Meiosis

Lesson Objectives

•Minds ON• Learning Goals • Yesterday’s Recap•What is Meiosis• Homework

Minds ON

Budding in Humans?

http://www.youtube.com/watch?v=NADx1ot0uhI

Learning Goals • Investigate the process of meiosis, using a microscope or

similar instrument, or a computer simulation and draw biological diagrams to help explain the main phases in the process (D2.2)

• Explain the phases in the process of meiosis in terms of cell division, the movement of chromosomes, and crossing over of genetic material (D3.1)

• Use appropriate terminology related to genetic processes: haploid, diploid, spindle (D2.1)

Yesterday’s RecapMitosis:- Occurs when a parent cell divides to produce two daughter

cells- Hereditary information stays the same during each division- Chromosomes are made of DNA and other proteins, which are

folded and condensed into chromosomes- Cell cycle: Interphase (growth and replication of DNA),

prophase, metaphase, anaphase and telophase , cytokinesis

Yesterday's Recap Continued…

Diploid Number: (2n) number of each chromosome in a body cell of an organism; having two chromosomes of each type per cell Haploid Number: (n) number of chromosomes in a cell that contains a single set of chromosomes (gametes or sex cells); one-half the diploid number

Di=2

Ha = half ½

Full complement; Humans: 2n = 46 (not actually distintive units

Number of unique chromosomes;Humans: n=23

Therefore, the number of homologous pairs is 23

Haploid/Diploid• In a humans, there are 22

pairs of autosomes (non-sex cells) and 1 pair of sex chromosomes (XX=female; XY=male) = 23 pairs in total

• Haploid numbered chromosomes usually occur in gametes or sex cells (sperm/eggs)

• Diploid numbered chromosomes usually occur in body cells (tissue cells, etc.)

Meiosis• Mitosis occurs in body cells and creates IDENTICAL daughter

cells• Meiosis occurs in reproductive tissues where sperms and eggs

are formed. It ensures VARIATION and ensures that the zygote has the correct number of chromosomes.

• Variation occurs in two ways: Independent (Random) Assortment and Crossing Over.

Mitosis and Meiosis in Life

Source: http://www.mysearch.org.uk/website1/html/433.Cycles.html

Independent (Random) Assortment

• When chromosomes line up side-by-side (tetrad) during Metaphase I, the paternal and maternal chromosomes line up randomly on the left and right

• = 8 388 608 different combinations!

So let’s try it…

• I need 4 males and 4 females to stand up and line up facing each other. • Is there more than one possible way to do

this?

MMMM FMFM MMFF FFMM F F F F MFMF FFMM MMFF

Crossing Over• While the dyads (X-shaped chromosomes) are in a tetrad

(lined up side-by-side), pieces of homologous chromatids can change places creating different chromosomes.

Note: If one piece gets misplaced, a genetic mutation will occur (genes are missing or misplaced).

So let’s try it…

• I need 4 males and 4 females to stand up and line up facing each other.• Take your genetic information (your cards) that

match your real characteristics and trade each other (make sure they are homologous traits). • What has happened? Explain.

Stages of MeiosisThere are two stages in Meiosis: Meiosis 1 and Meiosis II

Meiosis I• Homologous chromosomes

come together in synapsis (crossing over can occur here)

• Main difference from mitosis is that genetic sharing occurs; the rest of the function afterwards is similar to Mitosis

• Chromosomes are diploid at the beginning, become haploid by telophase I

Meiosis II• Sister chromatids line

up again, then separate• During telophase II,

four haploid cells result• Four haploid cells

result by the end of telophase II.

Meiosis – what it looks like

http://www.youtube.com/watch?v=D1_-mQS_FZ0

Homework• Sheets• Meiosis sheets

• Text book work (sheet from yesterday)• Lesson questions