Meiosis

Front 1

First step in Meiosis

Back 1

Interphase- DNA is replicated (each chromosome forms 2 chromatids held by a centromere). Forms bivalent

Front 2

Second step in Meiosis I (prophase)

Back 2

Chromatin condenses into visible chromosomes by a light microscope. The cell forms 2 centromeres and later form mitotic spindles (spindle fibres). Centromeres move to opposite poles of the cell.

Front 3

Next steps of Prophase (Meiosis I)

Back 3

Diploid cell forms when homologous chromosomes pair up (synapsis). There are parental (dad) and maternal (mum) chromosomes. The nuclear envelope also breaks down.

Front 4

Next steps of prophase (Meiosis I)

Back 4

Homologous chromosomes exchange genetic material (by crossing over). The point that they do this is a chiasmata. Also the microtubules attach to chromosomes at the centromere.

Front 5

Third step in Meiosis I (Metaphase)

Back 5

Homologous chromosomes line up at the equator, one pair on either side. Independent assortment occurs (arrangement of pairs on either side is random). So all maternal chromosomes may not be on the same side, they line up randomly)

Front 6

Fourth step in Meiosis I (Anaphase)

Back 6

Microtubules contract, chromosomes from each pair move to opposite poles. Centromeres don't divide so each chromosome is still 2 sister chromatids and due to independent assortment it is not necessarily identical

Front 7

Fifth step in Meiosis I (Telophase)

Back 7

Chromosomes decondense, nuclear membrane reforms, cytoplasm divides and there is interkinesis (short period of no action)

Front 8

First step in Meiosis II (Prophase)

Back 8

Chromatin condenses again into discrete chromosomes. The cell are still sister chromatids (have haploid number of chromosomes)

Front 9

Second step in Meiosis II (Metaphase)

Back 9

These chromosomes line up at the equator, the microtubules from opposite poles attach to the sister chromatids(each one)

Front 10

Third step in Meiosis II (Anaphase)

Back 10

Centromeres divide, sister chromatids become independent chromosomes and move to opposite poles of cell

Front 11

Fourth step in Meiosis II (Telophase)

Back 11

Chromosomes decondense, nuclear membrane reforms, cytokinesis happens and 4 genetically unique cells are produced from 2 in Telophase in Meiosis I.

Front 12

Why is Meiosis Important

Back 12

The two nuclear divisions form 4 genetically unique haploid daughter cells. This happens after interphase and ensures that somatic (body) cells produced from gametes have the full chromosome set.

Front 13

How does a chromosome form

Back 13

From 2 sister chromatids that are held by a centromere after DNA replication happens

Front 14

Ways that variation happens

Back 14

crossing over, independent assortment, mutation

Front 15

Crossing over

Back 15

Happens in Meiosis I (First division) where homologous chromosomes associate and adjacent chromosomes twist and cross over. DNA sections are exchanged at the chiasmata (a point) and enzyme action breaks the chromatids but they recombine. Genetic variety increases as there are more and new combinations of alleles on the chromosomes. Becomes bivalent homologous chromosomes.

Front 16

Independent Assortment

Back 16

Can happen in Meiosis 1 and 2. Maternal and paternal chromosomes assort independently but the combination of chromosomes that go into daughter cells is random.

Front 17

Formula for combination numbers when working with one cell

Back 17

2^n

Front 18

Formula for combination numbers when working with 2 cells (fertilisation)

Back 18

(2^n)^2

Front 19

Summary of Meiosis I

Back 19

Homologous chromosome form bivalents, chismata forms and crossing over happens, homologous pairs of chromosomes separate

Front 20

Summary of Meiosis II

Back 20

The chromosomes separate

Front 21

In early stages of mitosis, on 2 pairs of homologous chromosomes, are they identical or different?

Back 21

The same letter (they must be identical)