Chapter 6 Section 1
Chromosomes
Formation of New Cells by Cell Division
• Cell division, also called cell reproduction,
occurs in humans and other organisms at
different times of their life.
Formation of New Cells by Cell Division
• The formation of gametes
involves yet a special
type of cell division.
Gametes are an
organism’s reproductive
cells, such as sperm and
egg cells.
• When a cell divides, the
DNA is first copied and
then distributed.
Prokaryotic Cell Reproduction
• Prokaryotes reproduce
by a type of cell division
called binary fission.
• Binary fission is a form
of asexual reproduction
that produces identical
offspring.
Prokaryotic Cell Reproduction
• In asexual reproduction, a
single parent passes exact
copies of all its DNA to its
offspring.
• Binary fission occurs in two
stages: first, the DNA is copied
(so that each new cell will
have a copy of the genetic
information), and then the
cell divides.
Prokaryotic Cell Reproduction
• Eventually, the dividing cell is pinched into two
independent cells.
Eukaryotic Cell Reproduction
• A gene is a segment of DNA that codes for a
protein or RNA molecule.
Eukaryotic Cell Reproduction
• When genes are
being used the
DNA is stretched
out, so that the
information it
contains can be
used to direct
the synthesis of
proteins.
Eukaryotic Cell Reproduction
• As a eukaryotic cell prepares to divide, the
DNA and the proteins associated with the DNA
coil into a structure called a chromosome.
Eukaryotic Cell Reproduction
• The two exact
copies of DNA that
make up each
chromosome are
called chromatids.
• The two
chromatids of a
chromosome are
attached at a point
called a
centromere.
Eukaryotic Cell Reproduction
• The chromatids, which
become separated during
cell division and placed
into each new cell,
ensure that each new cell
will have the same
genetic information as
the original cell.
How Chromosomes Number and
Structure Affect Development
• Sets of Chromosomes
– Homologous
chromosomes are
chromosomes that
are similar in size,
shape, and genetic
content.
Sets of Chromosomes
• Each homologue in a
pair of homologous
chromosomes comes
from one of the two
parents.
• The 46 chromosomes
in human somatic cells
(body cells) are actually
two sets of 23
chromosomes.
Sets of Chromosomes
• When a cell such as a somatic cell contains
two sets of chromosomes, it is said to be
diploid.
Sets of Chromosomes
• When a cell, such as a gamete, contains one
set of chromosomes, it is said to be haploid.
Sets of Chromosomes
• The fusion of two haploid gametes – a process
called fertilization – forms a diploid zygote.
• A zygote is a fertilized egg cell.
Sex Chromosome
• Autosomes are chromosomes that are NOT
directly involved in determining the sex of an
individual.
• The sex chromosomes, one of the 23 pair of
chromosomes in humans, contain genes that
will determine the sex of the individual.
Sex Chromosomes
• In humans and
many organisms,
the two sex
chromosomes are
referred to as the
X and Y
chromosomes.
Sex Chromosomes
• Males have an X and a Y chromosome.
• Females have two X chromosomes.
What do these organisms genetically
have in common?
Change in Chromosome Number
• Humans who are
missing even 1 of the
46 chromosomes do
not survive.
• Humans with more
than two copies of a
chromosome, a
condition called
trisomy, will not
develop properly.
Change in Chromosome Number
• Abnormalities in chromosome number can be
detected by analyzing a karyotype, a photo of
the chromosomes in a dividing cell that shows
the chromosomes arranged by size.
Change in Chromosome Number
• Changes in an organism’s chromosome
structure are called mutations.
• Breakage of a chromosome can lead to four
types of mutations:
– Deletion mutation
– Duplication mutation
– Inversion mutation
– Translocation mutation