Structure of DNA

Learning Objectives

By the end of this class you should understand:

The experiments that determined the nature of genetic inheritance

The structure and chemistry of DNA and RNA

How to read and write complementary strands of DNA

The process of DNA replication

Why DNA and RNA can only be polymerized in one direction

How telomeres work and why they are linked to aging

Genetic Material

We have known since Mendel that some chemical carries information from parent to child

Determining which chemical was easier said than done! For a long time, many

believed it was protein, since there are 20 amino acids and it was believed they made an “alphabet”

Early Experiments

Information was gleaned step by step

“Nucleic acids” were found in the nucleus

Nucleic acids contain phosphorus whereas protein contains sulfur Both are also made with

carbon, hydrogen, oxygen, and nitrogen

Griffith Experiments

Hershey-Chase Experiments

Chemistry of Inheritance

These experiments established that nucleic acids carried the information for life Today it is taken as common

knowledge The structure and function of

nucleic acids still had not been determined until Watson and Crick And Franklin!

Chemistry Recap

All matter is made of many different kinds of atoms

Depending on the number of protons, atoms may wish to share electrons in a covalent bond Carbon, oxygen, hydrogen, and

nitrogen are very good at sharing electrons

Metals are not good at sharing electrons

Hydrogen Bonds

When an atom shares its electrons unequally with another atom it may be slightly positive or slightly negative

These small positive/negatives can attract each other in a hydrogen bond So called because hydrogen has

a weak pull on electrons and so is often the positive pole

Structure of DNA The various bonds between

adjacent bases of DNA and their backbones are all covalent bonds, as are bonds within other biomolecules

The two strands of DNA are held together by hydrogen bonds This should make sense

since the DNA cannot be read unless it is first unzipped

Bases (“letters”) of DNA

Backbone of DNA

Nucleic acid bases are attached to a special sugar to hold them in place Forms a sugar backbone

The sugars are attached to each other with a phosphate Individual DNA bases with a

sugar have three phosphates Phosphates carry energy

Nucleic Acids

There are two similar types of nucleic acids: DNA and RNA Deoxyribonucleic

acid (DNA) Sugar backbone

made with Deoxyribose

Ribonucleic acid (DNA)

Sugar backbone made with Ribose

Why Deoxyribose?

Deoxyribose is more stable than ribose Oxygen is reactive,

and removing one oxygen helps prevent unwanted chemical reactions

RNA is used for temporary copies, DNA is permanent

Directionality DNA is written in one

particular direction, just as we write left to write Or right to left in Hebrew.... Or top to bottom in

Chinese.... New DNA/RNA bases can

only be added to the 3' end of the DNA

PROBLEM: Complementary strands run antiparallel

Why Only 3' End?

Remember individual DNA bases come with three phosphates on the 5' end Remember also phosphates

store energy in their bonds The energy needed to attach a

new DNA base is partially paid for by the two extra phosphates There is no way to use those

phosphates when attaching to the 5' end!

Complementary Strands

Note the following sequences of complementary DNA:

5'-ACTTGCCATAGGA-3'

3'-TGAACGGTATCCT-5' If the DNA is opened, the template strand can

be copied with new DNA (replication) or with RNA (for protein synthesis)

5'-GGCCATAACCG-3'

3'-CCGGUAUUGGC-3' (RNA strand)

DNA Replication

Occurs during S phase of mitosis/meiosis

The complementary strands are opened and new DNA bases attach to each strand This is called

semiconservative replication

Semiconservative?

DNA Replication DNA replication is

accomplished with the use of DNA polymerase “Polymer” means

many chemicals strung together

“-ase” is a suffix used for enzymes (proteins that make chemical reactions happen faster)

Okazaki Fragments

Imagine painting a double yellow line on a road but you had to paint one line with forward strokes and the other with backwards strokes Forward one is easy, backwards one will be done in

many small strokes that link together Animation: http://www.youtube.com/watch?

v=teV62zrm2P0

RNA Transcription

More of this in chapter 9 Essentially the same

process as copying the leading strand of DNA only Uses RNA Polymerase

This RNA copy can then result in production of proteins

Telomere

The tips of chromosomes have long repeated sequences of DNA that are noncoding but protect the rest of the DNA DNA replication typically

fails to completely duplicate these telomeres

They function as the aglets on your shoelaces

Telomere Shortening

Every time a cell undergoes mitosis it loses a bit of its telomeres

Adult cells have shorter telomeres and telomere shortening may be linked to aging Dolly the sheep was cloned

from an adult cell and so may have showed premature aging

Telomerase

Immortal cells such as stem cells and cancer cells express an enzyme called telomerase Lengthens telomeres and

extends life of daughter cells

Research in stem cells and telomerase may provide clues into aging process

See you tomorrow!