Download Day 9: DNA Powerpoint

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!