1. Inquiry into Life Eleventh Edition Sylvia S. Mader
Chapter 5
Lecture Outline
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2. 5.1 Cell increase and decrease
Increase and decrease of cell numbers
Mitosis occurs in somatic cells for growth and repair
Meiosis occurs in the reproductive organs for the production of gametes.
Cell division increases number of somatic cells
Mitosis-division of nucleus of cell
Cytokinesis-division of cytoplasm
Occurs throughout life; for growth, development, and repair
Apoptosis- programmed cell death decreases cell number
Occurs throughout life also
Prevents abnormal cells from proliferating

3. Cell increase and decrease, cont’d.
The cell cycle
Set of events that occur between the time a cell divides and the time the resulting daughter cells divide.
Stages of interphase –longest phase of the cycle Most of the cell cycle is spent in interphase.
Normal cell functions occur as well as preparation for division
G1 stage-organelles double in number, accumulates materials needed for division
S stage-DNA replication
G2 stage-synthesis of proteins needed for division

4. The cell cycle

5. Cell increase and decrease, cont’d.
Mitotic stage
Follows interphase
Includes mitosis and cytokinesis
Control of cell cycle – certain checkpoints can stop the cell cycle if abnormalities are present
The protein cyclin must be present for stages to progress
G2 checkpoint-stops cycle if DNA is not done replicating or is damaged
M checkpoint-stops if chromosomes not aligned
G1 checkpoint-protein p53 stops cycle if DNA damaged

6. Control of the cell cycle

7. Cell increase and decrease, cont’d.
Apoptosis – programmed cell death.
Progressive series of events resulting in cell destruction
Cells rounds up, and loses contact with surrounding cells
Nucleus breaks up and cell undergoes fragmentation
Mediated by 2 sets of enzymes called capsases known as the initiators and the executioners.
Initiators are the set that receive the signal to initiate the events and activates the Executioners.
The Executioners are the set that activates enzymes that digest the cell

8. Apoptosis

9. 5.2 Maintaining the chromosome number
Maintaining the chromosome number- terminology
Chromatin-tangled mass of threadlike DNA in nondividing cell
Chromosomes-condensed rod-shaped DNA molecules during division
Diploid (2N) number-characteristic chromosome number, chromosomes in pairs
Haploid (N) number- half the diploid number, found in gametes

10. Maintaining the chromosome number cont’d.
Overview of Mitosis
One division that results in 2 diploid daughter cells identical to the parent cell.
DNA replicates before Nuclear division occurs. Nuclear Division then occurs and chromosome number stays constant. DNA replication produces duplicated chromosomes which are composed of 2 identical sister chromatids held together by a centromere
During mitosis, the centromeres divide and the sister chromatids of each chromosome separate, and become the nuclei of 2 daughter cells identical to the original cell

11. Mitosis overview

12. Maintaining the chromosome number cont’d.
Mitosis in detail-animal cells
Prophase-nuclear membrane disappears, centrosomes migrate, spindle fibers appear
Metaphase-chromosomes line up at equator, associated with spindle fibers
Anaphase-centromeres divide, sister chromatids migrate to opposite poles, cytokinesis begins
Telophase-nuclear membranes form, spindle disappears, cytokinesis occurs

13. Late interphase

14. Phases of animal cell mitosis

17. Maintaining the chromosome number cont’d.
Cytokinesis in animal cells
Cleavage furrow forms between daughter nuclei and contractile ring contracts deepening the furrow until separation is complete

18. Animal cell cytokinesis
Fig 5.8

19. Maintaining the chromosome number cont’d.
Mitosis in plant cells
Occurs in meristematic tissues
Same phases as animal cells except plant cells do not have centrioles or asters
Cytokinesis in plant cells
Flattened, small disk appears between daughter cells forming cell plate which will become new cell wall
Golgi apparatus produces vesicles which move to disk
Release molecules which build new cell walls
Vesicle membranes complete plasma membranes

20. Phases of plant cell mitosis
Fig 5.6

21. Plant cell cytokinesis

22. Maintaining the chromosome number cont’d.
Cell division in prokaryotes-binary fission
Prokaryotes have a single chromosome
Chromosomal replication occurs before division
Cell begins to elongate to twice its length
Cell membrane grows inward until division is complete

23. 5.3 Reducing the chromosome number
Overview of Meiosis
2 divisions resulting in 4 haploid daughter cells not identical to parent cells
Cells are diploid at beginning of meiosis
DNA replicates before Nuclear division occurs.
Pairs of chromosomes are called homologues
Meiosis I
Homologues line up side by side at equator-synapsis
When pairs separate, each daughter cell receives one member of the pair
Cells are now haploid

24. Overview of meiosis
Fig 5.9

25. Reducing the chromosome number cont’d.
Overview of meiosis, cont’d.
Meiosis II
No replication of DNA occurs in this division
Centromeres divide and sister chromatids migrate to opposite poles to become individual chromosomes
Each of the four daughter cells produced has the haploid chromosome number and each chromosome is composed of one chromatid

26. Reducing the chromosome number cont’d.
Meiosis in detail
Meiosis I- genetic recombination occurs in 2 ways
Crossing over-exchange of segments of DNA between homologues
Independent assortment of chromosome pairs

27. Independent alignment
Fig 5.11

28. Synapsis and crossing over
Fig 5.10

29. Reducing the chromosome number cont’d.
Phases of meiosis I
Prophase I
Synapsis occurs, nuclear membrane breaks down
Homologues line up side by side and crossing over occurs
Metaphase I
Homologous pairs line up at equator such that maternal or paternal member may be oriented toward either pole
Anaphase I
Homologous chromosomes (each still consisting of 2 chromatids) undergo independent assortment into daughter cells
Telophase I
Cytokinesis produces 2 daughter cells which are haploid

30. Meiosis I in animal cells

31. Meiosis I in animal cells

32. Reducing the chromosome number cont’d.
Interkinesis-period between meiosis I and meiosis II
Phases of meiosis II
Prophase II-
Cells have 1 member of each homologous pair
Metaphase II
Chromosomes line up at the equator
Anaphase II
Centromeres divide and daughter chromosomes migrate
Telophase II
Nuclei form, cytokinesis

33. Meiosis II in animal cells

34. Meiosis II in animal cells

35. Reducing the chromosome number cont’d.
Nondisjunction-causes various syndromes which result from abnormal chromosome numbers
Failure of homologous chromosomes to separate during anaphase
Failure of sister chromatids to separate during anaphase II
Ex: Down syndrome results from nondisjunction of chromosome 21
Genetic recombination
Promotes genetic variability
Independent alignment of paired chromosomes during metaphase I
Crossing over in prophase I
Both assure that gametes will contain different combinations of chromosomes
When fertilization occurs, the resulting offspring will be genetically unique

36. 5.4 Comparison of meiosis and mitosis
In comparison of meiosis to mitosis note that:
DNA replication occurs only once prior to both
Meiosis requires 2 divisions, mitosis only 1
Meiosis produces 4 daughter cells, mitosis produces 2
Daughter cells from meiosis are haploid, those from mitosis are diploid
Daughter cells from meiosis are genetically variable, while those from mitosis are genetically identical

37. Comparison of meiosis and mitosis cont’d.

40. Comparison of mitosis and meiosis con’td.

41. Comparison of mitosis and meiosis con’td.

42. 5.5 The human life cycle The human life cycle
Requires both mitosis and meiosis
In females meiosis is part of the process of oogenesis
In males meiosis is part of spermatogenesis
At fertilization, the resulting zygote divides by mitosis for the processes of growth and development
Mitosis is used for repair throughout life

43. Life cycle of humans

44. The human life cycle, cont’d.
Spermatogenesis
Begins at puberty and continues throughout life
Occurs in seminiferous tubules of testes
Primary spermatocytes (2n) divide in meiosis I to form 2 secondary spermatocytes (1n)
Secondary spermatocytes divide in meiosis II to produce 4 sperm

45. The human life cycle, cont’d.
Oogenesis
Occurs in the ovaries
Primary oocyte (2n) divides in meiosis I to produce 1 secondary oocyte (1n) and 1 polar body
Division is unequal as secondary oocyte receives most of the cell contents and half the chromosomes
Polar body functions only to receive half of the chromosomes
Secondary oocyte begins meiosis II but stops at metaphase II; polar body may also divide
At puberty, after ovulation secondary oocyte is activated if fertilized to complete division
Meiosis II produces 1 ovum and 1 polar body

46. The human life cycle, cont’d.
Oogenesis, cont’d.
Products of oogenesis are 1 large ovum and up to 3 small polar bodies
Ovum receives nearly all cytoplasm and organelles and half the chromosomes
Polar body gets the remaining half of the chromosomes
Allows ovum to have all the cellular “machinery” it needs for embryonic development

47. Spermatogenesis and oogenesis

48. The human life cycle, cont’d.
Summary
Spermatogenesis and oogenesis both utilize meiosis
Spermatogenesis begins at puberty and continues throughout life
Spermatogenesis produces 4 sperm per primary spermatocyte
Results in production of many sperm
Oogenesis results in 1 oocyte and up to 3 polar bodies per primary oocyte
Divisions are unequal, ovum receives most cell contents
Oogenesis begins prior to birth, stops until puberty, then resumes in a cyclic pattern with cyclic release of oocytes until menopause when the process stops