Meiosis

What happens in prophase 1? (1)

DNA condenses and becomes visible as chromosomes

DNA replication already occurred so each chromosome consists of 2 sister chromatids joined by a centromere

What happens in prophase 1? (2)

Chromosomes arrange side by side in homologous pairs (1 pair= a bivalent)

Crossing over of non-sister chromatids may occur, this is called chiasma

What happens in prophase 1? (3)

Centrioles migrate to opposite poles and the spindle is formed, nuclear envelope breaks down and nucleolus disintegrates

What happens in prophase 2?

Nuclear envelope breaks down and chromosomes condense, a spindle forms at a right angle to the old one

What happens in metaphase 1?

Bivalents line up along the equator of the spindle, with the spindle fibres attached to the centromeres

Maternal+paternal chromosomes in each pair position themselves independently of each other (independent assortment)

What controls the proportion of maternal/paternal chromosomes that end up on each side of the equator in metaphase 1?

It is due to chance

What happens in metaphase 2?

Chromosomes line up in single file along the equator of the spindle

What happens in anaphase 1?

Homologous pairs of chromosomes are separated as microtubules pull whole chromosomes to opposite ends of the spindle, centromeres do not divide

Wha happens in anaphase 2?

Centromeres divide and individual chromatids are pulled to opposite poles, this creates 4 groups of chromosomes w/ half the number of chromosomes in the parent cell

What happens in telophase 1?

Chromosomes arrive at opposite poles, spindle fibres start to break down, nuclear envelope form around the 2 groups of chromosomes and nucleoli reform

What do some plants not do in telophase 1?

They go straight into meiosis 1 without reformation of the nucleus in telophase 1

What happens in telophase 2?

Nuclear membranes form around each group of chromosomes

What happens in cytokinesis 1?

Division of cytoplasm, cell organelles get distributed between 2 developing cells, end product is 2 haploid cells

What happens in animal cells in cytokinesis 1?

Cell surface membrane pinches inwards creating a cleavage furrow in the middle of the cell which contracts, dividing the cytoplasm in half

What happens in plant cells in cytokinesis 1? (1)

Vesicles from the Golgi apparatus gather along the equator of the spindle, the vesicles merge to form cell surface membrane

What happens in plant cells in cytokinesis 1? (2)

Vesicles also secrete a layer of calcium pectate which becomes the middle lamella. Layers of cellulose are laid upon the middle lamella to form the primary and secondary walls of cell

What happens in cytokinesis 2?

Cytoplasm divides as new cell surface membranes are formed, creating 4 haploid cells