Chapter 8 Cell Division
1. Describe the structure of a chromosome.
A. New cells are formed by cell division – about 2 trillion cells are produced by an adult
human body every day
1) Bacterial cells divide by binary fission to reproduce = asexual reproduction that
produces identical offspring – an exact copy of DNA is passed from the parent cell to the
offspring
B. Before cell division cells make an exact copy of their DNA – both new cells need an exact
copy of the DNA - cell division = cell reproduction
1) DNA stores units called genes
a. A gene is a segment of DNA that codes for a protein or an RNA molecule
C. During cell division, the DNA in a eukaryotic cell’s nucleus is coiled into very compact
structures called chromosomes
1) Each chromosome is a single DNA molecule associated with proteins
2) Each half of the chromosome is called a chromatid.
a) The constricted area of each chromatid is called a centromere.
b) The centromere holds the two chromatids together until they separate during cell
division
3) The chromosomes in eukaryotic cells are linear rods with the DNA
wrapped tightly around proteins called histones
a. Histones help maintain the shape of the chromosome and aid in
the tight packing of DNA
b. Proteins called nonhistone proteins do not participate in the packing of DNA
c. Instead, they are involved in controlling the activity of specific regions of the
DNA
4) Humans have 46 chromosomes (23 pairs), Orangutans have 48, dogs have 78 –
consistent within the species
a. Sometimes we get extra copies of chromosomes
(1) Trisomy = three copies of a chromosome – trisomy 21 = Down’s
Syndrome
(2) A Karyotype – a photo of the chromosomes in a dividing cell can determine if there are any chromosomal abnormalities
b. Non-disjunction occurs when chromosomes fail to separate properly
(1) An abnormal gamete fuses with a normal gamete and the
offspring gets three copies of the chromosome
(2) Changes in chromosome structure is a mutation
(a) There are four types of chromosome mutations
i. Deletion – a piece of the chromosome breaks off
and is lost completely
ii. Duplication – a chromosome fragment attaches
to its homologous chromosome and is passed
onto the offspring as trisomy
iii. Inversion – a chromosome fragment breaks off
and reattaches itself in the reverse direction
iv. Translocation – a piece of chromosome breaks
off and reattaches to a non-homologous
chromosome
D. The DNA in prokaryotic cells takes the shape of a single circular chromoneme attached to
the inside of the cell membrane and kept in the nucleoid an area in the cytosol
1) No histones
2) No centromeres or telomeres
2. Explain the differences between sex chromosomes and autosomes.
A. Human and animal chromosomes are categorized as either sex chromosomes or autosomes
1) Sex chromosomes are chromosomes that determine the sex of an organism, and they
may also carry genes for other characteristics
a) There is one pair of sex chromosomes - Two of the 46 human chromosomes are sex
chromosomes
b) In humans, sex chromosomes are either X or Y
(1) Normal human females have two X chromosomes (XX), and normal males
have an X and a Y chromosome (XY)
(2) The sex of an offspring is determined by the male!
c) In grasshoppers XX = female and XO = male (missing a chromosome)
d) In birds, butterflies, and moths XX = male and X = female
2) All of the other chromosomes in an organism are called autosomes
a) The remaining 44 chromosomes are autosomes – are not involved in determining sex
1) Every cell of an organism produced by sexual reproduction has two copies of
each autosome
(a) The organism receives one copy of each autosome from each parent
(b) The two copies of each autosome are called homologous chromosomes, or
homologues
(1) Homologous chromosomes are the same size and shape and carry genes
for the same traits
3. Give examples of diploid and haploid cells.
E. Cells having two sets of chromosomes are said to be diploid - 2n = 46
1) Diploid cells have both chromosomes for each homologous pair
2) Diploid cells also have two sex chromosomes
a) All normal human body cells (somatic cells), except reproductive cells (sperm cells
and egg cells = gametes), are diploid cells
b) In humans, the diploid, or 2n, number of chromosomes is 46 – 22 pairs of
homologous chromosomes and 2 sex chromosomes
F. Human sperm and egg cells are haploid cells = 1n where n = 23
1) These cells contain only one set of chromosomes - one chromosome of each
homologous pair and only one sex chromosome
2) When a sperm cell (1n) and an egg cell (1n) combine to create the first cell of a new
organism, the new cell will be diploid (2n)
G. The fusion of two gametes results in a diploid zygote – a fertilized egg
4. Describe each phase of the cell cycle.
A. Cell division in eukaryotes is more complex than cell division in bacteria because it
involves dividing both the cytoplasm and the chromosomes inside the nucleus
B. The cell cycle is the repeating set of events that make up the life of a cell
1) Cell division is one phase of the cell cycle
2) The time between cell divisions is called interphase
a) Interphase is divided into three phases
(1) Stage 1. = G1 phase—offspring cells rapidly grow to mature size and
carries out its routine functions (cell growth). G1 stands for the time gap
following cell division and preceding DNA replication
(2) Stage 2. At a mature size, cells typically proceed into the next phase, called
the S phase. During which, the DNA is copied and each chromosome
consists of 2 chromatids
(3) Stage 3. The G2 phase represents the time gap following DNA synthesis (S
phase) and preceding cell division - a time during which the cell prepares
for cell division – microtubules are assembles (used to move the
chromosomes during mitosis)
3) During cell division, the chromosomes and cytoplasm are equally divided between two
offspring cells
a) Cell division is divided into two phases consisting of mitosis and cytokinesis
(1) Mitosis – M phase - the nucleus of the cell is divided into 2
nuclei - Each nucleus ends up with the same number and
kinds of chromosomes
(2) Cytokinesis – the cytoplasm divides creating 2 new cells
4) Cells can also exit the cell cycle (usually from the G1 phase) and enter
into a state called the G0 phase
a) During the G0 phase, cells do not copy their DNA and do not
prepare for cell division
b) Many cells in the human body are in the G0 phase
c) For example, fully developed cells in the central nervous system
stop dividing at maturity and normally never divide again
5) The cell cycle is carefully controlled
a) Done by “checkpoints”
(1) G1 checkpoint – makes the decision whether or not the cell
will divide
(2) G2 checkpoint – DNA replication is checked at this point by
DNA repair enzymes
(3) Mitosis checkpoint – This checkpoint triggers the exit from
mitosis - It signals the beginning of the G1 phase
b) When control is lost, cancer results
(1) Cancer- the uncontrolled growth of cells - It is disorder of
cell division
5. Summarize the phases of mitosis.
A. Mitosis is the division of the nucleus, which occurs during cell division – forms 2 nuclei,
each with a complete set of the cell’s chromosomes
1) The chromatids on each chromosome are physically moved to
opposite ends of the cell with the help of a spindle
2) Spindles are cell structures made up of both centrioles and
individual microtubule fibers
a) Forming the spindle
(1) Animal cells have a pair of centrioles
(a) They are at right angles of one another
(2) During the G2 phase, the centrioles are replicated, so each
cell now has 2 pairs
(a) During the mitotic phase, the centrioles move toward
opposite poles
(b) As the centrioles move, the spindles form
(c) Each spindle fiber is made up of an individual
microtubule
(d) Each centriole is made up of 9 triplets of microtubules
arranged in a circle
b) Separating chromatids by attaching spindle fibers
(1) The 2 sets of microtubules extend out toward opposite poles
(2) The microtubules attach to chromosomes
(3) Once the microtubules attach to the centromeres and poles, the 2
chromatids can be separated
(4) As soon as the chromatids separate from each other, they are called
chromosomes
B. Mitosis is a continuous process that is divided into four phases: prophase, metaphase,
anaphase, and telophase
1) Prophase
a) DNA shortens and coils tight of into rod-shaped chromosomes that can be seen with
a light microscope - each chromosome was copied in S phase
b) The two copies of each chromosome—called chromatids—stay connected to one
another by the centromere
c) nucleolus and the nuclear membrane break down and disappear
d) centrosomes appear next to the disappearing nucleus
e) the centrosomes move toward opposite poles of the cell
f) As the centrosomes separate, spindle fibers made of microtubules radiate from the
centrosomes in preparation for mitosis
g) This array of spindle fibers is called the mitotic spindle, which serves to equally
divide the chromatids between the two offspring cells during cell division
2) Metaphase
a) chromosomes are easier to identify using a microscope thus, karyotypes are typically
made from photomicrographs
b) kinetochore fibers (spindle fibers) link the chromatids of each homologous
chromosome to opposite poles and move the chromosomes to the center of the
dividing cell
c) Once in the center of the cell, each chromosome is held in place by the kinetochore
fibers
3) Anaphase
a) chromatids of each chromosome divide and separate at the centromere and slowly
move, centromere first, toward opposite poles as the spindle shortens
b) When the chromatids separate, they are considered to be individual chromosomes
4) Telophase
a) After the chromosomes reach opposite ends of the cell, the spindle fibers
disassemble and the chromosomes return to a less tightly coiled chromatin state
b) A nuclear envelope forms around each set of chromosomes, and a nucleolus forms in
each of the newly forming cells
5) Cytokinesis
a) During telophase, the cytoplasm of the cell divides by a process called
cytokinesis
b) In animal cells, cytokinesis begins with a pinching inward of the cell membrane
midway between the dividing cell’s two poles
c) In plant cells, vesicles formed by the Golgi apparatus fuse at the midline of the
dividing cell, forming a membrane-bound cell wall called the cell plate
d) In both animal cells and plant cells, offspring cells are approximately equal in
size
e) Each offspring cell receives an identical copy of the original cell’s chromosomes
and approximately one-half of the original cell’s cytoplasm and organelles
6. List and describe the phases of meiosis.
A. Meiosis is a process of nuclear division that reduces the number of chromosomes in new
cells to half the number in the original cell through two divisions of the nucleus
B. Meiosis produces haploid reproductive cells called gametes – sperm, eggs, spores
C. Stages of Meiosis
1) Cells preparing to divide by meiosis undergo the G1, S, and G2 phases of interphase
2) Because cells undergoing meiosis divide twice, diploid (2n) cells that divide meiotically
result in four haploid (1n) cells rather than two diploid (2n) cells
3) The stages of the first cell division are called meiosis I, and the stages of the second cell
division are called meiosis II
a. Meiosis I
1) Prophase I, DNA coils tightly into chromosomes, spindle fibers appear, the
nucleus and nucleolus disassemble, and every chromosome lines up next to its
homologue allowing crossing-over to occur
2) Metaphase I, the tetrads line up randomly along the midline of the dividing cell
a) The orientation of the homologous pair of chromosomes is random with
respect to the poles of the dividing cell
b) Spindle fibers from the poles attach to the centromeres of the homologous
chromosome
3) Anaphase I, each homologous chromosome (consisting of two chromatids
attached by a centromere) moves to an opposite pole
a) The random separation of the homologous chromosomes is called independent
assortment - results in a random separation of the maternal and paternal
chromosomes, which results in genetic variation
4) Telophase I
a) chromosomes reach the opposite ends of the cell, and cytokinesis I begins
b. During meiosis I, the original cell produces two new cells
1) Each new cell contains one chromosome from each homologous pair
2) The new cells contain half the number of chromosomes of the original cell =
haploid
3) However, each new cell contains two copies of the chromosome because the
original cell copied its DNA before meiosis I
c. Meiosis II
1) Meiosis II occurs in each cell formed during meiosis I and is not preceded by the
copying of DNA
2) Stages of Meiosis II
a) prophase II - spindle fibers form and begin to move the chromosomes toward the
midline of the dividing cell
b) metaphase II - the chromosomes move to the midline of the dividing cell, facing
opposite poles of the dividing cell
c) anaphase II - the chromatids separate and move toward opposite poles of the cell
d) telophase II - a nuclear membrane forms around the chromosomes in each of the
four new cells
e) Cytokinesis II occurs during telophase II, resulting in four new cells, each
containing half of the number of chromosomes
7. Compare the end products of mitosis with those of meiosis.
A. At the end of mitosis there are two cells identical to the parent cell
B. At the end of meiosis there are four cells each with half the number of chromosomes and
are genetically different from parent cell due to crossing over during Prophase I
8. Explain crossing-over and how it contributes to the production of unique individuals.
A. During prophase I when DNA coils tightly into chromosomes and Every chromosome
lines up next to its homologue in a process called synapsis Each pair of homologous
chromosomes is called a tetrad.
B. In each tetrad, chromatids of the homologous chromosomes are aligned lengthwise so that
the genes on one chromosome are adjacent to the corresponding genes on the other
chromosome and the pair twist around one another
C. Portions of chromatids may break off and attach to adjacent chromatids on the homologous
chromosome— a process called crossing-over
D. This process permits the exchange of genetic material between maternal and paternal
resulting in genetic recombination by producing a new mixture of genetic material
1) Remember that independent assortment and random fertilization also lead to new
genetic combinations
2) Genetic variation decreases the chances of genetic mutations and susceptibility to
diseases
9. Summarize the major characteristics of spermatogenesis and oogenesis.
A. Formation of Gametes
1) Meiosis produces haploid reproductive cells called gametes in animals
2) Because only those cells involved in the production of gametes divide by meiosis,
meiosis occurs only within the reproductive organs
3) In humans, meiosis occurs in the testes and in the ovaries
a) The production of sperm cells is called spermatogenesis – done in the testes of males
(1) Meiosis produces male gametes known as sperm cells or spermatozoa
(2) A diploid reproductive cell divides meiotically to form four haploid cells
called spermatids and each then develops into a mature sperm cell
(3) Men constantly make sperm throughout their reproductive years
b) Oogenesis is the production of mature egg cells, or ova in the ovaries of females
(1) A diploid reproductive cell divides meiotically to produce one
mature egg cell (ovum)
(2) During cytokinesis I and cytokinesis II of oogenesis, the cytoplasm of the
original cell is divided unequally between new cells
(a) One cell, which develops into a mature egg cell, receives most of the
cytoplasm of the original cell
(b) As a result, one egg cell is produced by meiosis and the other three
products of meiosis, called polar bodies, degenerate
(3) Women are born with their lifetime supply of eggs (as many as 2 million)
and this is why women only release 1 egg each month
10.Describe the differences between sexual and asexual reproduction
A. Similarities to parents is determined by the type of reproduction used
1. Asexual: a parent passes copies of all of its genes to its offspring
a. This produces a clone
2. Sexual: two haploid cells fuse to form a diploid offspring
a. The offspring will have traits of both parents
B. Types of Asexual reproduction
1. Fission: the separation of a parent into 2 or more individuals of equal size
2. Fragmentation: the body breaks into several pieces - Some of these
pieces develop into complete adults when the missing parts are regrown
3. Budding: new individuals split off from existing ones
C. Perks of Asexual Reproduction
1. Many offspring can be produced in a short period of time
2. Doesn’t use energy to produce gametes
3. No need to find a mate
D. Downfall of Asexual Reproduction
1. Lack of genetic variation leaves lots of room for genetic errors to occur
2. Because of lack of variation in DNA, organisms may not be able to readily adapt to
changing environments (no evolution)
E. Sexual Reproduction probably came about due to environmental pressures
1. In protists, the cells are haploid most of the time - They only form diploid cells in
response to changes in the environment that cause them stress
a. This happens because diploid cells can repair certain kinds of chromosomal
damages, like breaks in the DNA
1) As organisms got larger, they needed to keep this mechanism - This is done when
homologous chromosomes pair up during meiosis
F. Eukaryotes have 3 kinds of Sexual life cycles
1. Haploid life cycle
a. Haploid cells go through mitosis to form haploid gametes - These gametes fuse
to form a diploid zygote, which immediately undergoes meiosis to form haploid
individuals
2. Diploid life cycle
a. Parents produce gametes through meiosis - The gametes join during fertilization,
forming a diploid zygote - The zygote divides by mitosis
3. Alternation of generations
a. Plants, algae, and some protests have a life cycle that regularly alternates
between a haploid phase and a diploid phase
b. Sporophytes (diploid) give rise to spores (haploid)
c. Spores, which are haploid reproductive cells, are capable of developing into an
adult without fusing with another cell
d. Spores give rise to gametophytes
e. The gametophyte produces gametes that fuse and give rise to a diploid zygote
Chapter 8 Cell Reproduction
 Chromosomes are tightly coiled DNA molecules and associated proteins.
 In eukaryotes, histone proteins help maintain the compact structure of chromosomes.
 In dividing cells, chromosomes are composed of two identical chromatids constricted together
at a centromere.
 Chromosomes are categorized as either sex chromosomes or autosomes.
 Homologous chromosomes consist of one autosomal chromosome from each parent.
 Diploid (2n) is the number of chromosomes in cells that have homologous pairs of autosomes
and two sex chromosomes.
 Haploid cells (1n) have half the number of chromosomes that are present in diploid cells.
 Cell division is the process by which cells reproduce themselves.
 Binary fission is the process of cell division in prokaryotes.
 The cell cycle is the repeating of events that make up the life of a cell. The cell cycle consists
of cell division and interphase.
 Cell division in eukaryotes includes the division of the nucleus (mitosis) and the division of
the cytoplasm (cytokinesis).
 Interphase consists of a phase of growth (G1), a phase of DNA replication (S), and a phase of
preparation for cell division (G2).
 Mitosis is divided into prophase, metaphase, anaphase, and telophase. Mitosis results in two
offspring cells that are genetically identical to the original cell.
 During meiosis, a cell divides twice.
 Crossing-over during meiosis results in genetic recombination.
 Spermatogenesis is the process by which sperm cells are produced. Oogenesis is the process
that produces egg cells (ova).
 Asexual reproduction is the formation of offspring from one parent. Offspring produced by
asexual reproduction are genetically identical to the parent.
 Sexual reproduction is the formation of offspring through the union of a sperm and an egg.
Offspring produced by sexual reproduction are genetically different from the parents.
Vocabulary List
Asexual reproduction
Autosome
Binary fission
Cell cycle
Cell plate
Centriole
Centromere
Centrosome
Chromatid
Cleavage furrow
Crossing-over
Diploid
Gamete
Genetic recombination
Haploid
Homologous chromosome
Independent assortment
Karyotype
Kinetochore
Meiosis
Mitosis
Mitotic spindle
Oogenesis
Polar body
Sex chromosome
Sexual reproduction
Spermatid
Spermatogenesis
Spindle fiber
Synapsis
Tetrad