Meiosis

What’s the big idea?

DNA

• Every cell in your body has two complete sets of DNA. (One set from mom and one set from dad).

DNA

• Your two sets of DNA contain the same type of information, but they are NOT the same.– Ex. You have two sets of

DNA that both give instructions about your eye color. Dad’s may code for brown and mom’s may code for blue. If this is the case, you will have brown eyes, but you will still have the DNA needed to make blue eyes.

DNA

• During interphase, right before a cell gets ready to divide, it makes copies of all of its DNA.

• The identical copies are hooked together at the center and are called a chromosome.

Karyotype

• A “picture” of all the chromosomes in a cell is called a karyotype.

• The DNA pairs (one from mom and one from dad) receive the same number, and are called homologous pairs.

Karyotype

• Humans have 23 homologous chromosome PAIRS, which makes a total of 46 chromosomes.

DNA

• After your DNA replicates, you now have 4 complete copies of information (2 sets from mom and 2 sets from dad).

• Now your cell is ready to divide!

Why meiosis?

• Most of the cells in your body divide using mitosis, not meiosis.

• Remember mitosis?

Mitosis

• Mitosis starts with one cell and ends up with two identical cells that have the exact same DNA as the parent cell had.

Mitosis vs. meiosis

• Meiosis is used in sexual reproduction, and only occurs in the sex cells in your body. (mitosis occurs in all your other cells.)

Mitosis vs. meiosis

• Why wouldn’t you want to use mitosis to make cells for sexual reproduction (which involves two parents)?

Mitosis vs. meiosis

• TMI! (Too much information)

• The overload of DNA can be called polyploidy.

• Animal cells usually can’t survive with too much DNA. (Although many plant cells can)

Meiosis

• Meiosis is the process your cells use to make new cells that have only half as much DNA as a normal cell.

• These cells are called gametes

• When that cell combines with another ½ DNA cell, you now have a normal amount of DNA.

Meiosis

• Meiosis has 2 parts:– Meiosis 1– Meiosis 2

Meiosis 1

• Meiosis 1 has 4 phases:– Prophase 1– Metaphase 1– Anaphase 1– Telophase 1

Meiosis 1• Meiosis 1 is very

similar to mitosis EXCEPT that instead of pulling apart chromosomes during metaphase, homologous chromosome pairs are pulled apart.

Meiosis 1

• Homologous pairs don’t always pull apart the way they should. This failure to separate is called nondisjunction, and is the cause of many diseases including Down Syndrome.

Meiosis 1

• Down syndrome is cause by nondisjunction on chromosome #21.

• (Meaning that someone who has Down Syndrome has 1 extra copy of chromosome #21 in each of their cells.)

Meiosis 2

• Meiosis 2 has four phases:– Prophase 2– Metaphase 2– Anaphase 2– Telophase 2

• Meiosis 2 is very similar to mitosis.

Meiosis• Because Meiosis has 2

cell divisions, the new cells only have half as much DNA as the original cell had.

• These cells are now ready to combine with another cell to have the full amount of DNA again.

• When these cells combine, they form a new cell called a zygote.

Meiosis

• When two cells with half enough DNA combine to form 1 cell with full sets of DNA, (such as a sperm cell combining with an egg cell) it is called fertilization.

• There are over 64,000,000,000,000 possible combinations that could form from your 46 chromosomes.

Meiosis

• Meiosis:– Is used in sexual

reproduction– Begins with one cell and

ends up with 4 new cells– Results in 4 genetically

different cells.– Requires 2 parents to

produce one fully functioning cell.

– Produces offspring that are a mixture of both parents.

Meiosis

• Advantages:– Meiosis produces

variety.– Variety is a good thing!– Diversity is important,

and very little diversity would exist without meiosis.

Meiosis

• Disadvantages:– Organisms that

reproduce through sexual reproduction (meiosis) have to put in the effort to find and attract a mate in order to reproduce.

Mitosis

• Mitosis:– Is used in asexual

reproduction– Begins with 1 cell and ends

up with 2 new cells.– Results in 2 genetically

identical cells.– Requires only 1 parent to

produce 2 fully functioning cells.

– Is the process used in cloning

Cloning using mitosis

Mitosis

• Advantages:– Asexual reproduction

(through mitosis) saves an organism the time and energy of having to find a mate.

Mitosis

• Disadvantages:– Asexual reproduction

(through mitosis) produces a genetically identical offspring and does not create diversity.