THE MOLECULAR BASIS OF INHERITANCE

Chapter 16

THE SEARCH FOR GENETIC MATERIAL

Frederick Griffith (1928) – something changed normal cells into

pneumonia causing cells in miceAlfred Hershey & Martha Chase (1952)

– DNA of virus injected into bacteria, not protein

Figure 16.1 Transformation of bacteria

Figure 16.2a The Hershey-Chase experiment: phages

Figure 16.2ax Phages

Figure 16.2b The Hershey-Chase experiment

Erwin Chargaff (1947)– Amounts of G = C and A = T in DNA

Rosalind Franklin (1950’s)– Photographed DNA using X-ray

crystallographyJames Watson and Francis Crick (1953)

– DNA is a double helix with base pairs as the rungs

Figure 16.4 Rosalind Franklin and her X-ray diffraction photo of DNA

Figure 16.3 The structure of a DNA stand

Figure 16.0 Watson and Crick

DNA STRUCTURE

4 Bases– Purines: adenine and guanine– Pyrimidine: cytosine and thymine– A – T and C – G

Hydrogen bonds connect the base pairs to make the rungs

Deoxyribose and phosphate make up the uprights

Nucleotide – a base, a sugar, and a phosphate

Purine and pyridimine

Figure 16.6 Base pairing in DNA

Figure 16.5 The double helix

SEMICONSERVATIVE MODEL OF REPLICATION

When DNA copies itself, the resulting “copy” is one of the original strands with a new strand

Figure 16.7 A model for DNA replication: the basic concept

Figure 16.8 Three alternative models of DNA replication

Figure 16.9 The Meselson-Stahl experiment tested three models of DNA replication (Layer 4)

Figure 16.10 Origins of replication in eukaryotes

DNA REPLICATION

Why/when does DNA replicate? Every time a cell divides (mitosis or meiosis)

the DNA is copied in interphase (S of the cell cycle).

Copying the DNA ensures that the cells created will get the right number and kind of chromosomes during cell division (mitosis or meiosis).

DNA REPLICATION

Origins of replication – specific sequences of nucleotides that initiate replication

Replication fork – a Y shaped region where new nucleotides are added

DNA polymerases – enzymes that break H bonds (unwind helix) and add complementary nucleotides

Reaction driven by nucleoside triphosphates (like ATP)

Loose 2 phosphate groups to be added as a nucleotide

Figure 16.11 Incorporation of a nucleotide into a DNA strand

The 2 strands of a double helix are antiparallel

The sugar-phosphate backbones run in opposite directions

One is 5’ to 3’ while other is 3’ to 5’#1 is the C attached to a base and #5 is

the C attached to phosphate

Figure 16.12 The two strands of DNA are antiparallel

DNA polymerases can only attach nucleotides to the free 3’ end of existing an polynucleotide (already paired to complementary strand).

DNA polymerases add nucleotides in only the 5’ to 3’ direction

Leading strand – made by DNA polymerase following replication fork

Figure 16.13 Synthesis of leading and lagging strands during DNA replication

The other strand of DNA that is made is called the lagging strand

Polymerase makes a short strandAs bubble grows, more short strands are

made called Okazaki fragments

Figure 16.14 Priming DNA synthesis with RNA

Okazaki fragments are joined by DNA ligase

Priming: polymerase must attach nucleotides to an existing polynucleotide at a 3’ end

Primase (an enzyme) adds RNA nucleotides to make a primer ~10 bases long

Polymerase can then add its complementary nucleotides

Polymerase also replaces certain RNA nucleotides of primer

For lagging strand each fragment must be primed, but only one primer is needed for entire leading strand

DNA Polymerases

DNA polymerase I – removes primers from 5’ end of leading strand and each Okazaki fragment and replaces it with DNA nucleotides

DNA polymerase III – adds nucleotides to both leading and lagging strand

OTHER ENZYMES…

Helicase – untwists helix at replication fork

Topoisomerase – relieves the strain that untwisting causes ahead of the replication fork

Single-strand binding proteins – hold strands apart while replication occurs

Figure 16.15 The main proteins of DNA replication and their functions

Figure 16.16 A summary of DNA replication

PROOFREADING

Mismatch repairing – polymerase checks each addition

Excision pair – nuclease cuts out bad section of strand

Replication Animation 1Replication Animation 2 (can see multiple

replication origins)

Figure 16.17 Nucleotide excision repair of DNA damage

Figure 16.18 The end-replication problem

TELOMERES

Impossible on lagging strand to copy the end of 5’ strand

This leaves a gap that would shorten DNA every time it replicates

Telomeres (TTAGGG) repeated many times protects genes by postponing erosion of genes from this shortening effect

Telomerase – lengthens telomeres– Contains an RNA sequence that is the template

for a telomere– Present in germ cells (for future gametes)– Increased activity in cancer cells (allows for more

cell division)

Figure 16.19a Telomeres and telomerase: Telomeres of mouse chromosomes