and culminates in division. and culminates in - - PDF document

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• Cell division is necessary for reproduction, repair and growth.

Cell division is necessary for reproduction, repair and growth.

• The cell cycle is a continuum of processes undergone by cells

The cell cycle is a continuum of processes undergone by cells during their lifetime, which involves growth and functioning, during their lifetime, which involves growth and functioning, and culminates in division. and culminates in division.

• Mitosis produces two new identical cells.

Mitosis produces two new identical cells.

• Interactions of physical and chemical signals control the

Interactions of physical and chemical signals control the events of the cell cycle. events of the cell cycle.

• Cancer results from abnormal or lacking control signals of the

Cancer results from abnormal or lacking control signals of the cell cycle. cell cycle.

• Meiosis is a special kind of division that produces four (4)

Meiosis is a special kind of division that produces four (4) haploid, non haploid, non-

• identical cells.

identical cells.

• Errors may occur during cell division, producing cells with

Errors may occur during cell division, producing cells with abnormal chromosome number. abnormal chromosome number.

• anaphase
• centromere
• chromatin
• chromosome
• crossing-over
• cytokinesis
• diploid
• G1
• G2
• genome
• haploid
• homologue
• interphase
• kinetochore
• meiosis
• metaphase
• mitosis
• non-disjunction
• prophase
• sister chromatid
• spindle
• synthesis
• telophase
• tetrad
• Roles of Cell Division
• Growth and

development

• Reproduction
• Renewal and

repair

• Types of cell division
• Mitosis

– May have evolved from binary fission in prokaryotes – Method of asexual reproduction in unicellular eukaryotes

• Meiosis

– Responsible for production of gametes in multicellular eukaryotes

• Stages of the cell cycle

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• genome - genetic material of a cell
• chromatin – unorganized mass of DNA

and proteins that condense during cell division

• chromosomes – packaged DNA

molecules in nuclei

– somatic cells have 2 sets of chromosomes (2N, diploid) – gametes have 1 set of chromosomes (N, haploid)

• interphase – preparation for cell

division

– cell grows – DNA is replicated – centrosomes are replicated* – chromosomes condense

• 1 chromosome 2

sister chromatids, connected at a centromere, which separate during cell division

• mitosis – division of

the nucleus

• cytokinesis – division
• f cytoplasm

Mitosis consists of five distinct phases

– Prophase – Prometaphase

G2 OF INTERPHASE PROPHASE PROMETAPHASE Centrosomes (with centriole pairs) Chromatin (duplicated) Early mitotic spindle Aster Centromere Fragments

• f nuclear

envelope Kinetochore Nucleolus Nuclear envelope Plasma membrane Chromosome, consisting

• f two sister chromatids

Kinetochore microtubule Nonkinetochore microtubules

– Metaphase – Anaphase – Telophase

Centrosome at

• ne spindle pole

Daughter chromosomes METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS Spindle Metaphase plate Nucleolus forming Cleavage furrow Nuclear envelope forming

• 1

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

• f 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

• f the parent cell.

2 3 4 5

Nucleus Nucleolus Chromosome Chromatin condensing

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• Events in the cell cycle

triggered and coordinated by a molecular control system

• Checkpoints – critical

control points where stop and go-ahead signals can regulate the cycle

Restriction Point

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• 1. Telomeres

– Repeated DNA sequences at tips of chromosomes – TTAGGG sequences lost every time a cell divides – Restored to their original length by telomerase (normally found in gametes)

• 2.

Regulatory proteins

• Cyclins - concentration

cyclically fluctuates in the cell

• Cyclin-dependent kinases

(Cdks) – activate other

proteins in the presence of cyclin

• e.g. Maturation-promoting

factor (MPF)

– M-phase promoted

• Chromatin condensation
• Mitotic spindle formation
• Degradation of nuclear

envelope

– Deactivated when proteolytic enzymes digest the cyclin

• 3. Growth Factors
• proteins that stimulate other cells to divide
• promote the binding of cyclin to cdks
• ex. platelet-derived growth factor (PDGF)
• 4. Density-dependent

inhibition

• 5. Anchorage dependence
• Most animal cells must be

attached to a substrate before they can grow

• Cells do not heed

Cells do not heed normal signals to normal signals to STOP cell division STOP cell division

• Can invade

Can invade neighboring cells and neighboring cells and interfere with normal interfere with normal body function body function

• “immortal”

“immortal” – – can keep can keep dividing as long as dividing as long as nutrient supply is kept nutrient supply is kept constant constant

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• Numerous diverse

Numerous diverse causes causes

• Cancer cell

Cancer cell

• tumor

tumor

– Benign Benign – Malignant Malignant

• Metastasis

Metastasis

• Oncogenes

– Gene that enables transformation when mutated or expressed in high levels

• Viruses and bacteria

– e.g. HPV and cervical cancer; Hep B and C and liver cancer; H.pylori and stomach cancer

• Ionizing and UV

radiation

• Carcinogens
• Surgery
• Radiation
• Chemotherapy
• Immunotherapy

and Gene therapy

• !"
• Types of reproduction

– Asexual – Sexual

• Genes – hereditary

units of DNA

• Locus – gene’s specific

location in the chromosome

• !"#
• Generation-to-generation

sequence of stages in the reproductive history of an

• rganism
• Homologous chromosomes

– pair that has the same length, centromere position, staining pattern

• Humans: 22 pairs of

autosomes + 1 pair of sex chromosomes

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• Interphase

– G1, S, G2

• Meiosis

– Meiosis I – Meiosis II

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• %&

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• Homologues pair up

tetrad

• Synapsis “crossing-
• ver” that occurs at a

chiasma

• Does not normally

happen to sex chromosomes

• Purpose: to increase

genetic variation

• (

)(*

• Random fertilization

– 1/64 million

• Independent assortment

– 1/223

• Crossing-over

– occurs an average of 2-3 times per chromosome pair

+

1) Meiosis

• Gametes (n) formed from

embryonic primordial germ cells (PGC’s) via meiosis

• PGC’s (2n) meiosis

sex cells (n)

• Spermatogonium and
• ogonium

2) Maturation

– distinctive characteristics

• f sperm and egg cells are

formed

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!,-

SPERMATOGENESIS

process is continuous 100-650 million sperm cells produced

OOGENESIS

unequal cytokinesis time table

• nly 400 oocytes ovulated

between puberty & menopause

!,-

Suspended in prophase I One oocyte / month Halted at metaphase II until fertilization

2N N N

• -+.!!

2 million 1o oocytes in a fetus 1 million 1o oocytes in a newborn (at prophase I) 400,000 1o oocytes during puberty

(meiosis I completed in only one each month)

400 2o oocytes ovulated (at metaphase II) between puberty and menopause

(meiosis II completed only after fertilization)

mature ovum

Questions:

• How many sets of chromosomes are present in each of

the following cell types?

– an oogonium – a 1o spermatocyte – a spermatid – a cell during anaphase I, from either sex – a cell during anaphase II, from either sex – a 2o oocyte – a polar body derived from a 1o oocyte

• Why is it extremely unlikely that a child will be

genetically identical to a parent?

Questions:

• How do the structures of the male and female gametes

aid in their functions?

• A woman who is about 4 weeks pregnant suddenly

begins to bleed and pass some tissue through her

• vagina. After a physician examines the material, he

explains to her that a sperm fertilized a polar body instead of an ovum, and an embryo could not develop. What has happened? Why do you think a polar body cannot support the development of an embryo, whereas an ovum, which is genetically identical to it, can?

' Nondisjunction:

• Pairs of

homologous chromosomes do not separate normally during meiosis

• Gametes contain

two copies or no copies of a particular chromosome

Meiosis I Nondisjunction Meiosis II Nondisjunction Gametes n + 1 n + 1 n 1 n – 1 n + 1 n –1 n n Number of chromosomes Nondisjunction of homologous chromosomes in meiosis I Nondisjunction of sister chromatids in meiosis II (a) (b)

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• Aneuploidy

– Results from the fertilization of gametes in which nondisjunction

• ccurred

– Is a condition in which offspring have an abnormal number of a particular chromosome

• If a zygote is trisomic

– It has three copies of a particular chromosome

• If a zygote is monosomic

– It has only one copy of a particular chromosome

• Klinefelter (XXY), Turner (X0)
• The incidence of Down

syndrome in the general population is about 1 in every 770 births.

• Among women over the age
• f 35 years, however, the

incidence of delivering a child with Down syndrome increases.

• The correlation between

maternal age and Down syndrome risk is striking when the age distribution for all mothers for all mothers is compares to that of mothers

• f Down syndrome children.
• Polyploidy

– Extra sets of chromosomes (3n, 4n, 5n, 6n, 8n, 10n, 12n) – Caused by nondisjunction of all chromosomes

• Rare, usually fatal in

animals

• Common in plants (30-80%)

– Polyploids often thrive better and grow taller – Solution to hybrid sterility – May be preferred because of sterility