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