Cell Division
Cell Division Types
• Prokaryotes
– Binary fission
• Eukaryotes
– Mitosis:
- Growth, development & repair
- Asexual reproduction (yields genetically identical cells)
- Occurs in somatic (body) cells
– Meiosis:
- Sexual reproduction (yields genetically different cells with half the # of
chromosomes)
- Occurs in specific reproductive cells
- Yields ______________ (e.g., eggs & sperm) or spores
Mitotic cell division results in genetically identical daughter cells
• Cells duplicate their genetic material before they divide, ensuring that each daughter cell
receives an exact copy of the genetic material, DNA
• A dividing cell duplicates its DNA, allocates the two copies to opposite ends of the cell, and only
then splits into daughter cells
Cellular Organization of the Genetic Material
• A cell’s endowment of DNA (its genetic information) is called its genome
• DNA molecules* in a cell are packaged into chromosomes
• *Prokaryotes- circular DNA, Eukaryotes- _____________
DNA
• Every eukaryotic species has a characteristic number of
chromosomes in each cell nucleus
• _________________ (non-reproductive) cells (normally) have
two sets of chromosomes
• Gametes (reproductive cells: sperm and eggs) (and spores) have
half as many chromosomes as somatic cells
• Eukaryotic chromosomes consist of _________________, a
complex of DNA and protein that condenses during cell division
• DNA plus proteins is called chromatin.
• One half of a duplicated chromosome is a __________________.
• Sister chromatids are held together at the centromere.
• Telomeres protect DNA and do not include genes.
• DNA associates with special proteins to form more stable
structure called chromosomes (different proteins in prokaryotes
and eukaryotes, so chromosomes built different)
• Chromosomes are found inside nucleus in eukaryotes
• Human - ____ chromosomes, ____ pairs (1 set of 23 from egg, 1 set of 23 from sperm)
• Each chromosome contains many genes
• Gene is a segment of DNA that is responsible for controlling a trait (e.g., coding for a specific
protein)
Cell Cycle
The cell cycle is a repeated pattern of growth and
division that occurs in eukaryotic cells.
This cycle consists of the phases: _________________
• The main stages of the cell cycle are gap 1,
synthesis, gap 2, and mitosis.
• Gap 1 (G1): cell growth and normal functions
• DNA synthesis (S): ____________ DNA
• Gap 2 (G2): additional growth
• Mitosis (M): includes division of the cell
nucleus (mitosis) and division of the cell
cytoplasm (cytokinesis)
Distribution of Chromosomes During Cell Division
• In preparation for cell division, DNA is
replicated and the chromosomes condense
• Each duplicated chromosome has two sister
chromatids, which separate during cell division
• The centromere is the narrow “waist” of the
duplicated chromosome, where the two
chromatids are most closely attached
Interphase
• Cells spend the majority of their cell cycle in
interphase.
• The purpose of interphase is for cell _______________.
• By the end of interphase a cell has two full sets of DNA (chromosomes) and is large enough to
begin the division process.
• Interphase is divided into three phases. Each phase is
characterized by specific processes involving different
structures.
• During the G1 (gap 1) phase, the cell grows and
synthesizes proteins.
• During the S (synthesis) phase, chromosomes
replicate and divide to form identical sister
chromatids held together by a centromere.
• During the G2 (gap 2) phase, cells continue to grow
and produce the proteins necessary for cell
division.
Mitosis
• The purpose of mitosis is cell division: making two cells out
of one.
•
Each cell has to have its _______ cytoplasm and DNA.
Mitosis is divided into four phases. Each phase is characterized by specific processes involving different
structures.
1. ___________________
2. Metaphase
3. Anaphase
4. Telophase
Chromosomes condense at the start of mitosis. DNA wraps around proteins (histones) that condense it.
Prophase is characterized by four events:
1. Chromosomes condense and are more visible.
2. The nuclear membrane (envelope) disappears.
3. Centrioles have separated and taken positions on the opposite poles of the cell.
4. Spindle fibers form and radiate toward the center of the cell.
Metaphase (the shortest phase of mitosis) is characterized by two events:
1. Chromosomes line up across the ______________ of the cell at the metaphase plate
2. Spindle fibers connect the centromere of each sister chromatid to the poles of the cell.
Anaphase is characterized by three events:
1. Centromeres that join the sister chromatids ________________.
2. Sister chromatids separate becoming individual chromosomes.
3. Separated chromatids move to opposite poles of the cell.
Telophase (the last phase of mitosis) consists of four events:
1. Chromosomes (each consisting of a single chromatid) uncoil.
2. A nuclear envelope forms around the chromosomes at each pole of the cell.
3. Spindle fibers break down and dissolve.
4. Cytokinesis begins.
Cytokinesis
• Cytokinesis is the actual ______________ of the cytoplasm into two individual cells.
• The process of cytokinesis differs somewhat in plant and animal cells.
• In animal cells the cell membrane forms a cleavage furrow that eventually ___________ the cell
into two nearly equal parts, each part containing its own nucleus and cytoplasmic organelles.
• In plant cells a structure known as a cell ____________ forms midway between the divided
nuclei, which gradually develops into a separating membrane.
• The cell wall forms in the cell plate.
• Cells divide at different rates. The rate of
cell division varies with the need for those
types of cells.
• Some cells are unlikely to divide (G0).
• Cell size is limited. Volume increases
faster than surface area.
• Surface area must allow for adequate
exchange of materials.
• Cell growth is coordinated with division.
• Cells that must be large have unique shapes.
The cell cycle is regulated by a molecular control system
• The frequency of cell division varies with the type of cell
• These cell cycle differences result from regulation at the molecular level
• The cell cycle appears to be driven by specific chemical signals present in the cytoplasm
• The levels of these chemical signals are influenced by biotic & abiotic factors
• The sequential events of the cell cycle are directed by a distinct cell cycle control system, which
is similar to a clock
• The clock has specific _____________________ where the cell cycle stops until a go-ahead
signal is received
• For many cells, the G1 checkpoint seems to be the most important one
• An external signal is _______________-dependent inhibition, in which crowded cells stop
dividing
• Most animal cells also exhibit anchorage dependence, in which they must be attached to a
substrate in order to divide
Loss of Cell Cycle Controls in ___________________ Cells
• Cancer cells do not respond normally to the body’s control mechanisms
• Cancer cells form tumors, masses of abnormal cells within otherwise normal tissue
• If abnormal cells remain at the original site, the lump is called a benign tumor
• Malignant tumors invade surrounding tissues and can metastasize, exporting cancer cells to
other parts of the body, where they may form secondary tumors
Meiosis
 The form of cell division by which GAMETES, with HALF the number of CHROMOSOMES, are
produced
 DIPLOID (2n)  ________________ (n)
 Meiosis is SEXUAL reproduction
 TWO divisions (MEIOSIS I and MEIOSIS II)
 Sex cells divide to produce GAMETES (sperm or egg)
 Gametes have HALF the # of chromosomes
 Occurs only in GONADS (testes or ovaries)
 Male: SPERMATOGENESIS -sperm
 Female: OOGENESIS - egg or ova
Interphase I
 Similar to mitosis interphase.
 CHROMOSOMES (DNA) replicate in the S phase
 Each duplicated chromosome consist of two identical SISTER CHROMATIDS attached at their
CENTROMERES.
 CENTRIOLE pairs also replicate.
Meiosis I
 Cell division that reduces the chromosome number by __________.
 Four phases:
 Prophase I
 Metaphase I
 Anaphase I

Telophase I
Prophase I
 Longest and most complex phase (90%).
 Chromosomes condense.




______________ occurs - Homologous chromosomes come together
to form a tetrad.
Tetrad is two
chromosomes
or four chromatids (sister
and non-sister
chromatids)
Non-Sister ChromatidsHOMOLOGS
o Homologs contain
DNA that codes
for the same
genes , but
different
_______________ of those genes
o Genes occur at the same loci
Homologous Chromosomes
o Pair of chromosomes (maternal and paternal) that are similar in shape and size.
o Homologous pairs (tetrads) carry GENES controlling the SAME inherited traits.
o Each locus (position of a gene) is in the same position on homologues.
o Humans have 23 pairs of homologous chromosomes:
 First 22 pairs of autosomes
 Last pair of sex chromosomes

Crossing Over
o Crossing over may occur between non-sister chromatids at sites called chiasmata.
o Crossing over: segments of nonsister chromatids break and reattach to the other
chromatid.
o Chiasmata (chiasma) are where chromosomes touch each other and exchange genes
(crossing over.)
o Causes Genetic ________________________
Meiosis I – Homologs separate
Prophase I
 Nucleus & Nucleolus disappear
 Spindle forms
 Chromosomes coil & Synapsis (pairing) occurs
 Tetrads form & ______________ occurs
Metaphase I
 Shortest phase
 Tetrads align on the equator.
 Independent assortment occurs – chromosomes separate __________________ causing
GENETIC RECOMBINATION
 Formula: 2n
 Example:
2n = 4
 then 1n = 2
 thus 22 = 4 combinations
Anaphase I
 Homologous chromosomes ________________ and move towards the poles.
 Sister chromatids remain attached at their centromeres.
Telophase I
 Each pole now has _________________ (1n) set of chromosomes.
 Cytokinesis occurs and two haploid daughter cells are formed.
Meiosis II – sister chromatids separate
 No Interphase II
 No DNA Replication
 Meiosis II is similar to mitosis
Prophase II
 Same as Prophase in mitosis
 Nucleus & nucleolus disappear
 Chromosomes condense
 Spindle forms
Metaphase II
 Same as Metaphase in mitosis
 Chromosomes (not homologs) line up at metaphase plate
Anaphase II
 Same as Anaphase in mitosis
 SISTER CHROMATIDS separate
Telophase II- Same as Telophase in mitosis.
 Nuclei and Nucleoli reform, spindle disappears
 CYTOKINESIS occurs.
 Remember: _________________ HAPLOID DAUGHTER cells are produced.
 Called GAMETES (eggs and sperm)
Variation
 Also known as GENETIC RECOMBINATION
 Important to population as the raw material for
__________________________________________________.
 All organisms are NOT alike
 Strongest “most fit” survive to reproduce & pass on traits
Genetic Variation from…
 CROSSING OVER (prophase I)
 INDEPENDENT ASSORTMENT (metaphase I)
 _________________ FERTILIZATION