The Cell Cycle

The Cell Cycle

The Cell Cycle  A. The Role of Cell Division   Why do cells divide?   • Growth • Reproduction • Replacement of dying cells – skin, RBC • Reproduction in multi-cellular organisms – gamete formation (meiosis) In the case of growth, why divide, rather than simply get bigger? • Surface:volume ratio constraints

The Cell Cycle 1. 2. 3. 6. 7.

G1 Phase  1st growth phase S Phase  DNA duplicated G2 Phase  Final growth phase Mitosis Cytokinesis

Purpose of the first three phases (Interphase) – to duplicate cell contents; 90% of the cell’s growth cycle Purpose of Mitosis – to divide the genetic material into exact two halves Purpose of Cytokinesis – to divide all other contents (except nucleus) into two cells

Mitosis in an onion root

Binary fission in bacteria Not the best method of chromosome division – no microtubule and there is no division in exact halves of all genetic material. Bacteria have a single chromosome (versus the 46 human have).

Fig. 19.1

Important Notes DNA wraps some special proteins to form more stable structure called chromosomes Chromosome are found inside nucleus Human - 46 chromosomes, 23 pairs (1 set of 23 from egg, 1 set of 23 from sperm) Gene is a segment of DNA that is responsible for controlling a trait Each chromosome contains thousands of genes (these make up our traits)

Human female chromosomes

Human female karyotype

Human male chromosomes

Human male karyotype

Types of cell division • Mitosis: – Growth, development & repair – Asexual reproduction (yields identical cells) – Occurs in somatic (body) cells • Meiosis: – Sexual reproduction (yields different cells) – Occurs in specific reproductive cells

Process of mitosis   Aim: Two identical daughter cells exact copy of mother cell formed Requirements – the whole DNA must be copied exactly

Phases of Mitosis 1. Prophase 2. Prometaphase 3. Metaphase 4. Anaphase 5. Telophase

• Prophase -

Chromatin condenses, this causes the chromosomes to begin to become visible

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Centrosomes separate, moving to opposite ends of the nucleus

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The centrosomes start to form a framework used to separate the two sister chromatids called the mitotic spindle, that is made of microtubules

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Nucleolus disappears

  2. Prometaphase - Nuclear envelope fragments - Chromosomes become more condensed - A kinetochore is formed at the centromere, the point where the sister chromatids are attached - Microtubules attach at the kinetochores

3. Metaphase - Chromosomes align on an axis called the metaphase plate - Note: the spindle consists of microtubules, one attached to each chromosome

1. Anaphase - Each centromere splits making two chromatids free - Each chromatid moves toward a pole - Cell begins to elongate, caused by microtubules not associated with the kinetochore

5. Telophase • Formation of nuclear membrane and nucleolus • Short and thick chromosomes begin to elongate to form long and thin chromatin • Formation of the cleavage furrow - a shallow groove in the cell near the old metaphase plate • Cytokinesis = division of the cytoplasm

Mitosis

Mitosis in an onion root

The human life cycle

Three sexual life cycles differing in the timing of meiosis and fertilization

Preparation of a human karyotype

Meiosis • Type of cell division that halves number of chromosomes (e.g., 2N to 1N) • Process of 2 successive divisions • Product is gamete, essential for sexual reproduction

Overview of meiosis: how meiosis reduces chromosome number

The results of alternative arrangements of two homologous chromosome pairs on the metaphase plate in meiosis I

The results of crossing over during meiosis

A comparison of mitosis and meiosis

A comparison of mitosis and meiosis: summary

Evolutionary advantage • asexual reproduction (mitosis) – easy, rapid, effective way to reproduce – useful in stable environment – lack of genetic diversity among offspring • sexual reproduction (meiosis) – promotes genetic variability – useful in dynamic environment