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Chromosomes
Inheriting Traits
We inherit many of our physical
characteristics or traits from our parents.
This is known as heredity – the passing of
traits from one generation to the next.
In addition to our physical characteristics
we also inherit many other traits that may
not be as evident, but may be of particular
importance.
In scientific terminology, a trait is a
particular characteristic or feature of an
organism.
Why are traits inherited
Chromosomes contain the hereditary
(genetic) information in living cells.
All living cells and viruses contain genetic
information in chromosomes.
Each unique sequence of DNA (gene)
carries a particular instruction for a cell.
Genes vary in size from about 100 to
2.5million base pairs. The length of the
sequence of DNA and the precise order of
the base pairs in a gene are the critical
factors that determine what the gene
product (usually a protein) will be like and
what it will do in a cell.
Eukaryotes
Prokaryotes
Viruses
Location of
Chromosomes
In the nucleus
In the cytoplasm
Core
Structure of
Chromosomes
Double stranded
molecules of
DNA, with
attached protein
molecules.
Single, circular
chromosome
composed of
DNA, with very
few or no
attached proteins.
Contain either
DNA or RNA (not
both), with no
attached proteins
(naked nucleic
acids).
Number of
Chromosomes
Varies from
species to
species. Humans
have 46.
One.
One to many.
Reproduction
Nuclear DNA is
replicated prior to
cell division and
the chromosomes
distributed evenly
to daughter cells.
Single circular
chromosome is
replicated prior to
cell division.
Each daughter
cell receives one
copy of this
chromosome.
Reproduce only in
host cells. They
force host cells to
make copies of
their DNA or RNA.
Extrachromosomal
DNA
Mitochondria
contain DNA.
Small circular
DNA molecules
known as
plasmids.
Not applicable.
Eukaryotic Chromosomes
In eukaryotic cells, dark-staining
thread-like structures called
chromosomes are located in the
nucleus.
Chromosomes consist of DNA and
therefore carry genes.
Chromosomes constantly change
their appearance during the life of a
cell and come in various shapes and
sizes.
Packaging DNA into Chromosomes
In humans the average DNA molecule is about 6.5x107 base
pairs in length.
The nucleus of a human cells is just 6mm in diameter, yet it
contains 1.8m of DNA.
This can only be achieved because DNA in eukaryotes is
tightly packaged into chromosomes.
DNA is coiled around small proteins (histones).
Where the DNA is wrapped around a core of histone proteins
it forms a particle about 10nm in diameter called a
nucleosome.
The nucleosomes give the DNA strand the appearance of a
string of beads, and this arrangement of DNA wrapped
around histones serves to package the DNA efficiently and
protected from enzymatic degradation.
When a eukaryotic cell is preparing to divide, chromosomes
become very condensed and are visible under a light
microscope.
Packaging of DNA into
Chromosomes
Genes and Chromosomes
Each DNA molecule contains many genes.
Each gene has a particular position, called a
locus, on a specific chromosome.
Different forms of the same gene found in
members of the same species are called alleles
and will have the same locus on the same
chromosome. Alleles are the basis of heritable
variation.
The genes on each DNA molecule are separated
by regions called spacer DNA.
Spacer regions include DNA that does not encode
a protein product, and may function in spacing
genes apart so that enzymes or other molecules
can interact easily with them.
Chromosomes
Chromosomes in dividing cells consist of two
chromatids. Each chromatid contains an identical
copy of the DNA molecule.
Chromosomes in non-dividing cells chromosomes
exist as a single-armed structures (each is the
equivalent of one of the chromatids in a
metaphase chromosome prepared for cell
division).
Homologous chromosomes
The same genes are found at the same locations (loci) on the two
chromosomes in a pair. This is why they are referred to as homologous
chromosomes or homologues.
The two chromatids of a chromosome in a somatic cell contain identical
DNA molecules. While there can only be a maximum of two alleles (one
from each parent), there may be two copies of each.
Karyotypes
The karyotype of an organism is the
full set of chromosomes of a cell or
individual.
Karyotyping involves formally
arranging the chromosomes in
relation to their length and the
position of the centromere.
Karyotyping allows us to identify
changes in chromosome number and
structure. Changes of this nature
may be associated with genetic
abnormalities.
Classifying chromosomes
Chromosomes are identified by length, banding and position of the
centromere.
Banding occurs when eukaryotic chromosomes are stained with a
special dye during metaphase to create a characteristic pattern of
light and dark bands called G bands.
The bands reflect the regional differences in the amounts of A and T
versus G and C.
Chromosomes that the centromere centrally positioned, giving arms
of equal length, are metacentric.
Submetacentric chromosomes have the centromere towards one
end, resulting in arms of unequal length.
When the centromere is very close to one end, the chromosome is
acrocentric.
Human chromosomes have been assigned to seven groups (A-G)
according to their length and the position of the centromere.
The groups of autosomes in
humans
Group
Autosome
Number
Position of
centromere
Autosome
length
A
1-5
metacentric
long
B
4-5
submetacentric
long
C
6-12
metacentric,
submetacentric
medium
D
13-15
acrocentric
medium
E
16-18
metacentric
short
F
19-20
metacentric
shorter
G
21-22
acrocentric
shortest
Human Karyotype
In humans (and some other organisms),
sex chromosomes are distinguished from
the remaining chromosomes (autosomes).
Human somatic cells have a diploid
chromosome number of 2N=46 consisting
of 22 pairs of autosomes and one set of
sex chromosomes.
Human gametes have only one set of
chromosomes and are referred to as being
haploid.
Human female karyotype
When things go wrong…
Extra X Chromsome – Klinefelter Syndrome
Male, sterile, often some mental retardation, with female
secondary sex traits (e.g. breast enlargement)
Missing X Chromosome – Turner Syndrome
Female, short stature, no sexual maturation, webbing of skin
between neck and shoulder
Three copies of chromosome 13 – Patau Syndrome
Male or female, small skull, mentally retarded, harelip, cleft
palate, usually have heart defects, seldom survive more than
four months after birth
Three copies of chromosome 21 – Down Syndrome
Male or female, some mental retardation, characteristic palm
prints and facial features, usually sterile.