DNA
Deoxyribonucleic Acid
Structure and Function
Remember from the Genetics Unit…
• Chromosomes carry hereditary information (information
that is passed to offspring) that is present in distinct units
called genes arranged along chromosomes like beads.
• Those beads are called alleles.
• It is the combination of the alleles that create the gene,
which will eventually create the expression in the
organism.
Make Up of Chromosomes
• Chromosomes are made of chromatin, which describes
the combination of DNA and proteins wound tightly
together.
• They must be wound up tightly to fit inside of the nucleus.
• http://www.youtube.com/watch?v=eYrQ0EhVCYA
Nucleic acids
• 2 kinds:
• DNA = Deoxyribonucleic acid
• RNA = Ribonucleic acid
Both found in
the nucleus
of the cell
Let’s start with DNA…
• DNA is the hereditary material that is passed on
from one generation to the next through
reproduction
• No 2 people in the world have 100% the same
DNA.
• The Human Genome Project mapped the genes
of a single person and found that the human
genome contains more than 3.4 billion base pairs
and between 20 000 and 25 000 genes.
• It took 13 years to map the human genome and
was complete in 2003.
• http://www.youtube.com/watch?v=tZ_b800KcPo
DNA has three components
1.
5 carbon sugar (deoxyribose)
2.
Phosphate group (PO4)
3.
Nitrogenous base
There are 4 nitrogenous bases in DNA:
•
•
Adenine (A)
Guanine (G)
Thymine
Cytosine
(T)
(C)
• When the 3 components are combined together, they
create a nucleotide.
• This unit repeats and composes the majority of the DNA
structure.
• DNA nucleotides are composed of:
•
1 molecule deoxyribose sugar
•
1 molecule phosphate group
•
&
1 of the 4 nitrogenous bases (A,T,C or G)
• Nucleotides are arranged in a
ladder structure.
• Alternating phosphate and
sugar molecules make up the
sides of the ladder
• Pairs of nitrogenous bases
make up the rungs of the
ladder
The shape of DNA is called a Double Helix
Complementary base pairing rule
• In any pair of bases:
• Adenine will bond only to Thymine
• Guanine will bond only to Cytosine
• Nitrogenous bases are
attached to each other with
hydrogen bonds:
• There are 2 hydrogen bonds
between Adenine and Thymine
• There are 3 hydrogen bonds
between Guanine and Cytosine
• Each pairing must be with a ‘Purine’
and ‘Pyrimidine’.
• Adenine and Guanine are purines.
• Thymine and Cytosine are
pyrimidines.
• So, keeping the complementary base pairing rule in
mind: If the order of Nitrogenous bases along one
arm of DNA was…
A-A-A-C-G-A-T-A-T-T-G-C-C-G-A
T T T G C T A T A AC G GC T
• Question: If 12% of the nitrogenous bases in a persons
DNA was Adenine, then what percent of their nitrogenous
bases would be Guanine?
ADENINE 12
CYTOSINE 38
THYMINE 12
GUANINE 38
• The sequence of nitrogenous
bases acts as a code.
• The code determines what proteins
will be made by the cell.
• Proteins in turn determine the nature
and activities of the cell.
• This is why the amount of DNA is not
important.
• Sequence determines outcome.
Replication of DNA
• Due to the fact that cells divide so rapidly, DNA must be
replicated often and efficiently in order to keep up with
demand.
• DNA is replicated during interphase of either mitosis or
meiosis depending on the cell.
• Human cells take about 20 hours to replicate all 46
chromosomes.
• However, some can take only minutes. E.Coli bacteria
can replicate DNA and divide a cell in only 20 minutes.
Steps in Replication
1: The hydrogen bonds between the base pairs
(nitrogenous bases) break and the two strands
of the DNA molecule pull apart.
• this exposes the base pairs along each strand
• 2: Bases of free nucleotides (floating around in
the nucleus) fasten on to complementary bases
of the now exposed DNA chain.
• Result: complementary strand is created that is
identical to the one that split off earlier.
• Remember that this process is done for each of the
two original DNA strands.
• Think back to mitosis: when the chromosomes
doubled
• Replication does not actually begin at one end of the
DNA strand and end at the other.
• To speed up the process, Replication begins at many
points on the DNA strand that connect later.
• These are called Replication bubbles.
RNA
• Similar to in composition to DNA…
• The molecule consists of only 1 chain
(instead of 2).
• Half of the ‘ladder’ is present.
• The sugar is Ribose (not
deoxyribose)
• Thymine is replaced with Uracil
• (which eventually bonds with
adenine)
• RNA is involved in the synthesis of
proteins
A-A-A-C-G-A-T-A-T-T-G-C-C-G-A
U U UG C U A U A A C G G C U
RNA nucleotide
• How many different DNA & RNA nucleotides are there
total?