dna structure and function basis of heredity. chemical basis of life
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DNA Structure and Function
Basis of Heredity
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Chemical Basis of LifeMolecule Building Block Major Function
DNA NucleotidesHereditaryMaterial
RNA NucleotidesProteinSynthesis
Protein Amino AcidsCell Structureand Function
Lipid Fatty AcidsMembraneComponent
Carbohydrate SugarsEnergyProduction
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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
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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)
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• 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
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• specific base pairing• A and T• G and C• hydrogen bonds
• anti-parallel (5'-3')• complementary strands• template for synthesis
Double Strands
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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
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DNA Polymerase Activity• 'replication fork' moves along DNA• leading strand• lagging strand
• numerous replication forks act simultaneously
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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)
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Chromosome Replication• unfolding of chromosomes, DNA
replication, and refolding• chromosomal duplication• sister chromatids• joined at centromeres
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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
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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)
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Recombination
• independent chromosome assortment • 2n possible variants
• 'crossing-over'• recombination between
homologous chromosomes• at least once per
chromosome pair• positions ~ random
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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
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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