OAT

DNA is a double helix of two antiparallel polynucleotide strands. Each nucleotide contains a phosphate group, deoxyribose, and a nitrogenous base: adenine (A), thymine (T), guanine (G), or cytosine (C). A pairs with T through 2 hydrogen bonds, and G pairs with C through 3.

DNA Replication

Replication is semiconservative: each new molecule keeps one original strand and one new strand. It begins at origins of replication and proceeds as follows.

EnzymeorProtein	Role
Helicase	Unwindsthedoublehelix
Single-strand binding proteins	Stabilizetheseparatedstrands
Primase	SynthesizesRNAprimers
DNApolymeraseIII	Synthesizes new DNA 5′ → 3′; the leading strand is continuous and the lagging strand is
	madeinOkazakifragments
DNApolymeraseI	ReplacesRNAprimerswithDNA
DNAligase	JoinsOkazakifragments

Transcription, RNA Processing, and Translation

In transcription, RNA polymerase binds the promoter, reads the template strand 3′ → 5′, and builds RNA 5′ → 3′. Prokaryotes transcribe and translate simultaneously; in eukaryotes, transcription occurs in the nucleus and the transcript is processed before export: a 5′ cap (modified guanine) and a poly-A tail are added, and spliceosomes remove introns and join the exons.

Translation occurs on ribosomes in the cytoplasm. tRNA carries amino acids and pairs its anticodon with mRNA codons. The start codon is AUG (methionine); the stop codons are UAA, UAG, and UGA. The genetic code is universal, redundant (degenerate), and non-overlapping: 64 codons specify 20 amino acids plus stop signals.

➜

➜

Small subunit binds mRNA; initiator tRNA pairs

Aminoacyl-tRNA enters the A site; peptide

Release factor binds the stop codon; the

with AUG; large subunit joins

bond forms; ribosome translocates

polypeptide is released

Gene Regulation and Mutations

In prokaryotes, operons are gene clusters under a single promoter. The lac operon is inducible (lactose turns transcription on); the trp operon is repressible (tryptophan turns transcription off). Eukaryotic regulation works at many levels: chromatin remodeling, transcription factors binding promoters and enhancers, alternative splicing and mRNA stability, translational control, and post-translational modification.

Mutation	Effect
Silent	Pointmutation;thecodonstillspecifiesthesameaminoacid
Missense	Pointmutationthatchangesoneaminoacid
Nonsense	Pointmutationthatcreatesaprematurestopcodon
Frameshift	Insertionordeletion(notamultipleofthree)thatshiftsthereadingframedownstream

The cell cycle consists of interphase (G1 growth, S DNA synthesis, G2 preparation for division) and the mitotic phase (mitosis and cytokinesis). Cytokinesis divides the cytoplasm by a cleavage furrow in animal cells and a cell plate in plant cells.

Stage	KeyEvents
Prophase	Chromatincondenses;thenuclearenvelopebreaksdown;centriolesmigratetothepoles;spindlefibersform
Metaphase	Chromosomesalignatthemetaphaseplate;spindlefibersattachtokinetochores
Anaphase	Sisterchromatidsseparateandmovetooppositepoles
Telophase	Nuclearenvelopesreform;chromosomesdecondense;thespindledisassembles

Meiosis produces four haploid gametes from one diploid cell. Meiosis I separates homologous chromosomes: prophase I includes synapsis and crossing over, tetrads align in metaphase I, homologs separate in anaphase I, and two haploid cells form in telophase I. Meiosis II separates sister chromatids, much like mitosis.

Mitosisvs.Meiosis

Mitosisvs.Meiosis(Continued)

Feature	Mitosis	Meiosis
Homologpairing	No	Yes(synapsis crossingoverinprophaseI) ,
Purpose	Growth repair asexualreproduction , ,	Gameteformation

Genetic variation from sexual reproduction comes from crossing over, independent assortment, and random fertilization.