Unit 6: Gene Expression and Regulation

Ch. 16, 17, 18, 19, 20, 21, 27.1-2

Nucleotides

comprised of a 5-carbon sugar, a nitrogenous base, and a phosphate group.

Purines

(A,G) have two rings in their nitrogenous bases.

Pyrimidines

(C,T,U) have one ring.

Prokaryotes

1 circular chromosome (millions of base pairs), and usually at least one small, extrachromosomal plasmid (thousands of base pairs)

Eukaryotes

multiple linear chromosomes (e.g. humans have 46 - ~3 billion base pairs)

Viruses

DNA or RNA, single or double stranded (thousands of base pairs).

Exons

sections of DNA pulled out in order to be expressed

Introns

sections of DNA that are left behind

Spliceosome

enzyme that cuts DNA in order to pull out exons

Semi conservative model

• each strand of old molecule serves as a template for the synthesis of a new strand

Replisome

• All of the enzymes involved in replication that function together.

Helicase

opens the helix by breaking the (weak) phosphate bonds between nucleotides

Ligase

forms covalent bonds between adjacent nucleotides in one strand of DNA. Needed to join Okazaki fragments together.

Topoisomerase

Rotates the helix ahead of the replication fork to reduce stress.

DNA Polymerase III

• Responsible for the addition of free nucleotides to a new growing strand of DNA.

• Can only add nucleotides in the 5’ → 3’ direction of the growing strand.

• Makes mistakes but does proofreading to fix them.

• DNA pol. III synthesizes new strands by adding to RNA primer or pre-existing DNA strand only.

DNA Polymerase I

• Responsible for removing RNA nucleotides of primer from 5’ end and replacing them with DNA nucleotides.

• DNA Polymerase III & I can only add nucleotides to the 3’ end of a molecule (moving in the 5’ → 3’ direction of the growing strand).

• Note: Nucleotides must start building from a DNA primer, polymerase removes the primer and begins DNA replication

• Can only attach to a primer or base already present. Primer, made of RNA, acts like a flag to indicate where DNA synthesis begins. Finally, the U’s in the RNA are taken out by DNA polymerase (removing primer) so that the DNA is official.

Primase

Synthesizes RNA primer at 5’ end of leading strand and 5’ end of each Okazaki fragment

Leading Strand

• synthesized continuously in 1 piece

• synthesized as DNA polymerase moves along the template as the replication fork progresses

Lagging Strand

• synthesized discontinuously in multiple fragments, connected by ligase

• series of short segments, AKA Okazaki fragments,