4.1 Cell Division and Genetic Material
pg. 160
The Cell Theory is a central idea to Biology and it evolved in the
1800’s.
The Cell Theory States:
1. All living things are composed of one or more cells.
2. Cells are the smallest units of living organisms.
3. New cells come only from pre-existing cells by cell
division.
Cells that come from pre-existing cells must have genetic
information similar to the parent cell. Traits found in the parent
cell must be passed onto the daughter cells. These traits are passed
on by genetic material known as Deoxyribonucleic Acid (DNA).
Genetics is the study of heredity and variation of living organisms
and how genetic information is passed from one generation to the
next.
The Cell Cycle
Somatic Cell is a plant or animal cell that forms the body of the
organism; excludes reproductive sex cells (sperm & ovum).
The Cell Cycle is a process where the cell grows, performs
functions, and undergoes cell division. At the end of one complete
cycle one cell becomes two. The Cell Cycle has three functions in
multicellular organisms; growth of the organism, repair of tissues
and organs, and the maintenance to replace dying or dead cells.
The length of the cell cycle depends on the type of cell. Liver cells
can take up to a year to complete its cell cycle, red blood cells 120
days, and skin cells are replaced daily.
Stages of the Cell Cycle
It is important for the cell cycle to function properly and produce
normal and healthy cells. The cell has a series of checks and
balances that monitor the cells development. If some should
interfere with the cells development could lead to uncontrollable
growth and rapid cell division, cancer.
Interphase is the stage during which a cell carries out its normal
functions, grows, and makes copies of its genetic material in
preparation for cell division.
Interphase is divided into three phases;
G1 – this is the phase of major cell growth, increase in
volume, and number of organelles.
S – this is the phase where DNA is synthesized or
replicated. Doubling the number of chromosomes found
in the nucleus.
G2 – during this phase the cells continue to growth and
perform cellular functions in preparation for cell
division.
Figure 4.2: Interphase is the stage of growth and intense cell activity. Mitosis and
cytokinesis involve the division of genetic material and cell contents.
Mitosis (four phases of Nuclear division)
Mitosis is the stage during which a cell’s nucleus and genetic
material divide. Mitosis is made up of four phases.
During Mitosis the genetic material (double number of
chromosomes after replication) will be separated equally between
two new forming daughter cells.
The two daughter cells are genetically identical to each other and
the formal parent cell.
Figure 4.5: Chromosomes in prophase are actually pairs of sister chromatids that are
attached at the centromere.
Prophase
Chromosome is a structure in the nucleus that contains DNA.
Sister Chromatid is one of two chromosomes that are genetically
identical and held together at the centromere.
Centromere is the region where two sister chromatids are held
together.
Spindle Fibre is a microtubule structure that facilitates the
movement of chromosomes within a cell.
Centrosome (centriole) is a structure that helps to form the spindle
fibre.
During prophase the chromatin condenses into sister chromatids,
held together at their centromere.
The centrosomes start to migrate to opposite poles (north and
south).
Spindle fibres are formed and appear to radiate out from the
centrosome, creating a spindle apparatus.
The nuclear membrane and nucleolus disappears, releasing sister
chromatids into the cytoplasm.
Metaphase
During metaphase the spindle fibres appear to be attached to the
centromere of each pair of sister chromatids and aligns them up
along an imaginary equator. (Sister chromatids are considered to
be a single chromosomes if they are joined by the centromere).
Anaphase
During anaphase the spindle fibres appear to pull apart the sister
chromatids at their centromere. The individual chromosomes now
migrate towards opposite poles. By the end of anaphase there is a
complete set of chromosomes at each of the poles.
Telophase
During telophase the chromosomes start to lengthen and inter-wind
into chromatin. The spindle fibres disappear and the nuclear
membrane forms around the genetic material, forming a nucleus.
Figure 4.4: These illustrations and light micrographs show what happens during
interphase and mitosis.
Cytokinesis
Cytokinesis begins near the end of mitosis and involves the
division of the cell cytoplasm and pinching in of the cell
membrane creating two new daughter cells.
After mitosis, nuclear division, cytokinesis occurs. Here the
organelles and cytoplasm are divided up into the newly forming
daughter cells.
Once the two daughter cells have formed, they will begin their cell
cycle with interphase.
In animal and plants cells, cytokinesis is different.
Animal cells begin to pinch (cleavage) in at the equator. The
indentation will continue until two cells are formed.
Figure 4.6: Cytokinesis begins with a furrow that pinches the cell and eventually splits
the two cells apart. This transmission electron micrograph shows two identical kidney
cells forming.
Plant cells have a cell wall. Instead of pinching in, the cell wall
forma a cell plate between the two new nuclei. Cell walls form on
either side of the cell plate.
Prokaryotes do not have a nucleus and divide through a process
called Binary Fission.
Learning Check, questions 1 – 6, page 164
The Structure of Genetic Material
Genome is the complete DNA sequence of an organism.
DNA is a molecule of genetic information, consisting of a double
helical strand, with each strand running in opposite directions
(anti-parallel). DNA molecule is made up of repeating sub-units
called nucleotides. There are four different nucleotides. The
building blocks for a nucleotide are one phosphate group, a
deoxyribose sugar, and a nitrogen base (adenine, guanine, thymine
and cytosine). (purines and pyrimidines)
To create a double strand molecule, specific nucleotides can bond
to each other, adenine bonds to thymine and guanine bonds to
cytosine, at hydrogen bond sites. These are called complimentary
base pairs.
A genetic mutation occurs when there are errors in the
complementary base pairings.
DNA exists in the strands of chromatin fibre, the entangled genetic
mass found in the nucleus during interphase.
During mitosis chromatin condenses into distinct chromosomes (1
– 23).
Figure 4.7: DNA is part of chromatin fibre, which condenses to form chromosomes.
Making exact copies of DNA
During interphase, specifically S phase, DNA is replicated. The
double helix unwinds, and unzips, revealing each strand, which
acts as a template for a new strand. Each new DNA molecule
replicated is made up of one original strand bonded to a new strand.
This process is called semi-conservative replication.
Figure 4.8: A new DNA molecule has one original strand.
Chromosomes are Paired
Sex chromosomes are an X or Y chromosome, which determines
the genetic sex of an organism.
Autosome is a chromosome that is not involved in determining the
sex of an organism.
Each cell of an organism has the same type and number of
chromosomes in their nucleus. Different organisms have different
number and type of chromosomes.
Somatic cells in humans have 46 chromosomes (23 pairs, two of
each type) One chromosome is donated by the father and the other
from the mother (paternal and maternal). Chromosome pair
numbered 23 is known as the sex chromosomes, males (♂) have an
X and Y, and females (♀) have a pair of X’s.
Chromosomes numbered 1 through to 22 are known as autosomes.
Chromosomes are paired based on their length, centromere
location and similar characteristics.
Homologous Chromosomes Contain Alleles
Homologous Chromosome is a chromosome that contains the
same sequence of genes as another chromosome.
Gene is apart of a chromosome that governs the expression of a
trait and is passed onto offspring; it has a specific DNA sequence.
Allele is a different form of the same gene.
Karyotype is a photograph of pairs of homologous chromosomes in
a cell.
Figure 4.9: Homologous chromosomes have several characteristics in common.
However, they are not identical to one another. For example, they can carry different
forms of the same gene, called alleles.
Examining Chromosomes: The Karyotype
The chromosomes that a person has, is called the person’s
karyotype. The karyotype of a cell is collect when the cell cycle is
in mitosis, specifically metaphase. The chromosomes are then
sorted into their homologous pairs. Chromosomes 1 – 22 are
autosomes and chromosome 23, sex chromosomes X or Y.
Figure 4.10: This is a human karyotype. The chromosome pairs are arranged and
numbered in order of their length, from longest to shortest. The sex chromosomes are
placed last in a karyotype. Note that the banding patterns between homologous
chromosomes are different in this image because of the type of dye that is used.
Section 4.1 Review, questions 1 – 18, page 168