Lesson 6 Part 2: Mitosis and Meiosis

Lesson 6 Part 2: Mitosis and Meiosis

How cells makes new cells

Open Ended

After watching the video, what is the purpose of mitosis?

Mitosis - the replication of body cells

• Somatic (body) cells go through a cell division process called mitosis.

• This process allows organisms to grow and to repair themselves

Why does mitosis result in identical cells?

• Mitosis is responsible for growth and repair -- if your cells get damaged, the process of mitosis replaces them

• So the cells need to be the same!

Starting mitosis: DNA Replication

• Somatic cells will first need to replicate (double up) its DNA

• DNA has to replicate first so that each new somatic cell gets the exact same amount of DNA.

Mitosis: Getting Ready to Divide

• After DNA replication, the duplicated copies of chromosomes (DNA) are pulled apart and separated

• Chromosomes are pulled to opposite poles of the cell -- to make sure that chromosomes are in equal numbers for each new cell

Mitosis: Cell Division

• After the chromosomes are separated, the cell splits into two new cells

• Each new cell will be identical to the original cell

• 1 --> 2 identical cells with same # of chromosomes (46 --> 46)

Multiple Choice

True or false: Mitosis produces two different types of cells.

• True

• False

Multiple Select

Select all that occurs during mitosis.

• DNA replicates

• 2 genetically different cells are produced

• Chromosomes are separated into equal number and type

• 2 genetically identical cells are produced

Open Ended

Describe the process (what happens) and result (end product) of mitosis in your own words.

But sex cells have 1/2 the amount of chromosomes as body (somatic) cells.

So how does sex cell (gamete) formation differ from body cell formation?

Open Ended

How do you think the process of body cell division (which produces cells with all 46 chromosomes) will be different from sex cell division (which produces cells with only 23 chromosomes)?

Meiosis - the production of gametes

• Recall that gametes are sex cells (sperm and eggs)

• Sperm and eggs both have only 23 chromosomes each

• sperm [23] +egg [23] = zygote [46]

• So how does this happen?

Meiosis - Double Cell Division

• Meiosis starts very similarly to mitosis (DNA replicates and then separates)

• Results in 4 genetically different cells

• 1 --> 4 cells

DNA Replication - Homologous Chromosomes

• Homo = same; logous = having a correspondence or relation

• Homologous chromosomes are matching pairs of chromosomes

• Same length, same genes (!), etc.

DNA Replication - Homologous Chromosomes

• Homo = same; logous = having a correspondence or relation

• Homologous chromosomes are matching pairs of chromosomes

• Same length, same genes (!), etc.

• ! = alleles could be different (recall alleles are versions of genes ((blue eyes, brown eyes, green eyes, etc)

Why do I look different from my siblings if we have the same parents?

• The answer (part of it, at least): crossing over

• Homologous chromosomes will swap parts with their matching chromosome

• Crossing over makes more variation in genetic combinations

Open Ended

Describe crossing over in your own words.

Meiosis - Double Cell Division

• After the chromosomes replicate and exchange parts, the cell divides into two new cells

• New cells still have a full set of chromosomes so the two cells will need to divide one more time

• After 2 cells divide, there are 4 new cells --> each genetically different from each other

When does meiosis happen?

• Males: during puberty

• Females: two different time periods!

• Meiosis 1 (1st round of division) starts as a fetus in utero (still developing in the uterus)

• Meiosis 2 (2nd cell division) starts during puberty (this is when ovulation + menstruation starts)

Multiple Choice

How many divisions does a sperm/egg cell go through in meiosis?

• 0

• 1

• 2

• 4

Multiple Select

Meiosis produces _____________ that have ___________ chromosomes

• sex cells (gametes)

• body cells (somatic cells)

• 23 total

• 46 total

Open Ended

Why do siblings look different even though they come from the same parents?

What happens when meiosis goes wrong?

Non-disjunction events

Non-Disjunction Events

• Recall that every body cell should have 46 total chromosomes (2 of each pair of chromosomes)

• Sometimes cells don't evenly divide the sets of chromosomes

• This results in a non-disjunction event

• Causes genetic disorders that range from mild to severe (some with fetus not surviving gestation or child has very diminished life span)