DNA Structure and Function

Basis of Heredity

Chemical Basis of LifeMolecule Building Block Major Function

DNA NucleotidesHereditaryMaterial

RNA NucleotidesProteinSynthesis

Protein Amino AcidsCell Structureand Function

Lipid Fatty AcidsMembraneComponent

Carbohydrate SugarsEnergyProduction

The Central Dogmareplication

transcription processing translation

• cells can make exact copies of DNA• DNA encodes all of the information necessary

for cellular functions• RNA is made from a DNA template and

functions in protein synthesis• proteins are translated from messenger RNA

and carry out cellular functions

Nucleotides

DNA vs RNA2' oxygen

thymine (T) uracil (U)double

strandedsingle

stranded

DNA = deoxyribonucleic acidRNA = ribonucleic acid

• nucleotides are the building blocks of nucleic acids

• ribose + base + 5' phosphate• 5 different bases

• adenine (A)• guanine (G)• cytosine (C)• thymine (T)• uracil (U)

• nucleic acids are polymers of nucleotides

• nucleotides linked through phosphate bonds

• strand has polarity—5'-PO4 and 3'-OH ends

• order of the nucleotides defines a sequence

• 4 letter alphabet

Nucleic Acid Structure

• specific base pairing• A and T• G and C• hydrogen bonds

• anti-parallel (5'-3')• complementary strands• template for synthesis

Double Strands

DNA Replication• the DNA strands are separated• each strand serves as template• complementary strands are

synthesized (5'3')• yields 2 identical DNA molecules• semi-conservative replication

• carried out by cellular proteins (= enzymes)

• complex process involving many proteins

• DNA polymerase

DNA Polymerase Activity• 'replication fork' moves along DNA• leading strand• lagging strand

• numerous replication forks act simultaneously

Chromosomes• genomic DNA is very long•humans, 6 x 109 bp•~1.8 meters total length

• total length reduced via associates with proteins• individual DNA molecules

condensed into chromosomes• humans have 22 pairs + X/X

or X/Y (ie, diploid)

Chromosome Replication• unfolding of chromosomes, DNA

replication, and refolding• chromosomal duplication• sister chromatids• joined at centromeres

Mitosis• DNA synthesis• chromosomal duplication• separation of sister chromatids • cell division (cytokinesis) results

in 2 identical daughter cells• somatic cell reproduction• asexual process

• sexual reproduction involves germ line cells •oogenesis (egg formation)•spermatogenesis (sperm formation

Meiosis• two consecutive rounds of

cell division• meiosis I• DNA replication (2n4n)• separation of chromosome

pairs (4n2n)• meiosis II• no DNA replication• separation of chromatids

(2n1n)• fertilization (1n2n)

Recombination

• independent chromosome assortment • 2n possible variants

• 'crossing-over'• recombination between

homologous chromosomes• at least once per

chromosome pair• positions ~ random

Genes and Alleles• gene = unit of information (~protein)• locus = physical location on a chromosome• allele = different molecular forms (ie,

sequence) of a gene• homozygous = same allele on both

chromosomes• heterozygous = different alleles on

chromosome pairs

genotype = genetic makeup (ie, total genes)

phenotype = observable traits

Types of Inheritancedominant-recessive

dominant allele 'masks' recessive allele

incomplete dominance

phenotype between dominant and recessive traits

co-dominance shared expression of traits (eg, AB blood type)

polygenic several genes influence trait (often continuous variation)

pleiotrophy single gene affecting several traits