DNA Structure and ReplicationDNA Structure and Replication

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Chromosomes

• Contain all information for an organism

• Karyotype = arrangement of chromosomes

Eukaryotic Chromosomes

• Chromatin =– DNA – Histone proteins – condenses to

form strands called chromosomes,

– Strands occur just prior and during cell division

DNA

• Double stranded, coiled molecule

Discovery of Hereditary Material

• Friedrich Miescher (1869)– Isolates “nuclein” from nucleus– was not called DNA until the 1920’s– No notion it contained hereditary material

• Gregor Mendel (late 1800s)– Pioneering work in genetics– Traits are inherited discrete factors

Discovery of Hereditary Material

• Walter Sutton (early 1900s)– Suggested chromosomes held hereditary factors– No direct evidence for support

• Thomas Hunt Morgan (1910)– Provided experimental evidence to support Sutton– Identified X (sex) hormone– worked with chromosomes of fruit flies and eye

color

Discovery of Hereditary Material

• Phoebus Levene (1920s)

– Two types of nucleic acid in cells, composed of repeating nucleotides

– Both types with nearly identical structures

– DNA & RNA are the 2 types

Nucleotide Structure

• Three different components– Phosphate group– Nitrogen containing

base (A,T,G,C,U)– Five-carbon sugar

(ribose or deoxyribose)

Nitrogen Containing Bases

Discovery of Hereditary Material

• Oswald Avery (early 1940s)

– Preliminary evidence for DNA as hereditary material

– Transformed bacteria by transferring DNA

Discovery of Hereditary Material• Edwin Chargaff (late

1940s)– Proportion of bases

varies in the DNA of different type organisms

– Portions of bases roughly equal, (A=T & C =G)

– Purines = pyrimidines (A + G = C + T)

Discovery of Hereditary Material

• George Beadle & Edward Tatum (1950s)– One-gene-one-enzyme theory– Enzyme production under control of genes

• Alfred Hershey & Martha Chase (1952)– Building of Avery’s work, confirmedconfirmed DNA as

hereditary material

Discovery of Hereditary Material

• Rosalind Franklin & Maurice Wilkins (1953)– Developed double helix model for DNA– Evidence from X-ray diffraction

Discovery of Hereditary Material

• James Watson & Francis Crick (1953)– Structure of DNA– Synthesized previous

research– Nobel Prize in 1962,

along with Wilkins, Franklin died before nominations were made

Structure of DNA

Discovery of Genetic Code

DNA is Self-Replicating

• Before Cells divide

– Enzymes break bonds between bases

– Complementary strands separate

– Complementary bases are added to strands

– Copy of DNA results

Replication Mechanisms

• Replication begins at various points

• Proceeds in both directions

Replication Mechanisms• DNA molecule separates at its bases

• Forms split, or replication fork

• Each strand acts as a template

Replication Mechanisms

• Replication proceeds from 5‘ to 3‘ end

Genes

• One-gene-one-enzyme theory– Production of a given enzyme is under control

of a specific gene– Production of a given peptide is under control

of a single gene (revised statement)

Gene• The unit of heredity• Is a sequence of nucleotides• Codes for amino acid sequences of

polypeptides or for RNA

DNA Sequences

• Genome = all genetic information

• Promoters = “start here” part of sequence

• Introns = extra sequences between polypeptide-specifying portions, are not expressed, interrupt most eukaryotic genes

• Exons = portions of a gene that are expressed

Repeated Sequences

• Repetitive DNA

• 20-50% of eukaryotic DNA

• May play structural roles in chromosomes

Repeated Sequences

• Telomeres

– At ends of chromosomes

– Shorten as cell replicates

– Cells do not function when telomeres become too short Telomeres in yellow

Mutations • Change can occur in DNA• Point mutation or gene mutation

– Change in genetic message

• Chromosome mutation

Sources of Mutation• Ionizing radiation

• Ultraviolet radiation

• Some chemicals

• Transposable elements

Transposable Elements

• “jumping genes”

• Some DNA sequences move from one position to another

• Make up more than 40% of human genome

• First reported by Barbara McClintock

Transposable Elements

• Significance– Contribute to rate of mutation– May cause visible changes

• Example: mottling in corn

End Chapter 9