Meiosis Science 9 A Closer look at our

Meiosis Science 9

A Closer look at our Chromosomes � If you observe the chromosomes in a body cell through a microscope, you will see that our 46 chromosomes are arranged into 23 pairs that resemble each other in shape and size � These “matching” pairs of chromosomes are known as homologous pairs. � You receive one pair from your mother and one pair from your father.

Chromosomes cont’d. . . � Human body cells have 46 chromosomes � These cells are referred to as diploid, (“di” means double) � The human diploid number is 46 (2 x 23) � Human gametes have 23 chromosomes and are called haploid. (Think half the diploid number)

Reason for Meiosis cont’d. . . � In order for the human chromosome number to remain at 46, gametes must have one half of the chromosomes. � Only haploid gametes can combine during fertilization to form a diploid zygote. � Only a diploid zygote can function as the offspring’s first body cell.

Reason for Meiosis cont’d. . . � You know mitosis ensures that chromosome number does not change � The process that ensures each gamete contains only one-half set of chromosomes is called Meiosis

The process of Meiosis � Meiosis also ensures that each gamete has a different combination of the chromosomes that were present before meiosis � It is these non-identical nuclei that give the gametes the potential to produce offspring different from their parents. � How are some of the chromosomes different?

Crossing over � During the early part of meiosis , double stranded chromosomes come together. � During this time, an event called crossing over can occur � Crossing over is when the chromosomes that are close together actually exchange genetic information.

Crossing over

Now for Meiosis � The word meiosis comes from the Greek word “meioo”, meaning to lessen or reduce

Prophase 1 � Virtually identical to prophase in mitosis, involving the appearance of the chromosomes, the development of the spindle fibers, and the breakdown of the nuclear membrane (envelope). � This is where the homologous chromosomes pair up.

Meiosis I: Prophase I � Double stranded chromosomes match up to make homologous pairs.

Meiosis I: Prophase I � Microscope image:

Metaphase 1 � Here is where the critical difference occurs between Metaphase 1 in meiosis and metaphase in mitosis. � In mitosis, all the chromosomes line up on the metaphase plate in no particular order. � In metaphase 1, the chromosomes pairs (homologous pairs) are aligned on either side of the metaphase plate. � It is during this alignment that crossing over may occur in which pieces of DNA are exchanged.

Meiosis I: Metaphase I � The homologous pairs of double stranded chromosomes line up across the middle of the cell.

Meiosis I: Metaphase I � Microscope image:

Meiosis I: Anaphase I � The homologous pairs of double stranded chromosomes are pulled apart and move to opposite ends of the cell.

Meiosis I: Anaphase I � Microscope image:

Meiosis I: Telophase I � Two nuclei are formed each nucleus has one double stranded chromosome from the homologous pair. We end up with two daughter cells.

Meiosis I: Telophase I � Microscope image:

Prophase 2 �A new set of spindle fibers forms and the chromosomes begin to move toward the equator of the cell.

Meiosis II : Prophase II � Each cell has a set of double stranded chromosomes.

Meiosis II : Prophase II � Microscope image:

Meiosis II : Metaphase II � The double stranded chromosomes line up across the middle of the cell.

Meiosis II : Metaphase II � Microscope image:

Meiosis II : Anaphase II � The double stranded chromosomes are pulled apart at the centromere and move as single chromosomes to opposite ends of the cell.

Meiosis II : Anaphase II � Microscope image:

Meiosis II : Telophase II � Each of the two original cells splits into 2 cells leaving a total of four cells. Each new cell contains a half as many chromosomes as the original cell. They are haploid cells.

Meiosis II : Telophase II � Microscope image:

Where does meiosis occur? � Sexually reproducing animals produce gametes in reproductive organs called gonads. � The formation of gametes in males occurs in the testes (spermatocytes). The gametes formed are called sperm � The formation of gametes in females occurs in the ovaries (oocytes). The gametes formed are called eggs

Meiosis cont’d. . . � Although the process of meiosis is the same in males and females, the final outcome differs. � In males the process of meiosis produces 4 sperm cells. � However in females the process of meiosis produces only 1 egg cell along with 3 polar bodies.

Meiosis in males

Meiosis in females

Questions � On page 50 in your text book work on: ◦ # 1, 2, 3, 4, and 5