Cell Cycle

MitosisMeiosis

Genetic recombinationUncontrolled mitosis

Cell Cycle

• Life of a cell from time formed until division into 2 NEW cells

• Ends with division into 2

• Interphase covers most of cell cycle, followed by either mitosis or meiosis

• Last average 24 hours

Interphase

• 90% of cell cycle• G1 (First gap) phase• S (Synthesis) phase• G2 (Second gap) phase

• Low division or no division cells = G0

Gap phase : Division of organelle, proteins and cytoplasm material

Synthesis phase : Replication of chromosomes

G0 phase : Maintenance of cell condition for optimum function : Little to no growth

G0 Phase

• Most cells G0 phase

• If no cell signal to go G1 phase, then default is G0 phase

• Can be permanent : Muscle cells and nerve

• Can be temporary : Hepatocytes when molecular signal given

• Loss of control = Cancer

Chromosomes and Names!!

• Chromatin : Chromosome + proteins when NOT condensed. During non-division

• Chromosome : Condensed chromatin

• Chromatid : One of the chromosome copy after S phase : Comes in pairs joined at centromere (sister chromatids)

• Homolog pairing : Pairing up of chromosomes of the same type

Chromosomes• Humans have 23 pairs of chromosome• Made of 1 short arm and long arm, center

separated by centromere containing kinetochore proteins

• Protein (histones) + DNA soil into nucleosome, coiled and condensed.

Karyotype!!• 22 pairs somatic chromosome / autosomes• 1 pair sex chromosome

Centrosome• Microtubule organizing center (MTOC)• Centrioles arranged at 90 degrees to

each other within amorphous protein material (pericentriolar material - PCM)

> Centriole guides formation of microtubules > PCM contains microtubule building material

What the heck?!

• Centriole

• Centrosome

• Centromere

Mitosis

• Process of equal cell division, diploid 2n=46 where n = set of chromosomes

• Division of nucleus followed by cytoplasmic division• Occur in somatic cells1) Prophase2) Prometaphase3) Metaphase4) Anaphase5) Telophase Cytokinesis (Cytoplasm

movement)

1) Prophase

• Chromatin fibers condense, thickens - Visible as chromatid pairs joined at centromere• Nucleoli disappears• Mitotic spindle (spindle fiber) form from

centriole (with centriole center) > Kinetochore spindle > Non-kinetochore spindle• Centrosome starts moving to opposite poles

2) Prometaphase

• Nuclear membrane fragmentates• Chromosomes even more

condensed• Kinetochore spindle fiber move

into nuclear space, binds centromere kinetochore

> Sister chromatids move around by spindle fibers• Non-kinetochore spindles from

opposite poles interact

3) Metaphase

• Centrosome (centriole) arrive at opposite poles

• Centriole movement pulls chromatids to metaphse plate in the middle of nuclear space

• Each sister chromatid kinetochore attached to spindle fiber from opposite poles

4) Anaphase

• Proteins holding the sister chromatids are cleaved, allowing separation Chromosomes

• Kinetochore spindle fiber shortens, pulling chromosomes to each side centromere first

• Cell elongates as non-kinetochore spindle elongates

5) Telophase

• Nuclear membrane fragments shared among 2 cells, form 2 nuclear membrane

• Nucleoli reappear• Chromosome decondense• Spindle fibers depolymerise

• MITOSIS COMPLETE!!

Cytokinesis

• Pinching of 2 nucleus cell into 2 separate cells• Animals : Cleavage furrow formation - Actin and myosin form contractile ring, divide cell into 2 • Plants : Wall vesicles (phragmoplast) with cell wall material collect in middle of cell : Fuse, form cell plates Cell wall

Meiosis

• Division of sex cells to produce “half cells” or haploid n=23

- Chromosomes do not come in pairs • Form sex gametes or reproductive cells• During fertilization, fusion forms diploid (2n)

cells• Consists of 2 meiosis division phases (meiosis I

and II)

Meiosis I

• Prophase I

• Metaphse I

• Anaphase I

• Telophase I

• Cytokineses

Prophase I

• Chromosome begins to condense and homologous chromosomes pair loosely aligned gene by gene

• Homologs fuse at synapse (via synapsis) by a synaptic protein complex

> Crossing over and genetic exchange (genetic recombination) occur at sister chromatids at synapse > Synaptic protein complex disassemble after exchange sister chromatids still held loosely at chiasmata• Centrosome movement, nuclear fragmentation• Spindle forms, attach to kinetochore and moves

homologous pairs to metaphase plate

What the heck?!

• Crossing over

• Synapse

• Synaptic protein complex

• Chiasmata

Metaphase I

• Arrangement of homolog pair at metaphase plate

• Each homolog with sister chromatids attached to one centromere

Anaphase I

• Proteins holding chiasmata structure together broken down, separates homolog chromosome

• Spindle fibers shorten, homologs move towards opposite poles

P/S : Sister chromatids still attached to each other

Telophase I

• Nuclear membrane formation occur/doesn’t• Chromosomes decondense or not• Nucleoli forms

• Cytokinesis occur

Cells are haploid even thought each chromosome has 2 sister chromatids

Prophase II

• Spindle formation

• Spindle attach at centromere kinetochore

• Centrosome move start moving to opposite poles, moving chromatid pairs to metaphase plate

Metaphase II

• Metaphase plate arrangement of chromatid sisters

• Spindle fibers of opposite poles attach to each sister chromatid

Anaphase II

• Proteins holding sister chromatids broken down

• Chromatids separate on spindle shortening

• Chromatids move in opposite direction

Telophase II

• Nuclear membrane forms• Chromosome decondense• Spindle fibers depolymerize• Cytokinesis

Cells are now true haploid (n), 23 chromosomes

Need for telophase I and prophase II

Is it needed??? Discuss..

Genetic Variation in Meiosis

• Crossing over at chiasmata for genetic exchange during prophase I

• Random arrangement of homologous pairs at metaphase I and II

> Segregation of father mother chromatid pairs randomly > Segregation of sister chromatids randomly (yes or no synapse?)

• Crossing over:1) Contact and physically stick

together by cohesin2) Chromosome sections at

chiasmata break and recombine (genetic recombination)

3) Cohesin dissolved and crossing over ends

Uncontrolled Mitosis

• What if everything goes into G1 phase for fun..• CANCER!!• Cancer : Uncontrolled cell division which forms abnormal cells

• Metastasis : Movement of cancer cells from one site to another non-adjacent site : Usually via blood and lymph : Well vascularised areas affected easily : Eg Lung cell tumors found in brain

Cancer Therapy

• Surgery• Radiotherapy• Chemotherapy Guess what drugs.. Angiogenesis inhibitors• Immunotherapy – Anti-self vaccine• Hormone treatment eg: estrogen dependent breast cancer• Treat symptoms (Palliative care)