Answer the following questions

Cell Cycle

11. Write any three significance of mitosis

(1) Genetic stability - daughter cells are genetically identical to parent cells

(2) Growth - as multicellular organisms grow the number of cells making up their tissues increases - the new cells must be identical to the parent Eg. Yeast and Amoeba

(3) Repair of tissues - damages cells must be replaced by identical new cells by mitosis

12. Differentiate between mitosis and meiosis

Mitosis

• One division

• Number of chromosomes remains the same

• Homologous chromosomes line up separately on the metaphase plate

• Homologous chromosome do not pair up

• Chiasmata do not form and crossing over never occurs

• Daughter cells are genetically identical

• Two daughter cells are formed

Meiosis

• Two division

• Number of chromosomes is halved

• Homologous chromosomes line up in pairs at the metaphase plate

• Homologous chromosome pairup to form bivalent

• Chiasmata form and crossingover occurs

• Daughter cells are genetically different from the parent cells

• Four daughter cells are formed

13. Given an account of G0 phase

Sometime some cells exit G₁ and enters a quiescent stage called G_O.

• Cells are metabolically active

• No proliferation & no growth

• reduced rate of RNA and protein synthesis

• can remain in this stage for long period

• But this G_O is not permanent

Exception - Mature neuron & Skeletal muscles

• This G_O stage (quiescent stage) will get activated (because G_O cells are not dormant after getting appropriate growth factors or other extracellular signals.

• This idea is exploited in cloning experiments

Eg - Development of Dolly

14. Differentiate cytokinesis in plant cells and animal cells

Cytokinesis in Animals

• It is a contractile process

• A contractile ring is located at the plasma membrane

• A bundle of micro filaments assemble from actin and myosin generating a contractile force to draw the contractile ring inward forming a cleavage furrow dividing the cell into two

Cytokinesis in Plants

• Cell plate grows from centre, towards perphery in a centrifugal manner

• Golgi apparatus & ER produce

^|→ micro tubule

^|→ acting flament

^|→ Vesicles

• Golgi vesicles contain

^|→ Pectin

^|→ hemicellulose

• Microtubule of phragmoplast + Pectin & hemicellulose - Move

along to the equator fusing and forming a new plasma membrane the newly forming cell has cell plate eventually stretches across forming middle lamella

Cellulose deposits on each side form cell walls of 2 new cells.

Steps: phragmoplast → cell plate → middle lamella → cell wall

15. Write about Pachytene and Diplotene of Prophase I

Prophase I – longer 5 subtages

1. Leptalene

2. Zygolene

3. Pachytene

4. Diplotene

5. Diakinesis

3. Pachytene stage

• Chromosome appear as bivalent or tetrads

• 4 chromatids & 2 centromeres are seen

• Synapsis of homologous chromasomes between non sister chromatids completes except at chiasmata where crossing over occurs

• Recombination (exchange of chromosomal bits is completed by the end) - but chromosomes are linked at the sites of crossing over

• Enzyme - Recombinase mediates the process.

4. Diplotene

• Synaptonemal complex disassembled & dissolves

• Non sister chromatids of homologous chromosomes get attached where x like shape occur at Crossing over known as chiasmata holding the homologous chromosomes together the homologous chromosomes tend to separate except at chiasmata

• The sub stage last for days or years depending on the sex & organism

• follows Pachytene

• synaptical complex disassembled & dissolves 

• The chromosomes are actively transcribed in females as the eggs stores upmaterials for embryonic development

• Exception In Lamp brush chromosome prominent loops occur.