1. The Cell Cycle and How Cells Divide

2. Phases of the Cell Cycle
The cell cycle consists of
Interphase – normal cell activity
The mitotic phase – cell divsion
INTERPHASE
Growth
G 1
(DNA synthesis)
Growth G2
Cell Divsion

3. Functions of Cell Division
20 µm
100 µm
200 µm
(a) Reproduction. An amoeba, a single-celled eukaryote, is dividing into two cells. Each new cell will be an individual organism (LM).
(b) Growth and development This micrograph shows a sand dollar embryo shortly after the fertilized egg divided, forming two cells (LM).
(c) Tissue renewal. These dividing bone marrow cells (arrow) will give rise to new blood cells (LM).

4. Cell Division An integral part of the cell cycle
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

5. DNA Genetic information - genome Packaged into chromosomes Figure 12.3

6. DNA And Chromosomes
An average eukaryotic cell has about 1,000 times more DNA then an average prokaryotic cell.
The DNA in a eukaryotic cell is organized into several linear chromosomes, whose organization is much more complex than the single, circular DNA molecule in a prokaryotic cell

7. Chromosomes
All eukaryotic cells store genetic information in chromosomes.
Most eukaryotes have between 10 and 50 chromosomes in their body cells.
Human cells have 46 chromosomes.
23 nearly-identical pairs

8. Structure of Chromosomes
Chromosomes are composed of a complex of DNA and protein called chromatin that condenses during cell division
DNA exists as a single, long, double-stranded fiber extending chromosome’s entire length.
Each unduplicated chromosome contains one DNA molecule, which may be several inches long

9. Structure of Chromosomes
Every 200 nucleotide pairs, the DNA wraps twice around a group of 8 histone proteins to form a nucleosome.
Higher order coiling and supercoiling also help condense and package the chromatin inside the nucleus:

10. Structure of Chromosomes
The degree of coiling can vary in different regions of the chromatin:
Heterochromatin refers to highly coiled regions where genes aren’t expressed.
Euchromatin refers to loosely coiled regions where genes can be expressed.

11. Structure of Chromosomes
Prior to cell division each chromosome duplicates itself.
During this time, only the heterochromatin is visible, as dense granules inside the nucleus.
There is also a dense area of RNA production called the nucleolus:

12. Karyotype
An ordered, visual representation of the chromosomes in a cell
Chromosomes are photographed when they are highly condensed, then photos of the individual chromosomes are arranged in order of decreasing size:
In humans each somatic cell has 46 chromosomes, made up of two sets, one set of chromosomes comes from each parent
5 µm
Pair of homologous
chromosomes
Centromere
Sister
chromatids

13. Chromosomes Non-homologous chromosomes Look different
Control different traits
Sex chromosomes
Are distinct from each other in their characteristics
Are represented as X and Y
Determine the sex of the individual, XX being female, XY being male
In a diploid cell, the chromosomes occur in pairs. The 2 members of each pair are called homologous chromosomes or homologues.

14. Chromosomes A diploid cell has two sets of each of its chromosomes
A human has 46 chromosomes (2n = 46)
In a cell in which DNA synthesis has occurred all the chromosomes are duplicated and thus each consists of two identical sister chromatids
Maternal set of
chromosomes (n = 3)
Paternal set of
2n = 6
Two sister chromatids
of one replicated
chromosome
Two nonsister
chromatids in
a homologous pair
Pair of homologous
chromosomes
(one from each set)
Centromere

15. Homologues Homologous chromosomes: Look the same
Control the same traits
May code for different forms of each trait
Independent origin - each one was inherited from a different parent

16. Chromosome Duplication
In preparation for cell division, DNA is replicated and the chromosomes condense
Each duplicated chromosome has two sister chromatids, which separate during cell division
0.5 µm
Chromosome duplication (including DNA synthesis)
Centromere
Separation of sister chromatids
Sister chromatids
Centrometers
A eukaryotic cell has multiple chromosomes, one of which is represented here. Before duplication, each chromosome has a single DNA molecule.
Once duplicated, a chromosome consists of two sister chromatids connected at the centromere. Each chromatid contains a copy of the DNA molecule.
Mechanical processes separate the sister chromatids into two chromosomes and distribute them to two daughter cells.

17. Non-sister chromatids Two duplicated chromosomes
Chromosome Duplication
Because of duplication, each condensed chromosome consists of 2 identical chromatids joined by a centromere.
Each duplicated chromosome contains 2 identical DNA molecules (unless a mutation occurred), one in each chromatid:
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Two unduplicated
chromosomes
Centromere
Sister
chromatids
Duplication
Non-sister chromatids
Two duplicated chromosomes

18. Structure of Chromosomes
The centromere is a constricted region of the chromosome containing a specific DNA sequence, to which is bound 2 discs of protein called kinetochores.
Kinetochores serve as points of attachment for microtubules that move the chromosomes during cell division:
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Metaphase chromosome
Kinetochore
microtubules
Centromere
region of
chromosome
Sister Chromatids

19. Structure of Chromosomes
Diploid - A cell possessing two copies of each chromosome (human body cells).
Homologous chromosomes are made up of sister chromatids joined at the centromere.
Haploid - A cell possessing a single copy of each chromosome (human sex cells).

20. Phases of the Cell Cycle
Interphase
G1 - primary growth
S - genome replicated
G2 - secondary growth
M - mitosis
C - cytokinesis

21. Interphase G1 - Cells undergo majority of growth
S - Each chromosome replicates (Synthesizes) to produce sister chromatids
Attached at centromere
Contains attachment site (kinetochore)
G2 - Chromosomes condense - Assemble machinery for division such as centrioles

22. DNA duplication during interphase
Mitosis
Some haploid & diploid cells divide by mitosis.
Each new cell receives one copy of every chromosome that was present in the original cell.
Produces 2 new cells that are both genetically identical to the original cell.
DNA duplication during interphase
Mitosis
Diploid Cell

23. Mitotic Division of an Animal Cell
G2 OF INTERPHASE
PROPHASE
PROMETAPHASE
Centrosomes (with centriole pairs)
Chromatin (duplicated)
Early mitotic spindle
Aster
Centromere
Fragments of nuclear envelope
Kinetochore
Nucleolus
Nuclear envelope
Plasma membrane
Chromosome, consisting of two sister chromatids
Kinetochore microtubule
Nonkinetochore microtubules

24. Mitotic Division of an Animal Cell
METAPHASE
ANAPHASE
TELOPHASE AND CYTOKINESIS
Spindle
Metaphase plate
Nucleolus forming
Cleavage furrow
Nuclear envelope forming
Centrosome at one spindle pole
Daughter chromosomes

25. G2 of Interphase A nuclear envelope bounds the nucleus.
The nucleus contains one or more nucleoli (singular, nucleolus).
Two centrosomes have formed by replication of a single centrosome.
In animal cells, each centrosome features two centrioles.
Chromosomes, duplicated during S phase, cannot be seen individually because they have not yet condensed.
The light micrographs show dividing lung cells from a newt, which has 22 chromosomes in its somatic cells (chromosomes appear blue, microtubules green, intermediate filaments red). For simplicity, the drawings show only four chromosomes.
G2 OF INTERPHASE
Centrosomes (with centriole pairs)
Chromatin (duplicated)
Nucleolus
Nuclear envelope
Plasma membrane

26. Prophase
The chromatin fibers become more tightly coiled, condensing into discrete chromosomes observable with a light microscope.
The nucleoli disappear.
Each duplicated chromosome appears as two identical sister chromatids joined together.
The mitotic spindle begins to form It is composed of the centrosomes and the microtubules that extend from them. The radial arrays of shorter microtubules that extend from the centrosomes are called asters (“stars”).
The centrosomes move away from each other, apparently propelled by the lengthening microtubules between them.
PROPHASE
Early mitotic spindle
Aster
Centromere
Chromosome, consisting of two sister chromatids

27. METAPHASE
Spindle
Metaphase plate
Centrosome at one spindle pole
Metaphase
Metaphase is the longest stage of mitosis, lasting about 20 minutes.
The centrosomes are now at opposite ends of the cell.
The chromosomes convene on the metaphase plate, an imaginary plane that is equidistant between the spindle’s two poles. The chromosomes’ centromeres lie on the metaphase plate.
For each chromosome, the kinetochores of the sister chromatids are attached to kinetochore microtubules coming from opposite poles.
The entire apparatus of microtubules is called the spindle because of its shape.

28. The Mitotic Spindle
The spindle includes the centrosomes, the spindle microtubules, and the asters
The apparatus of microtubules controls chromosome movement during mitosis
The centrosome replicates, forming two centrosomes that migrate to opposite ends of the cell
Assembly of spindle microtubules begins in the centrosome, the microtubule organizing center
An aster (a radial array of short microtubules) extends from each centrosome

29. The Mitotic Spindle
Some spindle microtubules attach to the kinetochores of chromosomes and move the chromosomes to the metaphase plate
In anaphase, sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell
Microtubules
Chromosomes
Sister
chromatids
Aster
Centrosome
Metaphase
plate
Kineto-
chores
Kinetochore
microtubules
0.5 µm
Overlapping
nonkinetochore
1 µm

30. Anaphase
Anaphase is the shortest stage of mitosis, lasting only a few minutes.
Anaphase begins when the two sister chromatids of each pair suddenly part. Each chromatid thus becomes a full- fledged chromosome.
The two liberated chromosomes begin moving toward opposite ends of the cell, as their kinetochore microtubules shorten. Because these microtubules are attached at the centromere region, the chromosomes move centromere first (at about 1 µm/min).
The cell elongates as the nonkinetochore microtubules lengthen.
By the end of anaphase, the two ends of the cell have equivalent—and complete—collections of chromosomes.
ANAPHASE
Daughter chromosomes

31. TELOPHASE AND CYTOKINESIS
Nucleolus forming
Cleavage furrow
Nuclear envelope forming
Telophase
Two daughter nuclei begin to form in the cell.
Nuclear envelopes arise from the fragments of the parent cell’s nuclear envelope and other portions of the endomembrane system.
The chromosomes become less condensed.
Mitosis, the division of one nucleus into two genetically identical nuclei, is now complete.

32. Mitosis in a plant cell Nucleus Chromatine condensing Chromosome1
Prophase. The chromatin is condensing. The nucleolus is beginning to disappear. Although not yet visible in the micrograph, the mitotic spindle is staring to from.
Prometaphase. We now see discrete chromosomes; each consists of two identical sister chromatids. Later in prometaphase, the nuclear envelop will fragment.
Metaphase. The spindle is complete, and the chromosomes, attached to microtubules at their kinetochores, are all at the metaphase plate.
Anaphase. The chromatids of each chromosome have separated, and the daughter chromosomes are moving to the ends of cell as their kinetochore microtubles shorten.
Telophase. Daughter nuclei are forming. Meanwhile, cytokinesis has started: The cell plate, which will divided the cytoplasm in two, is growing toward the perimeter of the parent cell. 2 3 4 5
Nucleus
Nucleolus
Chromosome
Chromatine condensing

33. Cytokinesis Cleavage of cell into two halves Animal cells
Constriction belt of actin filaments
Plant cells
Cell plate
Fungi and protists
Mitosis occurs within the nucleus

34. Cytokinesis In Animal And Plant Cells
Cleavage furrow
Contractile ring of microfilaments
Daughter cells
100 µm
1 µm
Vesicles forming cell plate
Wall of patent cell
Cell plate
New cell wall
(a) Cleavage of an animal cell (SEM)
(b) Cell plate formation in a plant cell (SEM)
Daughter cells

36. Meiosis and Sexual Life Cycles
Living organisms are distinguished by their ability to reproduce their own kind
Heredity
Is the transmission of traits from one generation to the next
Variation
Shows that offspring differ somewhat in appearance from parents and siblings

37. Inheritance of Genes Genes are segments of DNA, units of heredity
Offspring acquire genes from parents by inheriting chromosomes
Genetics is the scientific study of heredity and hereditary variation

38. Inheritance of Genes
Each gene in an organism’s DNA has a specific locus on a certain chromosome
We inherit one set of chromosomes from our mother and one set from our father
Two parents give rise to offspring that have unique combinations of genes inherited from the two parents - sexual reproduction

39. Asexual Reproduction
In asexual reproduction, one parent produces genetically identical offspring by mitosis
Figure 13.2
Parent
Bud
0.5 mm

40. Sexual Reproduction
Fertilization and meiosis alternate in sexual life cycles
A life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism
Gametes
Diploid
multicellular
organism
Key
MEIOSIS
FERTILIZATION n 2n
Zygote
Haploid
Mitosis
(a) Animals

41. Sex Cells - Gametes
Unlike somatic cells, sperm and egg cells are haploid cells, containing only one set of chromosomes
At sexual maturity the ovaries and testes produce haploid gametes by meiosis

42. Sexual Reproduction - The Human Life Cycle
Haploid (n)
Diploid (2n)
Haploid gametes (n = 23)
Ovum (n)
Sperm
Cell (n)
MEIOSIS
FERTILIZATION
Ovary
Testis
Diploid
zygote
(2n = 46)
Mitosis and
development
Multicellular diploid
adults (2n = 46)
During fertilization, sperm and ovum fuse forming a diploid zygote
The zygote develops into an adult organism

43. Meiosis Reduces the chromosome number such that each daughter
Cell has a haploid set of chromosomes
Ensures that the next generation will have:
Diploid number of chromosome
Exchange of genetic information (combination of traits
that differs from that of either parent)

44. Meiosis Only diploid cells can divide by meiosis.
Prior to meiosis I, DNA replication occurs.
During meiosis, there will be two nuclear divisions, and the result will be four haploid nuclei.
No replication of DNA occurs between meiosis I and meiosis II.

45. Meiosis
Figure 13.7
Interphase
Homologous pair
of chromosomes
in diploid parent cell
Chromosomes
replicate
Homologous pair of replicated chromosomes
Sister
chromatids
Diploid cell with
replicated
chromosomes 1 2
Homologous
separate
Haploid cells with
replicated chromosomes
Sister chromatids
Haploid cells with unreplicated chromosomes
Meiosis I
Meiosis II
Meiosis reduces the number of chromosome sets from diploid to haploid
Meiosis takes place in two sets of divisions
Meiosis I reduces the number of chromosomes from diploid to haploid
Meiosis II produces four haploid daughter cells

46. Meiosis Phases Meiosis involves the same four phases seen in mitosis
prophase
metaphase
anaphase
telophase
They are repeated during both meiosis I and meiosis II.
The period of time between meiosis I and meiosis II is called interkinesis.
No replication of DNA occurs during interkinesis because the DNA is already duplicated.

47. Prophase I
Prophase I occupies more than 90% of the time required for meiosis
Chromosomes begin to condense
In synapsis, the 2 members of each homologous pair of chromosomes line up side-by-side, aligned gene by gene, to form a tetrad consisting of 4 chromatids
During synapsis, sometimes there is an exchange of homologous parts between non-sister chromatids. This exchange is called crossing over
Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred
Prophase I
of meiosis
Tetrad
Nonsister
chromatids
Chiasma,
site of
crossing
over

48. Metaphase I
At metaphase I, tetrads line up at the metaphase plate, with one chromosome facing each pole
Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad
Microtubules from the other pole are attached to the kinetochore of the other chromosome
Sister
chromatids
Chiasmata
Spindle
Centromere
(with kinetochore)
Metaphase
plate
Homologous
chromosomes
separate
Sister chromatids
remain attached
Microtubule
attached to
kinetochore
Tetrad
PROPHASE I
METAPHASE I
ANAPHASE I
Homologous chromosomes
(red and blue) pair and
exchange segments; 2n = 6
in this example
Pairs of homologous
chromosomes split up
Tetrads line up

49. Anaphase I In anaphase I, pairs of homologous chromosomes separate
One chromosome moves toward each pole, guided by the spindle apparatus
Sister chromatids remain attached at the centromere and move as one unit toward the pole
Sister
chromatids
Chiasmata
Spindle
Centromere
(with kinetochore)
Metaphase
plate
Homologous
chromosomes
separate
Sister chromatids
remain attached
Microtubule
attached to
kinetochore
Tetrad
PROPHASE I
METAPHASE I
ANAPHASE I
Homologous chromosomes
(red and blue) pair and
exchange segments; 2n = 6
in this example
Pairs of homologous
chromosomes split up
Tetrads line up

50. Telophase I and Cytokinesis
In the beginning of telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids
Cytokinesis usually occurs simultaneously, forming two haploid daughter cells
In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms
No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated

51. Prophase II Meiosis II is very similar to mitosis
In prophase II, a spindle apparatus forms
In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate
Cleavage
furrow
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE I AND
CYTOKINESIS
TELOPHASE II AND
Sister chromatids
separate
Haploid daughter cells
forming

52. Metaphase II
At metaphase II, the sister chromatids are at the metaphase plate
Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical
The kinetochores of sister chromatids attach to microtubules extending from opposite poles
Cleavage
furrow
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE I AND
CYTOKINESIS
TELOPHASE II AND
Sister chromatids
separate
Haploid daughter cells
forming

53. Anaphase II At anaphase II, the sister chromatids separate
The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles
Cleavage
furrow
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE I AND
CYTOKINESIS
TELOPHASE II AND
Sister chromatids
separate
Haploid daughter cells
forming

54. Telophase II and Cytokinesis
In telophase II, the chromosomes arrive at opposite poles
Nuclei form, and the chromosomes begin decondensing
Cytokinesis separates the cytoplasm
At the end of meiosis, there are four daughter cells, each with a haploid set of unreplicated chromosomes
Each daughter cell is genetically distinct from the others and from the parent cell
Cleavage
furrow
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE I AND
CYTOKINESIS
TELOPHASE II AND
Sister chromatids
separate
Haploid daughter cells
forming

55. A Comparison of Mitosis and Meiosis
Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell
Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell
The mechanism for separating sister chromatids is virtually identical in meiosis II and mitosis

56. A Comparison of Mitosis and Meiosis
Three events are unique to meiosis, and all three occur in meiosis l:
Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information
At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes
At anaphase I of meiosis, homologous pairs move toward opposite poles of the cell. In anaphase II of meiosis, the sister chromatids separate

57. A Comparison Of Mitosis And Meiosis
Prophase
Duplicated chromosome
(two sister chromatids)
Chromosome
replication
Parent cell
(before chromosome replication)
Chiasma (site of
crossing over)
MEIOSIS I
Prophase I
Tetrad formed by
synapsis of homologous
chromosomes
Metaphase
Chromosomes
positioned at the
metaphase plate
Tetrads
Metaphase I
Anaphase I
Telophase I
Haploid
n = 3
MEIOSIS II
Daughter
cells of
meiosis I
Homologues
separate
during
anaphase I;
sister
chromatids
remain together
Daughter cells of meiosis II n
Sister chromatids separate during anaphase II
Anaphase
Telophase
Sister chromatids
separate during
anaphase 2n
Daughter cells
of mitosis
2n = 6

58. Comparison Mitosis Meiosis Homologous chromosomes do not pair up
No genetic exchange between homologous chromosomes
One diploid cell produces 2 diploid cells or one haploid cell produces 2 haploid cells
New cells are genetically identical to original cell (except for mutation)
Meiosis
DNA duplication followed by 2 cell divisions
Sysnapsis
Crossing-over
One diploid cell produces 4 haploid cells
Each new cell has a unique combination of genes

59. Sexual Reproduction - The Human Life Cycle
Haploid (n)
Diploid (2n)
Haploid gametes (n = 23)
Ovum (n)
Sperm
Cell (n)
MEIOSIS
FERTILIZATION
Ovary
Testis
Diploid
zygote
(2n = 46)
Mitosis and
development
Multicellular diploid
adults (2n = 46)
During fertilization, sperm and ovum fuse forming a diploid zygote
The zygote develops into an adult organism

60. Spermatocytes to Spermatids
Primary spermatocytes undergo meiosis I, forming two haploid cells called secondary spermatocytes
Secondary spermatocytes undergo meiosis II and their daughter cells are called spermatids
Spermatids are small round cells seen close to the lumen of the tubule
Late in spermatogenesis, spermatids are nonmotile
Spermiogenesis – spermatids lose excess cytoplasm and form a tail, becoming motile sperm

61. Spermatogenesis
Figure 27.8b, c

62. Oogenesis Production of female sex cells by meiosis
In the fetal period, oogonia (2n ovarian stem cells) multiply by mitosis and store nutrients
Primordial follicles appear as oogonia are transformed into primary oocytes
Primary oocytes begin meiosis but stall in prophase I
From puberty, each month one activated primary oocyte completes meiosis one to produce two haploid cells
The first polar body
The secondary oocyte
The secondary oocyte arrests in metaphase II and is ovulated
If penetrated by sperm the second oocyte completes meiosis II, yielding:
One large ovum (the functional gamete)
A tiny second polar body

63. Oogenesis