CHAPTER 16

The Molecular Basis of Inheritance

What is DNA?

DNA stands for deoxyribonucleic acid.DNA is what makes our genes, and along

with protein, makes our chromosomes.It encodes our hereditary information.It directs the development of our anatomical,

physiological, and behavioral traits.

How did we find DNA?

Thomas Hunt Morgan, in the early 1900s, studied fruit flies and showed that genes are on chromosomes.

How did we find DNA?

In 1928, Frederick Griffith saw that when an organism gets some external bacteria, it changes, which is called transformation.

How did we find DNA?

In 1944, Avery, McCarty, and MacLeod figured out that the bacteria had DNA in it, and that’s what caused transformation.

How did we find DNA?

In 1952, Hershey and Chase showed that DNA is the genetic material of our cells.

How did we find DNA?

In 1947, Erwin Chargaff figured out that DNA had four nitrogenous bases, A, T, C and G.

How did we find DNA?

In the 1950s, Rosalind Franklin took an x-ray crystallography picture of DNA.

How did we find DNA?

Watson and Crick used her picture to determine that DNA is a double helix.

The Structure of DNA

DNA is a double helix.It is a polymer made of

monomers called nucleotides.

Each nucleotide is made of a nitrogenous base, a pentose sugar called deoxyribose, and a phosphate group.

The backbone of DNA is called “sugar phosphate” and has bases attached to it like rungs of a ladder.

The Structure of DNA

DNA is “right handed” and curves to the right.

Hydrogen bonds hold the bases together

The 5’ end has a phosphate group

The 3’ end has an OH group

Strands always line up with one 5’ strand face up attached to a 3’ strand

The Structure of DNA

There are 4 different nitrogenous bases:

Adenine (A)Thymine (T)Guanine (G)Cytosine (C)

The Structure of DNA

Adenine always pairs with Thymine

Cytosine always pairs with Guanine

Purines

Purines are nitrogenous bases with 2 organic rings.

G and A are purines

Pyrimidines

Pyrimidines are nitrogenous bases with only 1 organic ring

Cytosine and thymine

DNA Replication

http://www.hhmi.org/biointeractive/media/DNAi_replication_schematic-lg.wmv

http://www.hhmi.org/biointeractive/media/DNAi_replication_schematic-lg.wmv

DNA Replication

DNA replicates during the S phase of interphase, prior to cell division (mitosis).

DNA replication is semi-conservative, meaning that new DNA strands are made of one new daughter strand attached to one old parent strand.

Meselson and Stahl figured this out in late 1950s.

DNA Replication

DNA replication begins at special sites called origins of replication, by opening up a replication bubble.

At each end of the replication bubble there is a replication fork, a Y-shaped region where DNA is being replicated.

DNA Replication

DNA polymerases are special enzymes that add complementary bases to the unzipped DNA.

DNA Replication

DNA replication can ONLY go from 5’ to 3’

So replication is antiparallel, one strand elongates normally, called the leading strand.

The other is going away from the replication fork, called the lagging strand.

DNA Replication

As the bubble of replication grows, the lagging strand is made bit by bit in fragments, called Okazaki fragments.

These are eventually joined by an enzyme called DNA ligase.

Proteins that help DNA replication

A primer is an initial bit of nucleotide that helps the new base attach to the DNA strand.

Primase is an enzyme that joins nucleotides together.

Proteins that help DNA replication

Helicase is an enzyme that unwinds DNA at the replication fork.

Topoisomerase is an enzyme that relieves the strain of untwisting the DNA

Single strand binding proteins bind to unwound DNA strands and stabilize them until replication is done

DNA Replication

http://www.hhmi.org/biointeractive/media/DNAi_replication_vo2-lg.wmv

http://www.hhmi.org/biointeractive/media/DNAi_replication_vo2-lg.wmv

Proofreading and Repairing DNA

1 in every 100,000 base pairs gets paired incorrectly

Special enzymes repair mistakesA nuclease is an enzyme that cuts out bad

DNA during nucleotide excision repair

Telomeres

Telomeres are nucleotides at the end of our DNA strands

Each time DNA replicates (as we age), telomeres get shorter

Shorter or missing telomeres results in cancer and other effects from aging.

Tall peas are dominant to short peas.

Cross two plants that are heterozygous for height.

If they have 100 pea babies, how many would you expect to be tall? short?