Genetics and Molecular Biology (AAMC Categories 1B, 1C, 2)

DNA Replication Timing

Occurs in the S-phase of the cell cycle.

DNA Helicase

Unzips the DNA double helix during replication.

Topoisomerase (DNA Gyrase)

Relieves supercoiling tension ahead of the replication fork.

DNA Primase

Lays down an RNA primer to provide a free 3'-OH group for DNA Polymerase.

DNA Polymerase

Builds the new DNA strand in the 5' to 3' direction and reads the template 3' to 5'.

DNA Ligase

Seals Okazaki fragments on the lagging strand.

Eukaryotic Promoters

Often contain a TATA box (~25-30 bp upstream), CAAT boxes, and GC boxes (which bind Sp1 transcription factors).

Prokaryotic Promoters

Characterized by the Pribnow box (-10 region) and the -35 region.

Eukaryotic RNA Processing

Pre-mRNA receives a 5' 7-methylguanylate Cap, a 3' Poly-A Tail, and undergoes Splicing before leaving the nucleus.

Spliceosome

Uses snRNPs and snRNAs to cut out non-coding introns (as lariats) and stitch coding exons together.

Translation Directionality

Ribosomes synthesize proteins strictly from the N-terminus to the C-terminus.

Ribosome Sizes

Prokaryotes = 70S (50S + 30S subunits). Eukaryotes = 80S (60S + 40S subunits).

Start Codon

AUG (Codes for Methionine).

Stop Codons

UAA, UAG, UGA ("U Are Annoying, U Go Away, U Are Gone").

Silent Mutation

A point mutation that results in no change to the amino acid due to the wobble effect.

Missense Mutation

A point mutation that swaps one amino acid for another.

Nonsense Mutation

A point mutation that prematurely codes for a STOP codon, truncating the protein.

Frameshift Mutation

An insertion or deletion that shifts the reading frame, ruining the entire downstream sequence.

Mismatch Repair

Active in the G2 phase; fixes errors missed by DNA polymerase (uses MSH2/MLH1 genes).

Nucleotide Excision Repair (NER)

Fixes bulky, helix-deforming lesions like Thymine Dimers (caused by UV light) using an excision endonuclease.

Base Excision Repair (BER)

Fixes non-deforming lesions (like deaminated cytosine). A glycosylase removes the base (creating an AP site), and an AP endonuclease cuts the backbone.

Histone Acetyltransferases (HATs)

Add acetyl groups to basic lysine residues on histones, neutralizing positive charge and relaxing DNA into transcriptionally active Euchromatin.

Histone Deacetylases (HDACs)

Remove acetyl groups from histones, restoring positive charge and tightening DNA into silent Heterochromatin.

DNA Methylation

Adding methyl groups to cytosine bases (CpG islands) to directly and often permanently silence gene expression.

Genomic Imprinting

An epigenetic phenomenon where only one allele (maternal or paternal) is expressed, while the other is silenced via methylation (e.g., Prader-Willi and Angelman syndromes).

Hardy-Weinberg Equations

Allele frequency: p + q = 1. Phenotype frequency: p^2 + 2pq + q^2 = 1.

Hardy-Weinberg Requirements

No mutations, random mating, no gene flow, large population size, no natural selection.

Sex-Linked Traits

Traits located on the X chromosome. Males (XY) are far more likely to express X-linked recessive disorders because they are hemizygous.

Cell Cycle Phases

G1 (growth), S (DNA replication), G2 (growth/prep), M (Mitosis).

G1/S Checkpoint (Restriction Point)

Ensures DNA is intact before replication. Heavily controlled by the p53 protein. If damaged, arrests cycle or triggers apoptosis.

Cyclins and CDKs

Cyclins bind Cyclin-Dependent Kinases to form active complexes. These phosphorylate transcription factors to push the cell cycle forward.