1. Cell Division
Chapter 8

2. Cell Division Doubling organelles and proteins DNA replication
Nuclear division
Cytoplasmic division

3. Cell Increase and Decrease
Maintain homeostasis
Cell numbers kept in check by this mechanism
Through cell division of somatic cells and cell death
Cell division – interphase, mitosis and cytokinesis
Cell death – apoptosis

4. Cell Increase and Decrease
Somatic cells
Asexual reproduction
increase in number of somatic cells
Increase in number unicellular organisms
Germ cells
Sexual reproduction
requires the production of eggs and sperm

5. Important terms:
DNA
Chromosomes
Chromatin
Chromatid

6. Mitosis verse Meiosis?? Mitosis
Cell division mechanism that occurs in nonreproductive cells
somatic cell nuclei
Meiosis
Cell division mechanism that occurs in cells that participate in sexual reproduction
gamete nuclei

7. Cell Cycle – Somatic cells
Set of stages that involves cell growth and nuclear division
Consists of:
Interphase G1 S G2
Meiotic stage
Mitosis and Cytokinesis

8. Interphase When the cell carries on its usual functions Main stages:
Gap before DNA synthesis begins S
Time when DNA duplicated*************** G2
Gap between time DNA duplication ends and mitosis begins

9. Interphase in Meiosis? Meiosis is a "one-way" process
Cannot be said to engage in a cell cycle as mitosis does
Preparatory steps that lead up to meiosis are identical in pattern and name to the interphase of the mitotic cell cycle

10. Cytokinesis in Animal Cells
Cytoplasmic cleavage
Accompanies mitosis
Separate process
Cleavage furrow forms between daughter nuclei
Contractile ring contracts deepening the furrow
Continues until separation is complete

11. Human DNA in somatic cells
22-23 pairs of homologous chromosomes
Difference?
Autosomes (1-22)
Sex chromosomes (23)
Somatic cells have 46 chromosomes
Diploid 2n

12. Human DNA in gametes Due to reductional division
Halves the diploid number (2n) to a haploid number (n)
23 total chromosomes

13. Division of the Nucleus
Nucleus must be divided
Parent cell’s DNA into 2 nuclei
2 ways nucleus can divide:
Mitosis
Meiosis

14. Maintaining the Chromosome Number
Mitosis
Maintaining the Chromosome Number

15. DNA Replication…Somatic cells
Duplicated chromosome
Composed of 2 sister chromatids
held together by a centromere
Sister chromatids
Genetically identical
When separate, each daughter nucleus gets a chromosome
DNA copied
DNA divided

16. Chromosomes Chromosomes are paired in somatic cells
homologous chromosomes, homologues
contain information about the same traits but the information may vary
Cells that have two of each type of chromosome are called diploid cells
one chromosome of each pair is inherited from the mother and the other is inherited from the father

17. The difference between homologous chromosomes and sister chromatids

18. Mitosis
Mitosis
4 main stages:
Prophase
Metaphase
Anaphase
Telophase

19. How the cell cycle works

20. Mitosis 1. Prophase – Mitosis begins! Threadlike form
Spindle fibers appear
DNA start to condense
Aster formed
Nuclear envelope starts to break apart
Centrioles move to opposite sides of the cell

21. Mitosis
2. Metaphase
Duplicated chromosomes aligned midway between the poles
Associated with spindle fibers

22. Mitosis
3. Anaphase
Sister chromatids separate from each other and move to opposite poles
Become daughter chromosomes

23. Mitosis 4. Telophase Return to threadlike form as in prophase
New nuclear envelope separates each chromosome cluster
2 new nuclei!!!!!

24. Reducing the Chromosome Number
Meiosis
Reducing the Chromosome Number

25. Meiosis Mechanism for dividing the nucleus of germ cells
Oogonia and spermatogonia 2n
Meiosis must take place prior to formation of gametes
Sperm and eggs n
First stage in sexual reproduction

26. Meiosis Overview
Occurs in the life cycle of sexually reproducing organisms
Reduces the chromosome number
2 divisions, 4 daughter cells
Cells are diploid at beginning of meiosis
TWO consecutive divisions
Result is 4 haploid nuclei
Divided into:
Meiosis I
Meiosis II

28. Reducing the Chromosome Number
Genetic Recombination
Promotes genetic variability
Happens by:
Crossing Over
Independent Assortment of paired chromosomes
Random Fertilization

29. Meiosis comparison Meiosis I Meiosis II Crossing over
Homologous chromosomes line up
Homologous chromosomes split
Two haploid cells formed
Cytokinesis occurs
Essentially the same as Meiosis I
Starts with a haploid cell that has NOT undergone chromosome duplication

30. Comparisons between males and females
Spermatogenesis
Begins at puberty and continues throughout life
Oogenesis
Begins in the fetus
Primary oocytes are arrested in prophase I
At puberty, one primary oocyte continues the process of meiosis during each menstrual cycle

31. Comparisons Mitosis Meiosis DNA replication occurs only once
Requires only one division
Produces two daughter cells
Diploid daughter cells 2n
Genetically identical cells produced
Occurs all the time
Meiosis
DNA replication occurs only once
Requires two divisions
Produces four daughter cells
Haploid daughter cells n
Genetically variable cells produced
Occurs only at certain times

32. (before chromosome duplication) Daughter cells of meiosis II
MITOSIS
MEIOSIS
Parent cell
(before chromosome duplication)
Site of
crossing over
MEIOSIS I
Prophase I
Prophase
Tetrad formed
by synapsis of
homologous
chromosomes
Duplicated
chromosome
(two sister
chromatids)
Chromosome
duplication
Chromosome
duplication
2n = 4
Chromosomes
align at the
metaphase plate
Tetrads
align at the
metaphase plate
Metaphase
Metaphase I
Anaphase
Telophase
Anaphase I
Telophase I
Sister chromatids
separate during
anaphase
Homologous
chromosomes
separate
(anaphase I);
sister chroma-
tids remain
together
Figure 8.15 Comparison of mitosis and meiosis.
Haploid
n = 2
Daughter
cells of
meiosis I
No further
chromosomal
duplication;
sister
chromatids
separate
(anaphase II) 2n 2n
MEIOSIS II
Daughter cells
of mitosis n n n n
Daughter cells of meiosis II

33. Comparisons

34. Overview of the Life Cycle of Humans

35. Cell division in other organisms

36. How Plant Cells Divide Occurs in meristematic tissues
Same phases as animal cells
Plant cells do not have centrioles or asters 36

37. Plant Cells

38. Cytokinesis in Plant Cells
Flattened, small disk appears between daughter cells
Golgi apparatus produces vesicles which move to disk
Release molecules which build new cell walls
Vesicle membranes complete plasma membranes 38

39. Prokaryotes Have a Simple Cell Cycle
Cell division in prokaryotes takes place in two stages (simple cell cycle)
copy the DNA
this process is called replication
split the cell in two to form daughter cells
this process is called binary fission

40. Cell Division in Prokaryotes
Binary Fission
Prokaryotes have a single chromosome
Chromosomal replication occurs before division
Cell elongates to twice its length
Cell membrane grows inward until division is complete 40

41. Alterations of chromosome number and structure

42. Extra copy of chromosome 21 causes Down syndrome
Trisomy 21 involves the inheritance of three copies of chromosome 21
Trisomy 21 is the most common human chromosome abnormality
Imbalance in chromosome number causes Down syndrome, which is characterized by
Characteristic facial features
Susceptibility to disease
Shortened life span
Mental retardation
Variation in characteristics
The incidence increases with the age of the mother
Student Misconceptions and Concerns
1. Before addressing karyotyping and nondisjunction events, consider reviewing the general structure and terminology associated with replicated chromosomes and the arrangement of chromosomes during metaphase of mitosis, meiosis I, and meiosis II. Figures 8.4B and 8.15 will be particularly helpful. A firm foundation in chromosome basics is necessary to understand the irregularities discussed in Modules 8.20–8.23.
Teaching Tips
1. The Genetic Interest Group has a website devoted to human genetic disorders at Its long list of Internet links is both extensive and comprehensive.

43. Figure 8.20A A karyotype for trisomy 21 (Down syndrome).
This figure shows the karyotype of a female with Down syndrome. Chromosomes 1–20, 22, and 23 are shown in pairs while chromosome 21 is present in three copies.

44. Infants with Down syndrome90 80 70 60
Infants with Down syndrome
(per 1,000 births) 50 40 30 20
Figure 8.20C Maternal age and incidence of Down syndrome.
This figure shows the rise in incidence of Down syndrome with increasing maternal age. Studies demonstrate that a high frequency of cases is related to nondisjunction during meiosis I, but the mechanism for this increased occurrence in aging eggs has not yet been elucidated. As described in the text, there may be an age-related error in one of the checkpoints that coordinate the meiotic process. There is also some evidence for nondisjunction during sperm production since the incidence of Down syndrome is further increased when both the mother and father are over age 40. 10 20 25 30 35 40 45 50
Age of mother

45. Accidents during meiosis can alter chromosome number
Nondisjunction is the failure of chromosomes or chromatids to separate during meiosis
Fertilization after nondisjunction yields zygotes with altered numbers of chromosomes
Student Misconceptions and Concerns
1. Before addressing karyotyping and nondisjunction events, consider reviewing the general structure and terminology associated with replicated chromosomes and the arrangement of chromosomes during metaphase of mitosis, meiosis I, and meiosis II. Figures 8.4B and 8.15 will be particularly helpful. A firm foundation in chromosome basics is necessary to understand the irregularities discussed in Modules 8.20–8.23.
Teaching Tips
1. The Genetic Interest Group has a website devoted to human genetic disorders at Its long list of Internet links is both extensive and comprehensive.
2. 4. Students might be confused by the term nondisjunction. But simply put, it is an error in the sorting of chromosomes during mitosis or meiosis. Figure 8.21 illustrates two types of nondisjunction errors in meiosis.

46. Abnormal numbers of sex chromosomes
Sex chromosome abnormalities tend to be less severe as a result of
Small size of the Y chromosome
X-chromosome inactivation
In each cell of a human female, one of the two X chromosomes becomes tightly coiled and inactive
random process that inactivates either the maternal or paternal chromosome
Inactivation promotes a balance between the number of X chromosomes and autosomes
Student Misconceptions and Concerns
1. Before addressing karyotyping and nondisjunction events, consider reviewing the general structure and terminology associated with replicated chromosomes and the arrangement of chromosomes during metaphase of mitosis, meiosis I, and meiosis II. Figures 8.4B and 8.15 will be particularly helpful. A firm foundation in chromosome basics is necessary to understand the irregularities discussed in Modules 8.20–8.23.
Teaching Tips
1. The Genetic Interest Group has a website devoted to human genetic disorders at Its long list of Internet links is both extensive and comprehensive.
2. 4. Students might be confused by the term nondisjunction. But simply put, it is an error in the sorting of chromosomes during mitosis or meiosis. Figure 8.21 illustrates two types of nondisjunction errors in meiosis.

47. Table 8.22 Abnormalities of Sex Chromosome Number in Humans.
At fertilization, humans are chromosomally male or female but have presumptive gonads that can be influenced to become testes or ovaries. If the Y chromosome has been inherited, a series of genetic changes influences testis development. In the absence of a Y chromosome, the gonads become ovaries.
Klinefelter syndrome results from two or more X chromosomes with one Y chromosome.
Individuals with Turner syndrome (XO) are sterile, showing the importance of two X chromosomes during early development for the formation of functional sex organs. This influence must be exerted before X-chromosome inactivation, otherwise XX females would also be sterile.

48. New species can arise from errors in cell division
Polyploid species have more than two chromosome sets
Observed in many plant species
Seen less frequently in animals
Example
Diploid gametes are produced by failures in meiosis
Diploid gamete + Diploid gamete  Tetraploid offspring
The tetraploid offspring have four chromosome sets
Animal examples include fish, amphibians, and one species of rat.
Student Misconceptions and Concerns
1. Before addressing karyotyping and nondisjunction events, consider reviewing the general structure and terminology associated with replicated chromosomes and the arrangement of chromosomes during metaphase of mitosis, meiosis I, and meiosis II. Figures 8.4B and 8.15 will be particularly helpful. A firm foundation in chromosome basics is necessary to understand the irregularities discussed in Modules 8.20–8.23.
Teaching Tips
1. The Genetic Interest Group has a website devoted to human genetic disorders at Its long list of Internet links is both extensive and comprehensive.
2. 4. Students might be confused by the term nondisjunction. But simply put, it is an error in the sorting of chromosomes during mitosis or meiosis. Figure 8.21 illustrates two types of nondisjunction errors in meiosis.

49. Alterations of chromosome structure can cause birth defects and cancer
Structure changes result from breakage and rejoining of chromosome segments
Deletion is the loss of a chromosome segment
Duplication is the repeat of a chromosome segment
Inversion is the reversal of a chromosome segment
Translocation is the attachment of a segment to a nonhomologous chromosome; can be reciprocal
Altered chromosomes carried by gametes cause birth defects
Chromosomal alterations in somatic cells can cause cancer
Reciprocal translocations involve exchange of segments between nonhomologous chromosomes, but the sizes of the segments do not need to be the same.
Teaching Tips
1. Challenge students to create a sentence and then modify that sentence to represent (a) a deletion, (b) a duplication, and (c) an inversion as an analogy to these changes to a chromosome.

50. Homologous chromosomes
Deletion
Duplication
Homologous
chromosomes
Figure 8.24A Alterations of chromosome structure involving one chromosome or a homologous pair.
Inversion

51. “Philadelphia chromosome”
Reciprocal
translocation
Chromosome 22
Figure 8.24C The translocation associated with chronic myelogenous leukemia.
Familial Down syndrome is the result of a Robertsonian translocation. This is when the long arms of two nonhomologous chromosomes are joined to the same centromere. A translocation carrier for Down syndrome would have a translocated chromosome that has the long arm of chromosome 21 attached to another chromosome, such as chromosome 15. That individual would also have one complete copy of chromosome 21 and one complete copy of chromosome 15. Due to the translocated chromosome, this individual would have 45 chromosomes. If this parent produces a gamete containing the translocated chromosome along with the complete copy of 21, and the other parent provides single copies of 15 and 21, the offspring will have Down syndrome. The translocated chromosome provides a nearly complete third copy of chromosome 21.
“Philadelphia chromosome”
Activated cancer-causing gene