STRUCTURE OF
CHROMOSOME
KEVAL Y. RAVAL
INTRODUCTION
• When a cell divides, each of the two new cells gets a full, perfect
copy of genetic material.
• Mistakes during copying, or unequal division of the genetic
material between cells, can lead to cells that are unhealthy or
dysfunctional (and may lead to diseases such as cancer).
But what exactly is this genetic material, and how does it behave
over the course of a cell division?
DNA AND GENOMES
• DNA (deoxyribonucleic acid) is the genetic material of living
organisms.
• In humans, DNA is found in almost all the cells of the body and
provides the instructions.
• When a cell in the body divides, it will pass on a copy of its DNA to
each of its daughter cells.
• DNA is also passed on at the level of organisms, with the DNA in
sperm and egg cells combining to form a new organism that has
genetic material from both its parents.
DNA AND GENOMES
• DNA is a long string of paired chemical units (nucleotides) that come in four
different types, abbreviated A, T, C, and G, and it carries information organized into
units called genes.
• Genes typically provide instructions for making proteins, which give cells and
organisms their functional characteristics.
DNA AND GENOMES
• In eukaryotes such as plants and animals, the majority of DNA is found in the
nucleus and is called nuclear DNA.
• Mitochondria, organelles that harvest energy for the cell, contain their
own mitochondrial DNA, and chloroplasts, organelles that carry out
photosynthesis in plant cells, also have chloroplast DNA.
• The amounts of DNA found in mitochondria and chloroplasts are much smaller
than the amount found in the nucleus.
DNA AND GENOMES
• A cell’s set of DNA is called its genome.
• Since all of the cells in an organism (with a few exceptions) contain the same DNA,
- you can also say that an organism has its own genome,
- and since the members of a species typically have similar genomes, you can also describe
the genome of a species.
• When we refer to the human genome, or any other eukaryotic genome, it means
the set of DNA found in the nucleus.
• Mitochondria and chloroplasts are considered to have their own separate
genomes.
CHROMATIN
• In a cell, DNA does not usually exist by itself, but instead
associates with specialized proteins that organize it and give it
structure.
• In eukaryotes, these proteins include the histones, a group of basic
(positively charged) proteins that form “bobbins” around which
negatively charged DNA can wrap.
• The complex of DNA plus histones and other structural proteins is
called chromatin.
CHROMATIN
CHROMATIN
• For most of the life of the cell, chromatin is decondensed, meaning
that it exists in long, thin strings that look like a loop under the
microscope.
• Condensation takes place when the cell is about to divide.
• When chromatin condenses, you can see that eukaryotic DNA is
not just one long string.
• Instead, it’s broken up into separate, linear pieces
called chromosomes.
• Bacteria also have chromosomes, but their chromosomes are
typically circular.
CHROMOSOMES
• Each species has its own characteristic number of chromosomes.
• Humans, for instance, have 46 chromosomes in a typical body cell
(somatic cell), while dogs have 78.
• Like many species of animals and plants, humans are diploid (2n),
meaning that most of their chromosomes come in matched sets
known as homologous pairs.
• The 46 chromosomes of a human cell are organized into 23 pairs,
and the two members of each pair are said to be homologues of
one another.
CHROMOSOMES
• Human sperm and eggs, which have only one homologous
chromosome from each pair, are said to be haploid (1n).
• When a sperm and egg fuse, their genetic material combines to
form one complete, diploid set of chromosomes.
• So, for each homologous pair of chromosomes in genome, one of
the homologues comes from mother and the other from father.
CHROMOSOMES
• The two chromosomes in a homologous pair are very similar to
one another and have the same size and shape.
• Most importantly, they carry the same type of genetic
information: that is, they have the same genes in the same
locations.
• However, they don't necessarily have the same versions of genes.
• That's because inherited two different gene versions from mother
and father.
CHROMOSOMES
• The sex chromosomes, X and Y, determine a person's biological sex: XX specifies female
and XY specifies male.
• These chromosomes are not true homologues and are an exception to the rule of the
same genes in the same places.
• The 44 non-sex chromosomes in humans are called autosomes.
CHROMOSOMES AND CELL DIVISION
• As a cell prepares to divide, it must make a copy of each of its
chromosomes.
• The two copies of a chromosome are called sister chromatids.
• The sister chromatids are identical to one another and are
attached to each other by proteins called cohesins.
CHROMOSOMES AND CELL DIVISION
• The attachment between sister chromatids is tightest at
the centromere, a region of DNA that is important for their
separation during later stages of cell division.
• As long as the sister chromatids are connected at the centromere,
they are still considered to be one chromosome.
• However, as soon as they are pulled apart during cell division, each
is considered a separate chromosome.
CHROMOSOMES AND CELL DIVISION
CHROMOSOMES AND CELL DIVISION
STRUCTURE OF CHROMOSOMES IN PROKARYOTES
• The term “prokaryote” means “primitive nucleus”. Cell in
prokaryotes have no nucleus.
• The prokaryotic chromosome is dispersed within the cell and is
not enclosed by a separate membrane.
• Prokaryotes are monoploid = they have only one set of genes (one
copy of the genome)
STRUCTURE OF CHROMOSOMES IN PROKARYOTES
• In most viruses and prokaryotes, the single set of genes is stored in
a single chromosome (single molecule either RNA or DNA).
• The DNA is packaged into a region of the cell known as the
nucleoid.
• Basically it is present in circular form.
STRUCTURE OF CHROMOSOMES IN EUKARYOTES
• 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).
STRUCTURE OF CHROMOSOMES IN EUKARYOTES
STRUCTURE OF CHROMOSOMES IN EUKARYOTES
• Chromosomes Found in the nucleus
• It is Condensed and visible during cell division
• At the beginning of mitosis they can be seen to consist of two threads
(sister chromatids) joined by a centromere.
• The sister chromatids are identical copies.
• DNA amount in eukaryotic species is greater than that in bacteria.
THANK YOU!
