AP Biology: Ch. 16The MOLECULAR BASIS OF

INHERITANCE

DNA can transform bacteria Frederick Griffith performed experiments

showing evidence that genetic material was a specific molecule (1928).

Showed that bacteria had been transformed; their phenotype changed due to the assimilation of external genetic material

Viral DNA can program cells Alfred Hershey and Martha Chase

conducted studies showing that DNA was the genetic material of a bacteriophage called T2. (1952)

They concluded that viral proteins stay outside the host cell, and that viral DNA is injected into the host cell.

Chargaff’s Rules In 1947, Erwin Chargaff analyzed the DNA

composition of different organisms using paper chromatography to separate nitrogenous bases.

He discovered that DNA composition is species-specific; amounts and ratios of nitrogenous bases vary among species

Chargaff, cont. In a DNA sample, the amount of adenine

(A) = the amount of thymine (T). Guanine (G) = Cytosine (C)

Watson and Cricks’ structural model for DNA explained these rules.

Base-pairing rules:

A—T

G—C

Rosalind Franklin at King’s College in London produced an X-ray photo of DNA (1950s)

Discovering the Double Helix Watson and Crick studied Franklin’s work

and suggested that: DNA is a double helix with uniform width (2

nm) Purine and pyrimidine bases are

stacked .34 nm apart. There are 10 layers of nitrogenous base

pairs in each turn of the helix.

To be consistent with a 2 nm width, a purine on one strand must pair by hydrogen bonding with a pyrimidine on the other

DNA Replication and Repair Watson and Crick proposed that genes on

the original DNA strand are copied by a specific pairing of complementary bases.

The complementary strand can then be used as a template to produce a copy of the original strand.

This is a semiconservative model of DNA replication.

DNA replication, cont. Enzymes and other proteins carry out DNA

replication. The process is complex, extremely rapid,

and accurate. DNA replication is similar in prokaryotes

and eukaryotes.

Origins of Replication DNA replication begins at special sites called

origins of replication that have a specific sequence of nucleotides.

Specific proteins required to initiate replication bind to each origin, causing the double helix to open.

Replication forks spread in both directions away from the origin creating a replication bubble.

*Bacterial or viral DNA may have only one replication

origin.

Elongating a new strand Enzymes called DNA polymerases catalyze

synthesis of a new DNA strand. According to base-pairing rules, new nucleotides

align along the template of the old DNA strand. DNA polymerase links the nucleotides to the

growing strand in the 5’3’ direction. Hydrolysis of nucleoside phosphates provides the

energy necessary to synthesize the new DNA strands.

Antiparallel DNA strands The sugar-phosphate backbones of the two

complementary DNA strands run in opposite directions.

DNA polymerase can only elongate strands in the 5’ 3’ direction.

The problem of antiparallel DNA strands is solved by the continuous synthesis of one strand (leading strand) and discontinuous synthesis of the complementary strand (lagging strand).

Leading vs. Lagging strands The leading DNA strand is synthesized as a

single polymer in the 5’ to 3’ direction towards the replication fork.

The lagging strand is synthesized against the overall direction of replication. It is produced as a series of short segments called Okazaki fragments.

The many fragments are connected by DNA ligase, which catalzes the formation of a covalent bond between the 3’ end of each new fragment to the 5’end of the growing chain.

Priming DNA synthesis Before new DNA strands can form, there must be

small preexisting primers to start the addition of new nucleotides.

Primers are short RNA segments (linked by primase enzymes) that are complementary to DNA segments. Needed to begin DNA replication.

Only one primer is needed for replication of the leading strand, but many are required to replicate the lagging strand. An RNA primer must initiate the synthesis of each Okazaki fragment.

Other proteins assisting in DNA replication

Helicases are enzymes which catalyze unwinding of the parental double helix to provide the template.

Single-strand binding proteins are proteins which keep the separated strands apart and stabilize the unwound DNA until a complementary strand can be synthesized.

DNA Proofreading and Repair DNA replication is highly accurate due to

base-pairing specificity and proofreading/repair mechanisms.

DNA can be repaired as it is being synthesized (mismatch repair) or after the accidental changes in existing DNA (excision repair)

DNA repair, cont. Mismatch repair- DNA polymerase proofreads

each newly added nucleotide against the template. Incorrectly paired nucleotides are removed and replaced before synthesis continues.

Excision repair- Segments damaged by physical or chemical agents are removed by a repair enzyme, then the gap is filled in by base-pairing nucleotides with the undamaged strand. DNA polymerase and DNA ligase catalyze the filling in process.

Telomeres DNA polymerase can only add nucleotides to the

3’ end of a preexisting DNA chain. Repeated replication produces shortened DNA

molecules, potentially deleting some gene sequences.

Telomeres are special nucleotide sequences (not containing genes) at the end of eukaryotic chromosome molecules that prevent this.

Telomerase is an enzyme that catalyzes the lengthening of telomeres

Telomeres (The yellow dots!)