DESCRIBE THE CELL CYCLE IN HUMANS

 

SUBJECT: PHYSIOLOGY

TOPIC: CELL PHYSIOLOGY

WRITER: GOGO ANDREW MNGUTSWENGA ‘amblo

QUESTION 27

DESCRIBE THE CELL CYCLE IN HUMANS

It is a series of gradual events that take place in a cell between two reproductive phases. It is controlled by DNA, leading to cell division and replication.

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PHASES

 It is broadly divided into two phases

1. Interphase
2. Cell division phase (M)

INTERPHASE

It is the non dividing phase

It constitute of 90% of the cell cycle

It occurs in both somatic cells and germ cells

It comprise of G1, S, G2, Go phases.

Watch cell cycle on YouTube

G1 PHASE

it is the longest phase

the cell grows (increase in size)

there is a check point to ensure that everything needed for DNA synthesis is intact.

S PHASE

Period of DNA replication

G2 PHASE

The cell continue to grow

There is a check point, here to ensure that every material needed for cell division is available.

There is accumulation of energy needed for cell division.

Go PHASE

When the cell is at rest, when it has left the cell cycle and stopped dividing.

CELL DIVISION PHASE

It is the dividing phase

It constitute about 10% of the cell cycle

Comprise of mitosis and meiosis

MITOSIS

It is a multiplication division, where a cell splits into two genetically and phenotypically identical daughter cells. It is divided into prophase, metaphase, anaphase and telophase.

PROPHASE

Chromatin condense and become visible

Centrosomal apparatus also is visible

The nucleolus and nuclear membrane appear

Daughter chromatin are held at the centromere

METAPHASE

Chromosome align at the equator

Complete fragmentation of nuclear material

Centriole come to lie at the opposite poles

ANAPHASE

Spindle fibres retract (shortened)

Daughter chromatin detach from the centromere

Pulling of the separated daughter chromatin to opposite poles

Kinetochore shortens

Non-kinetochore elongate

Cytoplasmic division starts

TELOPHASE

Decondensation of chromosomal apparatus

Cytokinesis begins (formation of cleavage furrow) with the help of endoplasmic reticulum or golgi apparatus.

Reformation of nuclear membrane and cell membrane.

MEIOSIS

It is a reduction division, where a cell splits into four daughter cells that are genetically and physically different. It is divided into meiosis I and meiosis II

MEIOSIS I

It is further divided into prophase I, Metaphase I, anaphase I and telophase I

PROPHASE I 

It is the most complex phase and further divided into

Leptotene : chromosome condense and becomes visible

Zygotene : homologous chromosome come and lie side by side (i.e pairing) 

Pachytene : homologous pair exchange genetic material i.e synapsing

Diplotene : separation of homologous chromosome, without splitting at the centromere

DIAKINESIS : characterized by migration of the equator

Metaphase I

Anaphase I

Telophase I

MEIOSIS II

Same as meiosis I, apart from prophase II

FUNCTION OF CELL CYCLE

Function in cell reproduction

Help to regulate the activities of the cell at various points

Aid in protein synthesis via action of the operon.