Molecular Genetics

What is this stuff?

•Simply: DNA contains the instructions for making proteins in a cell

• DNA = Deoxyribonucleic Acid• The genetic/hereditary

material• Referred to as a “blueprint”

for life• More of a “recipe”

DNA – The Genetic Material• DNA contains our genes:

• Gene: A segment of DNA that codes for a specific protein

• DNA is packaged with proteins to form chromosomes

• DNA wraps around 8 histone proteins nucleosomes

• Nucleosomes large, coiled loops, held by scaffolding proteins

• Loops interact Chromosomes

Why Study DNA?

• Medical purposes

• Producing better crops• Controversial, downsides

• DNA Forensics

• Evolutionary relationships

• Many, many more........

A Brief History of DNA•Hershey and Chase (1952)• Bacteriophages: attach to cell and inject own genetic material

• Cell uses genetic material to make more viruses• Labelled bacteriophage:

• Protein coat with 35S • DNA with 32P• Infect cells

•Agitate cells in a blender (shakes off bacteriophages)

• Centrifuge cells to separate from phage particles

• Only 32P found in cells, not 35S

• Conclusion: The active component of the bacteriophage that transmits the infective characteristic is the DNA. There is a clear

correlation between DNA and genetic information.

DNA Review!

•Has shape of helix or corkscrew•Is about 2 nm in diameter•2m of it in a nucleus!!•Makes a complete helical turn ever 3.4 nm•An awesome interactive review...http://learn.genetics.utah.edu/content/begin/

tour/

The Structure of DNA• DNA molecules consist of two polynucleotide chains

• The two chains, or strands, are twisted and arranged in a double-helix

• Ladder-like arrangement

• We will look at a single strand first

A Single Strand• Composed of nucleotide building blocks

• Three main components

• A pentose sugar: deoxyribose

• A phosphate

• A nitrogenous base

Turn up the Base(s)!

Structure of deoxyribose

A Single Strand• Composed of an alternating sugar-phosphate backbone: legs of ladder

• The bases stick into the middle:rungs of ladder

• The phosphate end is referred to as the 5’ (five prime) end

• Attached to Carbon 5

• The opposite end is referred to as the 3’ (three prime) end (Carbon 3)

•Consists of millions of nucleotides

DNA Is Double Stranded

• Recall the double helix

• The legs of the ladder = sugar-phosphate backbone

•The rungs = bases

• How do single strands stick together?

4 Types of Nitrogenous Bases• DNA has 4 kinds of nitrogenous bases:

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

• 2 Categories:

1)Purines: Two carbon rings• Adenine and Guanine

2)Pyrimidines: One carbon ring• Thymine and Cytosine

• Bases are attracted to each other by hydrogen bonds

• Weak, but millions of them in a DNA molecule

• This is what holds the strands together

• When bonds form:• Guanine always pairs with Cytosine – 3 bonds

• Adenine always pairs with Thymine – 2 bonds

• Base pairing is complimentary

DNA Is Double Stranded – Hydrogen Bonds

Hydrogen bonding between base pairs

DNA Is Double Stranded

• Chargaff’s Rule •1950 - Erwin Chargaff looked at percentages of bases in DNA samples

•1:1 ratio of purines to pyrimidines• Amount of G = C They always pair• Amount of A = T They always pair

DNA Is Double Stranded• Base pairing is complimentary: use one strand to predict the other

• Strands are anti-parallel: run in opposite directions

• The 3’ end terminates with the hydroxyl group of the deoxyribose sugar.

• The 5’ end terminates with a phosphate group

Finding the code...

•5’ - ATGCCGTTA - 3’

•3’ - TACGGCAAT - 5’•By convention, only the 5’ to 3’ strand is

written since the complementary strand can easily be deduced ...

•Try this one...•5’ - TGGACGCTT - 3’

•3’ - ACCTGCGAA - 5’

DNA Shape: The Double Helix• We use the analogy of a ladder that has been twisted into a helix

• Right Handed Helix: B-DNA, type generally found• Left-Handed Helix: Z-DNA

•0.34 nm distance between base pairs (10 base pairs in 1 helical turn – 3.4 nm)

•width: 2nm•Each cell has about 2 m of DNA.•How does all this DNA fit into the tiny nucleus?

• the DNA helix is coiled and long segments (200 nucleotides long) are wrapped around a complex of 8 histones (proteins)

• this bead-like structure is called a nucleosome

• DNA is negative (phosphate groups) and so is attracted to the positively charged histones

• the histones protect the DNA from breaking and tangling

•nucleosomes are packed as a solenoid, a 30 nm fiber that has at least 5 nucleosomes strands together wound helically

• this arrangement of DNA (chromatin) makes it more compact and easier to accommodate in the relatively small nucleus

• DNA (and protein) fibers are then further coiled (supercoiling) to form chromosomes

• a chromosome is one unbroken double-stranded DNA helix (only seen when they condense during cell division)

• Just before mitosis, 2 identical copies (sister chromatids) are visible, attached at a region called the centromere Packaging of DNA

V6/

Ending on a cooler note

• Each cell has about 2 m of DNA.

• The average human has 75 trillion cells.

• The average human has enough DNA to go from the earth to the sun more than 400 times.

• DNA has a diameter of only 0.000000002 m.

The earth is 150 billion mor 93 million miles from

the sun.

That’s it!

Homework...

•Answer DNA Handout Questions, Complete Build a DNA molecule Lab questions

•**DNA Extraction Lab Friday – Prelab questions p 230 of textbook.