module 6 bio

Function of DNA

DNA is the body’s instruction manual; it holds the directions for making proteins like insulin and glucose transporters.

Parts of a Nucleotide

A phosphate group, a deoxyribose sugar, and a nitrogenous base (A, T, C, or G).

Phosphodiester Bonds

Bonds that connect the phosphate of one nucleotide to the sugar of the next; form the DNA backbone.

Hydrogen Bonds

Bonds that connect the nitrogen bases (A–T and C–G) between the two DNA strands.

Complementary Base Pairing

A pairs with T; C pairs with G. Example: 5’ CGAT 3’ → 3’ GCTA 5’.

5’ End of DNA

Has a phosphate group attached.

3’ End of DNA

Has a hydroxyl (–OH) group; new nucleotides are added here during replication.

Importance of the 3’ End

DNA grows only in the 5’ → 3’ direction because new nucleotides attach to the 3’ hydroxyl group.

Semiconservative Replication

Each new DNA molecule has one original (parental) strand and one new (daughter) strand.

Origin of Replication (ORI)

The site where DNA replication begins.

Replication Bubble

The open area formed when DNA strands separate at the origin of replication.

Replication Fork

The Y-shaped region at each end of a replication bubble where DNA is copied.

Helicase

Unzips the DNA by breaking hydrogen bonds between bases.

Topoisomerase

Relieves twisting strain ahead of the replication fork.

Primase

Builds short RNA primers to start DNA synthesis.

DNA Polymerase III

Builds new DNA strands and proofreads for mistakes.

DNA Polymerase I

Removes RNA primers and replaces them with DNA nucleotides.

DNA Ligase

Seals gaps between Okazaki fragments on the lagging strand.

Leading Strand

Made continuously in the same direction as the replication fork (5’ → 3’).

Lagging Strand

Made in short Okazaki fragments opposite the fork’s direction.

Okazaki Fragments

Short DNA pieces formed on the lagging strand that are later joined by ligase.

DNA Proofreading

DNA Polymerase III corrects errors as it builds the new strand.

Nucleotide Excision Enzymes

Remove incorrectly copied DNA sections so they can be rebuilt correctly.