribose and not a ribose

  • adding dNTP releases a pyrophosphate which is hydrolyzed to 2 molecules of inorganic phosphate

antiparallel

DNA strands run antiparallel to one another (like a two way street)

leading strand

• is synthesized continuously towards the replication fork

  • is built as a sliding clamp protein and a single DNA polymerases exist eukaryotes move towards the replication fork

lagging strand

• cannot be synthesized continously since nucleotides are added away from the replication fork

• produces short fragments called Okazaki fragments

  • 100-200 dNTP (DNA nucleotides) long in eukaryotes

okazaki fragments

• short fragments synthesized in the lagging strand of DNA replication

• 100-200 dNTP (DNA nucleotides) long in eukaryotes

lagging strand synthesis steps

1. Primase binds and a short RNA primer is synthesized to create a useable 5’ end

2. a DNA polymerase adds dNTP to the 5’ primer end to create the first Okazaki fragment

3. DNA polymerase 3 and the sliding clamp protein disassociate when they reach the next RNA primer

4. Okazaki fragment 2 is synthesized closer to the replication fork

5. DNA polymerase 1 removes the RNA primer and replaces it with DNA nucleotides for fragment 1

6. DNA ligase creates a bond between the 5’ and 3’ end of neighbouring fragments which joins them together