1. Unislawski – Living Environment
Reproduction
Unislawski – Living Environment

2. The Key Roles of Cell Division
The ability to reproduce is one of the key features that separates life from non-life.
All cells have the ability to reproduce, by making exact copies of themselves.

3. In multicellular organisms, cell division is needed for:
In unicellular organisms, division of one cell reproduces the entire organism
In multicellular organisms, cell division is needed for:
Development of an embryo from a sperm/egg
Growth
Repair

4. LE 12-2 Reproduction Growth and development Tissue renewal 100 µm

5. Asexual Reproduction
Asexual reproduction is reproduction that involves a single parent producing an offspring.
The offspring produced are, in most cases, genetically identical to the single cell that produced them.
Asexual reproduction is a simple, efficient, and effective way for an organism to produce a large number of offspring.
Prokaryotic organisms (like bacteria) reproduce asexually, as do some eukaryotes (like sponges)

6. Sexual Reproduction
In sexual reproduction, offspring are produced by the fusion of two sex cells – one from each of two parents. These fuse into a single cell before the offspring can grow.
The offspring produced inherit some genetic information from both parents.
Most animals and plants, and many single-celled organisms, reproduce sexually.

7. Contrasting Reproduction Types

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9. Cell Duplication
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.

10. Chromosomes
A cell’s endowment of DNA (its genetic information) is called its genome.
DNA molecules in a cell are packaged into chromosomes.

11. Chromosomes
The genetic information that is passed on from one generation of cells to the next is carried by chromosomes.
Every cell must copy its genetic information before cell division begins.
Each daughter cell gets its own copy of that genetic information.
Cells of every organism have a specific number of chromosomes.

12. Prokaryotic Chromosomes
Prokaryotic cells lack nuclei. Instead, their DNA molecules are found in the cytoplasm.
Most prokaryotes contain a single, circular DNA molecule, or chromosome, that contains most of the cell’s genetic information.

13. Eukaryotic Chromosomes
In eukaryotic cells, chromosomes are located in the nucleus, and are made up of chromatin.

14. Chromatin is composed of DNA and histone proteins.

15. DNA coils around histone proteins to form nucleosomes.

16. The nucleosomes interact with one another to form coils and supercoils that make up chromosomes.

17. 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

18. LE 12-4 0.5 µm Chromosome duplication (including DNA synthesis)
Centromere
Sister
chromatids
Separation
of sister
chromatids
Centromeres
Sister chromatids

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20. Phases of the Cell Cycle
The cell cycle consists of
Mitotic (M) phase (mitosis and cytokinesis)
Interphase (cell growth and copying of chromosomes in preparation for cell division)
Interphase (about 90% of the cell cycle) can be divided into subphases:
G1 phase (“first gap”)
S phase (“synthesis”)
G2 phase (“second gap”)

21. INTERPHASE S (DNA synthesis)
LE 12-5
INTERPHASE S
(DNA synthesis) G1
Cytokinesis
Mitosis G2
MITOTIC
(M) PHASE

22. G1 Phase: Cell Growth
In the G1 phase, cells increase in size and synthesize new proteins and organelles.

23. S Phase: DNA Replication
In the S (or synthesis) phase, new DNA is synthesized when the chromosomes are replicated.

24. G2 Phase: Preparing for Cell Division
In the G2 phase, many of the organelles and molecules required for cell division are produced.

25. M Phase: Cell Division
In eukaryotes, cell division occurs in two stages: mitosis and cytokinesis.
Mitosis is the division of the cell nucleus.
Cytokinesis is the division of the cytoplasm.

26. Important Cell Structures Involved in Mitosis
Chromatid – each strand of a duplicated chromosome
Centromere – the area where each pair of chromatids is joined
Centrioles – tiny structures located in the cytoplasm of animal cells that help organize the spindle
Spindle – long proteins (part of the cytoskeleton) that the centrioles produce
Helps move the chromosomes into place.

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29. Prophase
During prophase, the first phase of mitosis, the duplicated chromosome condenses and becomes visible.

30. Prophase
The centrioles move to opposite sides of nucleus and help organize the spindle.

31. Prophase
The spindle forms and DNA strands attach at a point called their centromere.

32. Prophase
The nucleolus disappears and nuclear envelope breaks down.

33. Metaphase
During metaphase, the second phase of mitosis, the centromeres of the duplicated chromosomes line up across the center of the cell.

34. Metaphase
The spindle fibers connect the centromere of each chromosome to the two poles of the spindle.

35. Anaphase
During anaphase, the third phase of mitosis, the centromeres are pulled apart and the chromatids separate to become individual chromosomes.

36. Anaphase
The chromosomes separate into two groups near the poles of the spindle.

37. Telophase
During telophase, the fourth and final phase of mitosis, the chromosomes spread out into a tangle of chromatin.

38. Telophase
A nuclear envelope re-forms around each cluster of chromosomes.

39. Telophase
The spindle breaks apart, and a nucleolus becomes visible in each daughter nucleus.

40. Cytokinesis Cytokinesis is the division of the cytoplasm.
The process of cytokinesis is different in animal and plant cells.

41. Cytokinesis in Animal Cells
The cell membrane is drawn in until the cytoplasm is pinched into two equal parts.
Each part contains its own nucleus and organelles.

42. LE 12-9a
100 µm
Cleavage furrow
Contractile ring of
microfilaments
Daughter cells
Cleavage of an animal cell (SEM)

43. Cytokinesis in Animal Cells
In plants, the cell membrane is not flexible enough to draw inward because of the rigid cell wall.
Instead, a cell plate forms between the divided nuclei that develops into cell membranes.
A cell wall then forms in between the two new membranes.

44. LE 12-9b
Vesicles
forming
cell plate
Wall of
parent cell
1 µm
Cell plate
New cell wall
Daughter cells
Cell plate formation in a plant cell (TEM)

45. LE 12-10 Chromatin condensing Nucleus Chromosomes Cell plate 10 µm
Nucleolus
Prophase. The
chromatin is condensing.
The nucleolus is beginning to disappear.
Although not yet visible in the micrograph, the mitotic spindle is starting to form.
Prometaphase. We
now see discrete chromosomes; each
consists of two identical sister chromatids. Later
in prometaphase, the
nuclear envelope 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 the cell as their kinetochore micro-
tubules shorten.
Telophase. Daughter nuclei are forming. Meanwhile, cytokinesis has started: The cell plate, which will divide the cytoplasm in two, is growing toward the perimeter of the parent cell.

46. LE 12-6ca
INTERPHASE
PROPHASE
PROMETAPHASE

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48. Binary Fission
Prokaryotes (bacteria and archaea) reproduce by a type of cell division called binary fission
In binary fission, the chromosome replicates (beginning at the origin of replication), and the two daughter chromosomes actively move apart

49. Chromosome replication begins. Soon thereafter,
LE 12-11_1
Cell wall
Origin of
replication
Plasma
membrane
E. coli cell
Bacterial
chromosome
Chromosome replication begins. Soon thereafter,
one copy of the origin moves rapidly toward the other end of the cell.
Two copies
of origin

50. Chromosome replication begins. Soon thereafter,
LE 12-11_2
Cell wall
Origin of
replication
Plasma
membrane
E. coli cell
Bacterial
chromosome
Chromosome replication begins. Soon thereafter,
one copy of the origin moves rapidly toward the other end of the cell.
Two copies
of origin
Origin
Origin
Replication continues. One copy of the origin is now at each end of the cell.

51. LE 12-11_3 Cell wall Origin of replication Plasma membrane
E. coli cell
Bacterial
chromosome
Chromosome replication begins.
Soon thereafter,
one copy of the origin moves rapidly toward the other end of the cell.
Two copies
of origin
Origin
Origin
Replication continues. One copy of the origin is now at each end of the cell.
Replication finishes.
The plasma membrane grows inward, and
new cell wall is deposited.
Two daughter
cells result.

52. The Evolution of Mitosis
Since prokaryotes evolved before eukaryotes, mitosis probably evolved from binary fission
Certain protists exhibit types of cell division that seem intermediate between binary fission and mitosis

53. LE 12-12 Bacterial chromosome Prokaryotes Chromosomes Microtubules
Intact nuclear
envelope
Dinoflagellates (Type of plankton)
Kinetochore
microtubules
Intact nuclear
envelope
Diatoms (Type of Algae)
Kinetochore
microtubules
Centrosome
Fragments of
nuclear envelope
Most eukaryotes

54. The Cell Cycle Control System
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 checkpoints where the cell cycle stops until a go-ahead signal is received

55. G1 checkpoint Control system S G1 G2 M M checkpoint G2 checkpoint
LE 12-14
G1 checkpoint
Control
system S G1 G2 M
M checkpoint
G2 checkpoint

56. For many cells, the G1 checkpoint seems to be the most important one
If a cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the S, G2, and M phases and divide
If the cell does not receive the go-ahead signal, it will exit the cycle, switching into a nondividing state called the G0 phase

57. LE 12-15 G0
G1 checkpoint G1 G1
If a cell receives a go-ahead signal at the G1 checkpoint, the cell continues on in the cell cycle.
If a cell does not receive a go-ahead signal at the G1 checkpoint, the cell exits the cell cycle and goes into G0, a nondividing state.

58. An example of external signals is density-dependent inhibition, in which crowded cells stop dividing
Most animal cells also exhibit anchorage dependence, in which they must be attached to a substratum (connective tissue) in order to divide

59. Cells anchor to dish surface and divide (anchorage dependence).
LE 12-18a
Cells anchor to dish surface and
divide (anchorage dependence).
When cells have formed a complete
single layer, they stop dividing
(density-dependent inhibition).
If some cells are scraped away, the
remaining cells divide to fill the gap and
then stop (density-dependent inhibition).
25 µm
Normal mammalian cells

60. Cancer cells do not exhibit anchorage dependence
LE 12-18b
Cancer cells do not exhibit
anchorage dependence
or density-dependent inhibition.
25 µm
Cancer cells

61. Loss of Cell Cycle Controls in Cancer 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

62. LE 12-19 Lymph vessel Tumor Blood vessel Glandular tissue Metastatic
Cancer cell
A tumor grows from a
single cancer cell.
Cancer cells invade
neighboring tissue.
Cancer cells spread
through lymph and
blood vessels to
other parts of the
body.
A small percentage
of cancer cells may
survive and establish
a new tumor in another
part of the body.

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64. Meiosis

65. Chromosomes in Human Cells
Somatic cells include all cells in the human body except sperm and eggs.
Gametes are human sperm and egg cells.
Each human somatic cell has 23 pairs of chromosomes, 46 total.
Each pair of chromosomes are called homologous chromosomes.
Each homologous chromosome carries a copy of the same genes, either from the father or mother.

66. LE 13-3
Pair of homologous
chromosomes
5 µm
Centromere
Sister
chromatids
This is called a karyotype. All 23 pairs of homologous chromosomes are lined up.

67. Key Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of
LE 13-4
Key
Maternal set of
chromosomes (n = 3)
2n = 6
Paternal set of
chromosomes (n = 3)
Two sister chromatids
of one replicated
chromosomes
Centromere
Two nonsister
chromatids in
a homologous pair
Pair of homologous
chromosomes
(one from each set)

68. The sex chromosomes are called X and Y
Human females have two X chromosomes.
Human males have one X and one Y chromosome
The 22 pairs of chromosomes that do not determine sex are called autosomes.

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70. Inheritance of Genes
A gene is a unit of heredity that carries the information for a specific trait or body function.
A gene is made of a segment of DNA.
Each gene is located on a specific chromosome.
Everyone has two copies of each gene (one on each homologous chromosome).

71. A cell with a full pair of each chromosome is called diploid.
Diploid is written shorthand as 2n.
All somatic cells are diploid (46 chromosomes).
A cell with only one of each homologous chromosome is called haploid.
Haploid is written shorthand as n.
All gametes are haploid and have 23 total chromosomes.

72. Gametes are haploid cells, containing only one set of chromosomes
For humans, this means 23 total chromosomes (no pairs)
This includes 22 autosomes and a single sex chromosome
In an unfertilized egg (ovum), the sex chromosome is always X
In a sperm cell, the sex chromosome may be either X or Y

73. Chromosomes and the Human Sex Cycle
At sexual maturity, the ovaries and testes begin producing sperm and eggs through meiosis.
Gametes are the only types of human cells produced by meiosis, rather than mitosis
Meiosis is a form of cell division that results in one set of chromosomes in each gamete instead of two.
The resulting daughter cells are haploid.
When fertilization occurs, the haploid sperm and haploid egg fuse together to form a diploid embryo.

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75. Interphase
At the end of interphase, each cell has grown into its full size, produced a full set of organelles, and duplicated its DNA.
The cell is diploid at this point.
The nucleus contains 23 homologous chromosome pairs.
Each chromosome is made of two sister chromatids (copies).

76. Prophase I
The cells begin to divide, and the chromosomes pair up, forming a structure called a tetrad, which contains four chromatids.

77. Prophase I
As homologous chromosomes pair up and form tetrads, they undergo a process called crossing-over.
First, the chromatids of the homologous chromosomes overlap each other.
Then, the crossed sections of the chromatids are exchanged.
Crossing-over is important because it produces new combinations of genes in the cell.

78. Metaphase I
As prophase I ends, a spindle forms and attaches to each tetrad.
During metaphase I of meiosis, paired homologous chromosomes line up across the center of the cell.

79. Anaphase I
During anaphase I, spindle fibers pull each homologous chromosome pair toward opposite ends of the cell.
When anaphase I is complete, the separated chromosomes cluster at opposite ends of the cell.

80. Telophase I and Cytokinesis
During telophase I, a nuclear membrane forms around each cluster of chromosomes.
Cytokinesis follows telophase I, forming two new cells.

81. Summary of Meiosis I Two new haploid cells have been produced.
Each haploid cell contains one chromosome out of the original pair.
Each chromosome still contains two sister chromatids.

82. Prophase II
As the cells enter prophase II, their chromosomes—each consisting of two chromatids—become visible.
The chromosomes do not pair to form tetrads, because the homologous pairs were already separated during meiosis I.

83. Metaphase II
During metaphase of meiosis II, chromosomes line up in the center of each cell.

84. Anaphase II
As the cell enters anaphase, the paired chromatids separate.

85. Telophase II and Cytokinesis
The two daughter cells from Meiosis I divide, resulting in four daughter cells, each with two chromatids.
These four daughter cells now contain the haploid number (N)—just two chromosomes each.

86. Summary of Meiosis II A total of four cells have been produced.
Each cell is haploid and only contains one out of the original pairs of homologous chromosomes.
Each chromosome only contains a single chromatid.

87. A Comparison of Mitosis and Meiosis
Mitosis produces cells that are genetically identical to the parent cell.
Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid).
Meiosis allows crossing over of chromosomes.
This produces cells that are genetically different from the parents and each other.

88. 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, it is homologous chromosomes, instead of sister chromatids that separate and are carried to opposite poles of the cell

89. LE 13-9 Propase MITOSIS MEIOSIS Parent cell
(before chromosome replication)
Chiasma (site of
crossing over)
MEIOSIS I
Propase
Prophase I
Chromosome
replication
Chromosome
replication
Tetrad formed by
synapsis of homologous
chromosomes
Duplicated chromosome
(two sister chromatids)
2n = 6
Chromosomes
positioned at the
metaphase plate
Tetrads
positioned at the
metaphase plate
Metaphase
Metaphase I
Anaphase
Sister chromatids
separate during
anaphase
Homologues
separate
during
anaphase I;
sister
chromatids
remain together
Anaphase I
Telophase
Telophase I
Haploid
n = 3
Daughter
cells of
meiosis I 2n 2n
MEIOSIS II
Daughter cells
of mitosis n n n n
Daughter cells of meiosis II
Sister chromatids separate during anaphase II

90. Synapsis and crossing over Daughter cells, genetic composition
Mitosis
Meiosis
DNA replication
During interphase
Divisions
One
Two
Synapsis and crossing over
Do not occur
Form tetrads in prophase I
Daughter cells, genetic composition
Two diploid, identical to parent cell
Four haploid, different from parent cell and each other
Role in animal body
Produces cells for growth and tissue repair
Produces gametes

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92. Genetic Variation Among Offspring
The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation
Three mechanisms contribute to genetic variation:
Independent assortment of chromosomes
Crossing over
Random fertilization

93. Independent Assortment of Chromosomes
In independent assortment, each pair of chromosomes sorts maternal and paternal homologous chromosomes into daughter cells independently of the other pairs.
Example:
One human sperm cell could contain 15 chromosomes from his father, and 8 from his mother
Another contains 20 from the mother, 3 from the father.

94. LE 13-10
Key
Maternal set of
chromosomes
Possibility 1
Possibility 2
Paternal set of
chromosomes
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Daughter
cells
Combination 1
Combination 2
Combination 3
Combination 4

95. Independent Assortment of Chromosomes
The number of combinations possible when chromosomes assort independently into gametes is calculated by 2n, where n is the haploid number
For humans (n = 23):
223 = 8,388,608 possible combinations!

96. Crossing Over
Crossing over produces new chromosomes with a mixture of genes from each parent.
Instead of a chromosome that is 100% from the person’s father or mother, it might now be 95% from the father, 5% from the mother.

97. LE 13-11 Prophase I Nonsister of meiosis chromatids Tetrad Chiasma,
site of
crossing
over
Metaphase I
Metaphase II
Daughter
cells
Recombinant
chromosomes

98. Random Fertilization
Random fertilization adds to genetic variation because any sperm can fuse with any egg.

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100. Genetic Diversity
How many possible combinations of genes are there from two parents?
Independent assortment:
223 = 8,388,608 combinations of chromosomes in each sperm or egg cell.
Random assortment:
8.4 million possible sperm combinations
+ 8.4 million possible egg combinations
= 16.8 trillion possible embryos

101. Genetic Diversity
How many possible combinations of genes are there from two parents?
Crossing over
Average of 1,000 genes in each chromosome
At the most, about half of the chromosome can cross over to its homologous partner.
This results in 3.3 novemquardragintillion (1 followed by 150 zeros) gene combinations for each chromosome pair crossing over.

102. Genetic Diversity
How many possible combinations of genes are there from two parents?
Total
3.3 novemquardragintillion possible chromosome combinations
x 23 chromosomes
x 16.8 trillion possible sperm-egg combinations
=1.3 quinquinquagintillion (1 followed by 168 zeros) possible different genetic combinations for two people.