Rhonda Alexander IC Science Robert E. Lee
B.6.A identify components of DNA, and describe how information for specifying the traits
of an organism is carried in the DNA
DNA serves as the molecular basis of heredity
In living organisms genetic information is stored in the molecule deoxyribonucleic acid, or
DNA. DNA is made and resides in the nucleus of living cells. DNA gets its name from the
sugar molecule contained in its backbone (deoxyribose), however it gets its significance
from its unique structure. There are four different nucleotide bases that occur in DNA:
adenine (A), cytosine (C), guanine (G) and thymine (T).
Chemical Structure of the DNA Nucleotides
Each nucleotide has three parts.
– a phosphate group
– a deoxyribose sugar
– a nitrogen-containing base
These nucleotides bind to the sugar backbone of the molecule as follows:
A
T
G
C
sugar phosphate sugar phosphate sugar phosphate sugar ...
Watson and Crick determined the three-dimensional structure of
DNA by building models. They realized that DNA is a double helix
that is made up of a sugar phosphate backbone on the outside with
bases on the inside.
The versatility of DNA comes from the fact that the molecule is actually double-stranded.
The nucleotide bases of the DNA molecule form complementary pairs: the nucleotides
hydrogen bond to another nucleotide base in a strand of DNA opposite to the original. This
bonding is specific, and adenine always bonds to thymine (and vice versa) and guanine
always bonds to cytosine (and vice versa). This bonding occurs across the molecule leading
to a double-stranded system.
sugarphosphatesugarphosphatesugarphosphatesugar...
T
A
C
G
¦
¦
¦
¦
A
T
G
C
sugarphosphatesugarphosphatesugarphosphatesugar...
The base-pairing rules show
how nucleotides always pair
up in DNA.
– A pairs with T
– C pairs with G
The genome is the collective name for all of the genes in a species, such as a human being.
It is a DNA blueprint which specifies how all the cells are built, how they will function and
how they might divide to produce new, daughter cells. The genome is coded in a DNA
molecule. The structure of DNA is built from four different nucleotide bases arranged in
a linear string. The bases themselves are: adenine (a), guanine (g), cytosine (c) and thymine
(t). Every cell in the organism’s body contains a complete copy of the genome which was
inherited from the parents.
The genetic code is made of triplets of bases called codons. A codon codes for a single
amino acid and a series of amino acids results in a protein. The series of amino acids
determine the identity of the protein (the stuff we are made of).
Chromosome Structure found in the Nucleus
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
In non dividing cells, the DNA molecules are stretched out like thin threads, with many proteins attached to it. Each
DNA thread and its proteins is called a Chromosome.
Each species has a specific number of chromosomes (Human somatic cells (non sex) have 46)
Gametes (sperm and egg) contain half the number of chromosomes of somatic cells. (Human gametes have 23
chromosomes)
When a cell prepares to divide the genetic information must be copied and packaged in order to evenly and
efficiently divide it among the two new cells.
A chromosome duplicates itself prior to cell division. The copies stay attached to each other until a particular point
in cell division. The chromosome and its duplicate are now called sister chromatids. They are attached to each other
at a point along their length called the centromere. This region contains attachment sites on each sister chromatid
for microtubules called a kinetochore, that will move the chromosomes where they need to go. The image that you
see is of a condensed duplicated chromosome
When a chromosome condenses after duplication it does so by winding around specific proteins called histones like
thread on a spool. Each histone and the DNA wrapped around it is called a nucleosome. Multiple nucleosomes coil
into a cylindrical fiber called a solenoid. Each solenoid then supercoils into the visible structure that can be seen
with a microscope.