Transcript Section 6-1 Chromosomes

Section 6-1
Chromosomes
• Cell division is the same as
reproduction of the cell.
• Gametes – an organism’s
reproductive cells
Females – eggs
Males – sperm
• Regardless of the type of cell
division occurring, all information
that is stored in DNA must be
present in the resulting cell.
• DNA stores the information that
tells cells which proteins to
make and when to make them.
• This information directs a cell’s
activities and determines its
characteristics.
• Thus, when a cell divides, the
DNA is first copied then
distributed.
• Each cell ends up with a
complete set or copy of the
DNA.
Bacterial Cells Divide to
Reproduce
• First remember…
Bacterial DNA is

single-stranded
 circular
 attached to the inner cell
membrane (no nuclear
envelope).
Bacterial Cells Divide to
Reproduce
• Bacteria reproduce by a type of cell
division called BINARY FISSION.
• Binary Fission – form of asexual
reproduction that produces
identical offspring
• Asexual Reproduction – a single
parent passes exact copies of all
its DNA to its offspring
Binary Fission Occurs in
Two Stages
1. DNA is copied so each cell has a
copy of the genetic information.
2. Cell divides – bacterium divides by
adding a new cell membrane to a
point on the membrane between
the two DNA copies. As new
material is added, the growing cell
membrane pushes inward and the
cell is constricted in the middle. It
will be pinched into two cells.
Eukaryotic Cells Form
Chromosomes Before Cell
Division
• Gene – segment of DNA that codes
for a protein or RNA molecule. How
DNA is organized.
• Thousands of genes are on a single
DNA molecule.
• Genes play an important role in
determining how a person’s body
develops and functions.
• As eukaryotic cells prepare to
divide, the DNA and proteins
associated with DNA coil into a
structure called a chromosome.
• Before the DNA coils up, the DNA
is copied.
• The two exact copies of DNA that
make up each chromosome –
chromatids
• Centromere – the point that the
two chromatids of a chromosome
are attached
• 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.
Chromosome Number and
Structure Affect Development
• Each human somatic cell contains
23 different chromosomes; two
copies each, totaling 46
chromosomes
• The 23 different chromosomes
differ in size, shape, and genes.
• Each chromosome contains
thousands of genes – genes
determine body development and
functions
Sets of Chromosomes
• Each of the 23 pairs of
chromosomes consists of two
homologous chromosomes or
homologues.
• Homologous chromosomes –
similar in size, shape, and
genetic content.
• Each homologue in a pair of
homologous chromosomes comes
from one of the two parents.
• Thus, 46 chromosomes in human
somatic cells are actually 23 pairs
of chromosomes.
• One set comes from the mother
and the other comes from the
father.
• All cells in the body other than the
gametes are SOMATIC CELLS.
• SOMATIC CELLS – contain 2 sets of
chromosomes (46 chromosomes or
23 pairs). These cells are DIPLOID.
Diploid cells are 2n. So… In
humans, 2n = 46.
• GAMETES – contain 1 set of
chromosomes (23 chromosomes).
These cells are HAPLOID. Haploid
cells are n. So… In humans, n = 23.
• Fusion of 2 haploid gametes
(fertilization) forms a diploid zygote.
• Zygote – a fertilized egg cell, or the
first cell of a new individual.
• Each organism has a
characteristic number of
chromosomes. It stays
constant in each species.
Chromosomes
Determine Your Sex
• Of the 23 pairs of chromosomes in
somatic cells, 22 pair are
autosomes and I pair are sex
chromosomes.
• AUTOSOMES – chromosomes not
directly involved in determining the
sex or gender of an individual.
• SEX CHROMOSOMES – contains
genes that will determine the sex of
an individual.
• Often sex chromosomes are referred to
as X (female) and Y (male).
• Genes that cause a fertilized egg to
develop into a male are on the Y
chromosome.
• XX – female; XY – male
• The mother always gives an X
chromosome. The father will give either
an X or Y chromosome, so he determines
the offspring’s gender.
• Some species have no Y chromosome, so
the males are characterized by XO.
Change in Chromosome
Number
• The presence of all 46
chromosomes essential for
normal developmental function.
• Humans missing one of the 46
chromosomes do not survive.
• Humans with more than two
copies of a chromosome will
not develop properly
(TRISOMY).
• KARYOTYPE – a photo of the
chromosomes in a dividing cell that
shows the chromosomes arranged
by size. It will detect abnormalities
in chromosome number.
Down Syndrome
• Down Syndrome occurs when
there is an extra copy of
chromosme 21. This is also
known as Trisomy 21.
• People with this have short
stature, a round face with upper
eyelids that cover the inner
corners of the eyes, and varying
degrees of mental retardation.
Down Syndrome
Karyotype
• What can cause an individual to
have an extra copy of a
chromosome?
• When a sperm and an egg cell form,
each chromosome and its
homologue separate –
DISJUNCTION. If one or more
chromosomes fail to separate
properly (NONDISJUNCTION), one
new gamete ends up receiving both
chromosomes, and the other one
does not get one. If the gamete
with both copies undergoes
fertilization, trisomy results.
Change in Chromosome
Structure
• Mutations – changes in
chromosome structure.
• Breakage of a chromosome can
lead to one of four types of
mutations.
Four Types of Mutations
1. Deletion – piece of chromosome breaks
off completely.
2. Duplication – Chromosome fragment
attaches to its homologous
chromosome, which will then carry 2
copies of a certain set of genes
3. Inversion – chromosome piece
reattaches to the original chromosome
but in a reverse orientation.
4. Translocation – chromosome piece
reattaches to a nonhomologous
chromosome.
HOMEWORK
SECTION 6-1 Review Questions
#1-#6