DNA Replication

In terms of DNA replication, what does it mean when we say its semi-conservative?

There is 1 template strand of DNA and 1 new strand of DNA

Who proposed that DNA replication is semi-conservative?

Meselson and Stahl

Who experimentally confirmed that DNA replication is semi-conservative?

Watson and Crick

Origin of Replication

Where DNA replication begins on a DNA strand

Prokaryotes vs Eukaryotes — origin of replication

Prokaryotes have a single origin; eukaryotes have multiple origins

Prokaryote’s replication bubble is (circular/linear)

Circular

Eukaroyote’s replication bubble is (circular/linear)

Linear

Replication Fork

Y-shaped region where the DNA double helix unwinds

When was DNA polymerase I discovered?

1956

• Originally was simply termed “DNA polymerase” until others were found

When DNA polymerase I was first discovered, what was it thought to do?

Thought to be the only enzyme responsible for copying DNA

What is DNA polymerase I responsible for?

• Removes RNA primers and replaces them with DNA

• Responsible for DNA repair

When was DNA polymerase II discovered?

1970

What is DNA polymerase II responsible for?

Involved in backup repair where DNA Polymerase I may have failed

When was DNA polymerase III discovered?

1971

What is DNA polymerase III responsible for?

Responsible for leading/lagging strand synthesis

Sliding Clamp

Forms a complex with DNA polymerase 3

Primase

Adds RNA primers

RNA Primer

Must start with RNA, as RNA can be built with nothing

What does DNA require to build from?

A 3’ hydroxyl group

Helicase

Unwinds DNA strands

Singe-stranded Binding Protein (SSBP)

Prevent reannealing of DNA after helicase unwinds

• Keeps original DNA strands from binding back together

Topoisomerase

Moves ahead of helicase to relieve pressure of unwinding

Where does synthesis of new DNA strands begin?

Origin of replication

DNA synthesis unwinds in what direction?

Both

Antiparallel Elongation in DNA strands

5’ —— 3’

3’ —— 5’

Regarding DNA polymerase III, what is the most important thing to note about its function?

It can only add nucleotides to the 3’ -OH end

Leading Strand Synthesis

• Goes from the 5’ to 3’ direction

• Moves toward the replication fork/helicase

• Topoisomerase is ahead of the helicase to stabilize the helix

• SSBPs prevent re-annealing of template strands

• Primase adds the RNA primer on the leading strand for continuous synthesis

• DNA polymerase III + Sliding Clamp form a complex

  • Adds nucleotides to the 3’ -OH end

Lagging Strand Synthesis

• Moves away from the replication fork/helicase

• SSBPs prevent re-annealing of template strands

• Results in multiple RNA primers

• DNA Polymerase III + Sliding Clamp form a complex

  • Adds nucleotides to the 3’ -OH end

• Okazaki fragments created between RNA primers

• DNA Polymerase I removes RNA primase and adds DNA to that spot

• DNA Ligase seals the two fragments

Prokaryote mitosis

Essentially gives rise to a new organism that is an exact clone

Unicellular Eukaryote mitosis

Essentially gives rise to a new organism that is an exact clone

Multicellular Eukaryote mitosis

Supports renewal, repair, and embryonic development

Mitosis

Cell division

Chromosomes

Form of packaging of genetic material

(T/F) — All animals have the same number of chorosomes

False

Chromatin

Packaging of complex DNA + histones

• Makes up chromosomes

• “Beads on a string” where each bead is called a nucleosome

Histones

Positively-charged proteins containing high amounts of lysine and arginine, allowing negatively-charged DNA to bind to them tightly

• Octamer (8 subunits) with 2 DNA “winding” around each histone complex

Somatic Cells

All body cells except for reproductive cells

• Each contains 46 chromosomes

  • 22 autosomes (non-sex) + 1 sex chromosome

    • 23 comes from each parent; n = 46

Gametes

Reproductive cells

• Contains only 23 chromosomes

• Derived from “germ cells”

  • Spermatogonia and oogonia

Germen

Latin for “seed”

During division, long, thing chromatin fibers are (condensed/uncondensed)

Uncondensed

After division, chromosomes start to (condense/uncondense)

Condense

Sister chromatids

Duplicated chromosomes consisting of two copies of the original chromosome

• Held together via cohesins (proteins)

• Greatest point of attachment is at the centromere