The Chromosome structure French bean(Phaseolus vulgaris)14

The condensed chromosome may be as Onion(Allium cepa)16

short as ¼ micron (in fungi and birds) or Cabbage(Brassica oleracea)18
as long as 30 microns in Trillium sp. Coffee(Coffea Arabica)44

Each chromosome includes a single, linear Chromosome Structure/Parts

molecule of DNA with its own set of genes. 1.Centromere or Primary Constriction
Chromosomes are numbered according to their - acts as the point of association between
size, and genes are arranged along them in a the
specific order. So a gene near the tip of the short chromosome and the spindle.
arm of chromosome 4 in one person will be found
in the same location in everyone. This means that - is a permanent well-defined region of the
each of our genes can be mapped to a specific chromosome where the kinetochore
place on a specific chromosome. proteins attach to

- The spindle fibers bind the kinetochore

and the depolymerization of the spindle
fibers enable the chromosome to move to
the opposite poles.
The position of the centromere along the
length the of the chromosome contributes to
the shape of the chromosome during cell
division:

● Metacentric- when the centromere is

located at the middle,

Humans have 23 pairs of chromosomes. The ● Submetacentric- when the centromere is

first 22 pairs are called autosomes. The last pair close to the middle,
are called sex chromosomes, and they are
different between males and females. Most ● Acrocentric- when the centromere is
females have two X chromosomes (XX), and located near one end;
most males have an X and a Y (XY).
● Telocentric- when the centromere is at the
DNA is covered with proteins called histones. end of the chromosome.
DNA wraps around the histones the way sewing
thread wraps around a spool. By wrapping DNA
more or less tightly, histones control which genes
are active (being read to build proteins) and which
genes are not.

When a cell gets ready to divide, additional

histones attach to DNA, coiling it even tighte

Common name Specific name Chromosomal

number (2n)

Fruity fly (Drosophila )8 2.Secondary Constriction

Frog (Rana pipiens) 20
Gorilla(Gorilla gorilla)48 - More than one constriction may be
Monkey(Macaca mulatta)42 observed in some chromosomes and
Man(Homo sapiens)46 these are referred to as secondary
Garden pea(Pisum sativum)14 constrictions.
 process is essential for growth and repair in the
- The pinching off of a small chromosomal body.
section forms the satellite. On the other hand, meiosis occurs in germ cells
3.Nucleolus-Organizing region and produces four non-identical gametes with a
haploid (n) number of chromosomes. These
- The organization of the nucleolus is the gametes, such as sperm and egg cells, are
function of a specific point on a particular crucial for sexual reproduction and genetic
chromosome. diversity.

- When a nucleolus is visible, it can be seen Meiosis, on the other hand, is used for just one
to be attached to this nucleolus-organizing purpose in the human body: the production of
region. gametes—sex cells, or sperm and eggs. Its goal
is to make daughter cells with exactly half as
- The chromosome where this region is many chromosomes as the starting cell.
located is known as the nucleolus To put that another way, meiosis in humans is a
organizer division process that takes us from a diploid
cell—one with two sets of chromosomes—to
haploid cells—ones with a single set of
chromosomes. In humans, the haploid cells made
in meiosis are sperm and eggs. When a sperm
and an egg join in fertilization, the two haploid
sets of chromosomes form a complete diploid set:
a new genome.

Phases
In many ways, meiosis is a lot like mitosis. The
cell goes through similar stages and uses similar
strategies to organize and separate
chromosomes. In meiosis, however, the cell has a
4.Chromomeres and Knobs
more complex task. It still needs to separate
sister chromatids (the two halves of a duplicated
- When a mitotic chromosome is stretched
chromosome), as in mitosis. But it must also
out it would be observed to consist of a
separate homologous chromosomes, the similar
string of characteristic particles of unequal
but nonidentical chromosome pairs an organism
sizes at unequal distances apart.
receives from its two parents.
These goals are accomplished in meiosis using a
- The smaller “beads on the string” are
two-step division process. Homologous pairs
called chromomeres; the larger ones are
separate during a first round of cell division,
called knobs.
called meiosis I. Sister chromatids separate
during a second round, called meiosis II.
Cell Division
Since cell division occurs twice during meiosis,
one starting cell can produce four gametes (eggs
or sperm). In each round of division, cells go
through four stages: prophase, metaphase,
anaphase, and telophase.

Prophase I
During prophase I, differences from mitosis begin
Mitosis and meiosis are two different types of
to appear. As in mitosis, the chromosomes begin
cell division. Mitosis occurs in somatic cells and
to condense, but in meiosis I, they also pair up.
results in two identical daughter cells with a
Each chromosome carefully aligns with its
diploid (2n) number of chromosomes. This
homologue partner so that the two match up at
corresponding positions along their full length.
 Finally, in telophase I, the chromosomes arrive at
opposite poles of the cell. In some organisms, the
nuclear membrane re-forms and the
chromosomes decondense, although in others,
this step is skipped—since cells will soon go
through another round of division, meiosis II (2,3)
Cytokinesis usually occurs at the same time as
This process, in which homologous chromosomes
telophase I, forming two haploid daughter cells
trade parts, is called crossing over.
You can see crossovers under a microscope as
Meiosis II
chiasmata, cross-shaped structures where
Cells move from meiosis I to meiosis II
homologues are linked together. Chiasmata keep
without copying their DNA. Meiosis II is a shorter
the homologues connected to each other after the
and simpler process than meiosis I, and you may
synaptonemal complex breaks down, so each
find it helpful to think of meiosis II as “mitosis for
homologous pair needs at least one.
haploid cells."
The cells that enter meiosis II are the ones made
After crossing over, the spindle begins to
in meiosis I. These cells are haploid—have just
capture chromosomes and move them towards
one chromosome from each homologue pair—but
the center of the cell (metaphase plate). This may
their chromosomes still consist of two sister
seem familiar from mitosis, but there is a twist.
chromatids. In meiosis II, the sister chromatids
Each chromosome attaches to microtubules from
separate, making haploid cells with
just one pole of the spindle, and the two
non-duplicated chromosomes.
homologues of a pair bind to microtubules from
opposite poles. So, during metaphase I,
homologue pairs—not individual
chromosomes—line up at the metaphase plate for
separation.

When the homologous pairs line up at the

metaphase plate, the orientation of each pair is
random. For instance, in the diagram above, the
pink version of the big chromosome and the
purple version of the little chromosome happen to
be positioned towards the same pole and go into
the same cell. But the orientation could have During prophase II, chromosomes condense and
equally well been flipped, so that both purple the nuclear envelope breaks down, if needed.
chromosomes went into the cell together. The centrosomes move apart, the spindle forms
between them, and the spindle microtubules
In anaphase I, the homologues are pulled apart begin to capture chromosomes.
and move apart to opposite ends of the cell. The
sister chromatids of each chromosome, however, The two sister chromatids of each chromosome
remain attached to one another and don't come are captured by microtubules from opposite
apart. spindle poles. In metaphase II, the chromosomes
line up individually along the metaphase plate. In
anaphase II, the sister chromatids separate and
are pulled towards opposite poles of the cell.

In telophase II, nuclear membranes form around

each set of chromosomes, and the chromosomes
decondense. Cytokinesis splits the chromosome
sets into new cells, forming the final products of
meiosis: four haploid cells in which each
chromosome has just one chromatid. In humans,
the products of meiosis are sperm or egg cells.

Highlights in Meiosis I
- Synapsis and crossing over
● Homologous chromosomes
physically connect and exchange
genetic information
- Tetrads on the metaphase plate
● At metaphase I of meiosis, paired
homologous chromosomes
(tetrads) are positioned on the
metaphase plates
- Separation of homologues
● At anaphase I of meiosis,
homologous pairs move toward In animals
opposite poles of the cell - Meiosis occurs during gamete formation
- Gametes are the only haploid cells

Plants and some algae

- Exhibit an alternation of generations
 - The life cycle includes both diploid and
haploid multicellular stages
In most fungi and some protists
- Meiosis produces haploid cells that give
rise to a haploid multicellular adult
organism
- The haploid adult carries out mitosis,
producing cells that will become gametes

Terms
Mutations
Are the original source of genetic variation
Sexual reproduction
Produces new combinations of variant genes,
adding more genetic diversity
Genetic variation
Is the raw material for evolution by natural
selection.
Genetic variation produced in sexual life cycles
contributes to evolution
Reshuffling of genetic material in meiosis
Produces genetic variation

A cell’s endowment of DNA, its genetic

information is called its genome

The DNA molecules in a cell

Are packaged into chromosomes
Mitosis
 MENDEL’S POSTULATES OR PRINCIPLES OF
INHERITANCE
Unit Factors in Pairs
● Genetic characters are controlled by unit
factors that exist in pairs in individual
organisms.

Dominance/Recessiveness

● When two unlike unit factors responsible

for a single character are present in a
single individual, one unit factor is
dominant to the other, which is said to be
recessive.
Segregation

● During the formation of gametes, the

paired unit factors separate or segregate
randomly so that each gamete receives
one or the other with equal likelihood.

Independent Assortment

● During gamete formation, segregating

pairs of unit factors assort independently
of each other.
Mendelian Genetics

BRILLIANT INSIGHTS AND METHODS OF

GREGOR MENDEL
● The choice of his experimental organism
(Pisum sativum)
● He followed seven visible (discrete)
features represented by two contrasting
forms, or trait.
Punnett Squares- named after Reginald C.
● He restricted his examination to one or Punnett
very few traits in each experiment
-Uses a checkerboard or table format to
● He kept accurate quantitative records show the genotypes of possible gametes from
each parent. The vertical columns represent
● From the analysis of his data, he derived those of the female parent, and the horizontal
the principles of inheritance rows represent those of the male parent.

- By filling out the Punnett square, we are

listing all possible random fertilization events.
Probability

- expresses the likelihood of the occurrence

of a particular event.
THE ADDITION RULE
● States that the probability of any one of
- It is the number of times that a particular
two or more mutually exclusive events is
event takes place, divided by the number
calculated by adding the probabilities of
of all possible outcomes.
these events

- For example: With a normal coin, the

chance of tossing head is ½ and the
chance of tossing tail is ½. Another
example, the probability of rolling the
number 3 with a die, which is six-sided, is
1/6 while the chance of rolling number
other than 3 is 5/6.

- The probabilities of all possible outcomes

for an event must add up to 1

- The outcome of any particular toss is

unaffected by what has happened on
previous trials

- The phenomena such as successive coin

tosses, or simultaneous tosses of several
coins, are known as independent events.

THE MULTIPLICATION RULE

● States that the probability of two or more
independent events taking place together
is calculated by multiplying their
independent probabilities