CHROMOSOMES ARE MATCHED IN
In humans, somatic cells (body cells) have:
• 23 pairs of homologous chromosomes and
• one member of each pair from each parent.
The human sex chromosomes (Gonosomes)
X and Y differ in size and genetic composition.
The other 22 pairs of chromosomes are autosomes with the same size
and genetic composition.
HOMOLOGOUS CHROMOSOMES ARE
• centromere position, and
• gene locations (locus).
A locus (plural, loci) is the position of a gene.
Different versions (alleles) of a gene may be found at the
same locus on maternal and paternal chromosomes.
GAMETES HAVE A SINGLE SET OF
• All organisms have different
numbers of chromosomes.
• A body cell in an adult fruit fly has
8 chromosomes: 4 from the fruit
fly's male parent, and 4 from its
•These two sets of chromosomes are
•Each of the 4 chromosomes that came
from the male parent has a
corresponding chromosome from the
• A cell that contains both sets of
homologous chromosomes is said to be
• The number of chromosomes in a diploid
cell is sometimes represented by the
• For Drosophila, the diploid number is 8,
which can be written as 2N=8.
• Di= two sets
• Cell that contains both sets of homologus chromosomes
• Cell contains
• 2 complete sets of chromosome
• 2 complete sets of genes
• Number of chrms in diploid cell represented by 2N
• For Drosophilia (fruit fly) 2N=8
• Mendel said:
• Each adult cell contains two copies of each gene
• The gametes of sexually reproducing
organisms contain only a single set of
chromosomes, and therefore only a single
set of genes.
• These cells are haploid. Haploid cells are
represented by the symbol N.
• For Drosophila, the haploid number is 4,
which can be written as N=4.
• Means “one set”
• Refers to cells that contain only one set of chromosomes
• Gametes (sex cells)
• Represented by N
• Drosophilia fruit fly
WHAT IS MEIOSIS?
• Process of reduction division in which the number of chromosomes per cell
is cut in half through the separation of homologous chromosomes in a
WHAT HAPENS DURING MEIOSIS?
• Each organism must inherit a single copy of every gene from each of its
• Gametes are formed by a process that separates the two sets of genes so that
each gamete ends up with just one set.
MEIOSIS HAS TWO STAGES
MEIOSIS I consisting of 5 phases:
Interphase I, Prophase I, Metaphase I, Anaphase I,
MEIOSIS II consisting of 4 phases
Prophase II, Metaphase II, Anaphase II, Telophase
Interphase I Prophase I Metaphase I Anaphase I Telophase I
Cell build up energy
DNA Replication (to
Cell doesn’t change
• Each chromosome pairs with its
chromosome to form a tetrad.
• There are 4 chromatids in a tetrad.
• Non-sister chromatids exchange
genetic material through the process
of crossing over to ensure genetic
• Centrioli move to opposite poles with
spindle fibres between them.
• When homologous
chromosomes form tetrads in
meiosis I, they exchange
portions of their chromatids
in a process called crossing
• Crossing-over produces new
combinations of alleles.
Centrioli has reached the poles.
Homologous pairs align at the
The two chromosomes attach to
one spindle fiber by means of
the kinetochore of the
• The spindles pull homologous
chromosomes apart to opposite
TELOPHASE I AND CYTOKINESIS
• Duplicated chromosomes have
reached the poles.
• A nuclear envelope and nucleolus re-
forms around chromosomes.
• Each nucleus now has the haploid
number of chromosomes.
• Cell invaginates forming a cleavage
furrow, which extends to for 2 separate
• The two cells produced by meiosis I now enter a second
• Unlike meiosis I, neither cell goes through chromosome
• Each of the cell’s chromosomes has 2 chromatids.
Telophase I and
Cytokinesis I Prophase II Metaphase II Anaphase II Telophase II
• Chromosomes coil and
become compact (if uncoiled
after telophase I).
• Nuclear envelope and
nucleolus, if re-formed,
• Centrioli move to opposite
poles, forming spindle fibers
• Individual duplicated
chromosomes align on the
• One chromosome per
spindle fiber attached by
means of kinetochore of
• Centrioli has reached the
• Spindle fibers contract.
• Duplicated chromosomes
split in half (centromere
dividing in 2)
chromosomes move to
TELOPHASE II AND CYTOKINESIS
• Daughter chromosomes has reached
• Two cells invaginate and form 4
daughter haploid cells (gametes)
• They uncoil and form chromatin.
• Nuclear envelope and nucleolus
for around chromatin again.
• Centrioli for centrosome.
DIFFERENCES BETWEEN MITOSIS ANS
• Cells produced by mitosis have the
same number of chromosomes and
alleles as the original cell.
• Mitosis allows an organism to grow
and replace cells.
• Some organisms
reproduce asexually by mitosis.
• Cells produced by meiosis have half
the number of chromosomes as the
• These cells are genetically different
from the diploid cell and from each
• Meiosis is how sexually-reproducing
organisms produce gametes.
ALTERATION IN CHROMOSOME
An extra copy of chromosome 21 causes Down
syndrome or also known as TRISOMY 21.
A. Trisomy 21
• involves the inheritance of three copies of
chromosome 21 and
• is the most common human chromosome abnormality.
Trisomy 21 produces a characteristic set of symptoms,
• mental retardation,
• characteristic facial features,
• short stature,
• heart defects,
• susceptibility to respiratory infections, leukemia, and
Alzheimer’s disease, and
• shortened life span.
The incidence increases with the age of the mother.
Accidents during meiosis can alter
Nondisjunction is the failure of chromosomes or chromatids to
separate normally during meiosis. This can happen during:
• meiosis I, if both members of a homologous pair go to one
• meiosis II if both sister chromatids go to one pole.
Fertilization after nondisjunction yields zygotes with altered
numbers of chromosomes.
Alterations of chromosome structure
Chromosome breakage can lead to
rearrangements that can produce:
• genetic disorders or,
• if changes occur in somatic cells, cancer.
These rearrangements may include:
• a deletion, the loss of a chromosome segment,
• a duplication, the repeat of a chromosome segment,
• an inversion, the reversal of a chromosome segment, or
• a translocation, the attachment of a segment to a non-
homologous chromosome that can be reciprocal.