Cell Reproduction Notes

Mitosis: Stage

Events occurring

Interphase

DNA molecule duplicates

46 chromosomes (2n) Prophase

Nucleoli disappear; nuclear membrane breaks down; centrioles migrate to opposite poles; chromosomes appear as pairs of chromatids tightly coiled; spindle forms

46 chromosomes (2n) Metaphase

Chromosomes line up on the spindle at the equator of the cell – centromeres of each pair attach to a spindle fibre

92 chromatids Anaphase

Centromeres divide; chromatids become chromosomes & move to opposite ends of the spindle

92 chromosomes Telophase

two diploid cells (2n)

Spindle disappears; nuclear membranes and nucleoli form; centrioles divide; chromosomes uncoil and disappear; during this phase cytokinesis (cytoplasm divides; each with nucleus) occurs

Mitosis (produces cells for growth and repair) • It is vital that new cells produced must have the same genetic material as the parent cell • Spindle: fibres (microtubules) that radiate out from the poles of a cell Meiosis: 2 divisions (produces gametes for sexual reproduction) • Human cells contain 46 chromosomes • Takes place in sexual reproducing organisms • Each 4 daughter cells have haploid number of chromosomes (23) • Homologous chromosomes: pairs of chromosomes with the same genes • n – the number of different chromosomes = 23 • 2n = 46 chromosomes = in humans • 1st division: members of pairs separate • 2nd division: chromatids separate

Meiosis; First division

Events occurring

Prophase I

Chromosomes become visible; chromosomes gradually move so that homologous pairs lie next to each other; chromosomes shrinks & thickens; spindle forms; paired chromosomes move towards spindle fibres

2n

2n

Metaphase I

Paired chromosomes arrange on spindle fibres across equator

Anaphase I

Each member of the pairs of homologous chromosomes move to opposite poles

Telophase I

n n

Cytokinesis occurs; each daughter cell contain haploid number of chromosomes (23)

Meiosis; Second division

Events occurring

Prophase II

New spindle forms at each end of original spindle; chromosomes gradually move towards equator; nuclear membrane dissolves

Metaphase II

Chromosomes arrange on new spindle; centromeres divide (so that each sister chromatid is now a separate chromosome)

Anaphase II

New chromosomes move to opposite poles

Telophase II

n

n n

n

Cytokinesis occurs; 4 haploid cells are formed