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Inquiry into LifeEleventh Edition

Sylvia S. MaderChapter 5

Lecture Outline

Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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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

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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

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The cell cycle

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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

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Control of the cell cycle

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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

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Apoptosis

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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

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Maintaining the chromosome number cont’d.

• Overview of MitosisOne 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

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Mitosis overview

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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

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Late interphase

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Phases of animal cell mitosis

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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

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Animal cell cytokinesis

• Fig 5.8

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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

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Phases of plant cell mitosis

• Fig 5.6

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Plant cell cytokinesis

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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

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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

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Overview of meiosis

• Fig 5.9

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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

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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

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Independent alignment

• Fig 5.11

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Synapsis and crossing over

• Fig 5.10

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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

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Meiosis I in animal

cells

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Meiosis I in animal

cells

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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

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Meiosis II in animal cells

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Meiosis II in animal cells

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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

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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

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Comparison of meiosis and mitosis cont’d.

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Comparison of mitosis and meiosis con’td.

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Comparison of mitosis and meiosis con’td.

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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

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Life cycle of humans

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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

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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

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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

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Spermatogenesis and oogenesis

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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