DNA & Chromosomes
DNA – deoxyribonucleic [dee-ox-ee-ri-bo-new-clay-ik] acid
DNA is the molecule which contains the instructions or
code for every characteristic or trait. The structure of the
DNA molecule was first discovered by Francis Crick and
James Watson in 1953.
DNA is made up of repeating units called nucleotides they are
made up of three different components:



A deoxyribose (pentose) sugar, a 5 carbon
monosaccharide.
A phosphate: PO43- molecule
A base which can either be:
adenine (A), thymine (T), guanine (G) or cytosine (C)
A nucleotide is represented in the
diagram above, where each
component is shown by a different
shape.
Long chains of nucleotide (a polynucleotide) form a DNA molecule. It is called a double helix
because it resembles two intertwining spirals. Untwisted, the
molecule is like a ladder.
Structure of DNA:
The ‘rails’ of the ladder are made up of millions of sugar and
phosphate groups arranged alternately.
The ‘rungs’ or ‘steps’ of the ladder are made up of molecules
called bases. Each base is known by its first letter.
A = adenine
T = thymine
C = cytosine
G = guanine
Bases bond together in pairs to form the rungs. A always pairs
with T and C with G (A ---- T and C --- G). This is the base
pairing rule.
The specific order of the bases (base sequence or genetic
code) is what determines an organism’s phenotype.
A DNA molecule is, comparatively, very long, about 2-5 cm (that doesn’t seem big but remember
an average human cheek cell is only 0.05 mm [50 μm] wide).
The total length of DNA present in one adult human is 2.0 ×
1013 (20,000,000,000,000) meters!! That is the equivalent of
nearly 70 trips from the earth to the sun and back.
(Almost) every cell in an organism contains the entire
compliment of DNA required to make the whole organism.
Since each DNA molecule is so long is must very tightly and
densely coiled until it forms a structure called a
chromosome.
Chromosomes
Chromosomes are made up of long, thin strands of DNA, which are coiled tightly.
Chromosomes come in pairs of similar shape and size, called homologous
pairs. In sexually reproducing organisms the homologous pair contains one
chromosome from the mother and the other one comes from the father.
Human cells have 46 chromosomes – 23 pairs. (Other organisms have a different numbers of
chromosomes e.g. a banana has 22 chromosomes – 11 pairs).
A karyotype is a record of the full set of chromosomes from a cell. The chromosomes are
arranged in pairs in order of size.
This karyotype shows a human female.
You will see chromosomes drawn as single structures like the picture on the right or a double
structure (replicated/duplicated chromosomes or ‘X’s) like the picture on the left.
Genes
A specific section of a chromosome (a length of DNA) is called a gene.
A gene is an area of a chromosome that contains the specific base sequence that controls a
particular phenotype/trait/characteristic.
Since chromosomes are in pairs, genes are also in pairs.
The genes for any one characteristic are at identical positions on a particular chromosome for all
members of the species.
A gene consists of hundreds to thousands of DNA bases, arranged in a particular order.
Genetics 101: What are genes
https://www.khanacademy.org/science/biology/heredity-andgenetics/v/genetics-101-part-1--what-are-genes
Alleles
Different versions of a gene are called alleles. For example, there is a gene for pod shape in pea
plants, inflated (full) or constricted (flat) are the possible alleles. All pea plants have the same
section of a chromosome (gene locus) where the gene for pod shape will be found , but…
…at this gene locus the pea plant may have the allele for inflated (full) pod shape (F) or
constricted (flat) pod shape (f).
A sequence of 3 bases in an allele (called a triplet) code for 1 amino acid.
1 amino
acid
A
T
C
T
G
G
C
A
A
T
A
C
www.chemguide.co.uk/organic props/aminoacids
There are 20 different amino acids.
These combine to form long chain molecules called proteins.
The proteins are enzymes (protease, lipase, cytochrome oxidase…) or structural proteins
(keratine, collagen) which determine the phenotypes/traits/ characteristics of an organism such as
leaf shape, hair type, etc.
There are slight differences in the sequence of bases that makes up each allele and this gives a
different sequence of amino acids and therefore a different protein.
Each allele codes for a different form or version of the same characteristic. E.g. for cheek dimples
one child could end up producing a long cheek protein which results in no dimples while another
could produce a shorter version and have dimples.
Since chromosomes come in pairs and genes come in pairs so do alleles.
For most phenotypes/characteristics/traits, 2 alleles are inherited – one from each parent.
Alleles can be dominant or recessive.
Alleles are represented by letters.
The dominant allele is given an upper case/capital letter and the recessive allele is written using
lower case/small letter.
For example, inflated pods in pea plants are dominant to constricted pods.
So F = inflated and f = constricted.
If a person inherits a dominant allele from 1 parent and a recessive allele from the other parent,
only the dominant feature will “show up”. The recessive trait is “hidden”.
More on this later…
Glossary
allele
Amino acid
base
Base-paring rule
base sequence
Characteristic /
Feature / trait /
phenotype
chromosome
deoxyribose
(pentose) sugar
DNA
Gene
karyotype
Triplet
Variation
One of a number of possible versions of a gene. An organism has two alleles
for each characteristic.
A sub-unit of a protein. There are different 20 amino acids.
adenine (A), thymine (T), guanine (G) or cytosine (C)
A always pairs with T, C with G (A ---- T and C --- G).
The specific order of the bases is what determines an organism’s phenotype.
These four words are often used interchangeably to mean appearance or
physical expression of the genotype.
Chromosomes are made up of long, thin strands of DNA, which are coiled
tightly.
a 5 carbon monosaccharide.
Stands for Deoxyribonucleic Acid and is a long chain molecule that carries
the genetic code.
A specific section of a chromosome (a length of DNA) that codes for a
characteristic.
A record of the full set of chromosomes from a cell. The chromosomes are
arranged in pairs in order of size.
A sequence of 3 bases in an allele. This codes for one amino acid.
Differences between organisms of the same species.