Bio: 8.1-8.2 - DNA & Replication

How are DNA, genes, and chromosomes related?

**DNA** makes up our **genes**, which are found on **chromosomes**

DNA Function

stores the genetic info of an organism as a “code” of bases

Gene

A segment of DNA that contains the info to make a specific protein

How are proteins and phenotypes related?

proteins = phenotypes, so different genes → different order of bases → different proteins → different phenotypes

DNA Structure

made of repeating nucleotides which are made of

* 5 carbon sugar (deoxyribose)
* phosphate group
* nitrogenous bases (a small organic molecule w/ a core ring made of carbon and nitrogen)

Deoxyribose

5 carbon sugar:

* 1’ carbon = nitrogenous base attaches
* 3’ carbon = -OH group
* 5’ carbon = phosphate attaches

Nitrogenous Bases

Each nucleotide has a different one:

* Adenine
* Guanine
* Cytosine
* Thymine

Purines

2 rings of C & N, larger → A & G

Pyrimidines

1 ring of C & N → C & T

Complementary Base Pairing

chemical structures “complement” each other

* A w/ T → 2 Hydrogen bonds
* g w/ C → 3 Hydrogen bonds

a purine **must** pair w/ a pyrimidine - consistent width

Chargaff’s Rule

\# of A = # of T (and same w/ G & C) - small difference result from mutations

Sides of DNA

2 strands of alternating sugar & phosphates connected via covalent bonds (strong)

Anti-Parellel

side-by-side, but running in opposite directions

Ends of DNA

5’ ends = a phosphate

3’ ends = sugar w/ a “free” -OH groups at the 3’ C atom

When does DNA Replication occur?

during interphase of the cell cycle - before mitosis

Semi-conservative

each replicated DNA is made of one “old” and one “new” strand, but identical in the genetic info they hold

DNA Replication Location

begins at **specific** **origins of replication** along the DNA

Helicase

* binds to an origin of replication - breaks H bonds between base pairs
* separates the 2 strands of DNA (like unzipping a zipper)

→ **creates replication fork**

Primase

* builds **RNA Primers** - small pieces of RNA (a molecule similar to DNA
* provides a starting point for DNA to be built - has a 3’ end, so more nucleotides can attach

DNA Polymerase Short Definition

adds nucleotides to build new strands of DNA

DNA Polymerase Process

* builds new DNA off the 3’ ends of the primers - “reads” the old template DNA
* adds complementary bases in the 5’ → 3’ direction to build new DNA
* new nucleotides add to the 3’ ends via dehydration synthesis (H + OH → H2O)
* eventually removed and replaced w/ DNA

Leading Strand

replicated continuously toward the fork

Lagging Strand

replicated in **Okazaki fragments** away from the fork - b/c original DNA molecules is antiparallel

Ligase

connects the gaps in the sugar-phosphate backbone by creating covalent bonds between sugars & phosphates