6.5a The Stages of Meiosis I Flashcards
What is meiosis?
Meiosis is a form of nuclear division that results in the production of haploid cells from diploid cells
What does meiosis produce?
It produces gametes in plants and animals that are used in sexual reproduction
In terms of stages, how is it different from mitosis?
It has many similarities to mitosis however it has two divisions: meiosis I and meiosis II
Within each division there are the following stages: prophase, metaphase, anaphase and telophase
What happens in prophase I?
DNA condenses and becomes visible as chromosomes
DNA replication has already occurred so each chromosome consists of two sister chromatids joined together by a centromere - the chromosomes are arranged side by side in homologous pairs
A pair of homologous chromosomes is called a bivalent
As the homologous chromosomes are very close together the crossing over of non-sister chromatids may occur. The point at which the crossing over occurs is called the chiasma (chiasmata; plural)
In this stage centrioles migrate to opposite poles and the spindle is formed
The nuclear envelope breaks down and the nucleolus disintegrates
What happens in metaphase I?
The bivalents line up along the equator of the spindle, with the spindle fibres attached to the centromeres
The maternal and paternal chromosomes in each pair position themselves independently of the others; this is independent assortment
This means that the proportion of paternal or maternal chromosomes that end up on each side of the equator is due to chance
What happens in anaphase I?
The homologous pairs of chromosomes are separated as microtubules pull whole chromosomes to opposite ends of the spindle
The centromeres do not divide
What happens in telophase I?
The chromosomes arrive at opposite poles
Spindle fibres start to break down
Nuclear envelopes form around the two groups of chromosomes and nucleoli reform
Some plant cells go straight into meiosis II without reformation of the nucleus in telophase I
What happens during cytokinesis in meiosis in animal cells?
This is when the division of the cytoplasm occurs
Cell organelles also get distributed between the two developing cells
In animal cells: the cell surface membrane pinches inwards creating a cleavage furrow in the middle of the cell which contracts, dividing the cytoplasm in half
The end product of cytokinesis in meiosis I is two haploid cells
What happens during cytokinesis in meiosis in plant cells?
This is when the division of the cytoplasm occurs
Cell organelles also get distributed between the two developing cells
In plant cells, vesicles from the Golgi apparatus gather along the equator of the spindle (the cell plate). The vesicles merge with each other to form the new cell surface membrane and 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 the cell
The end product of cytokinesis in meiosis I is two haploid cells
