Download Meiosis - BEHS Science

Meiosis
Meiosis
• How is meiosis going to be different
from mitosis?
– Meiosis is part of sexual reproduction
• How is sexual reproduction different from
asexual reproduction?
Grab a book pg. 273
• List your observations
• Discuss: What looks different?
What are the goals of Meiosis?
• Make cells for reproduction
• Reduce chromosome numbers
• Create genetic variation
Making cells for reproduction
• Which cells go through meiosis?
• Gametes
– Egg cells and sperm cells
Reducing chromosome
numbers
• How many chromosomes do humans
have?
46
Reducing chromosome
numbers
• That’s 2 sets of chromosomes with 23
chromosomes in each set
• Why do we have 2 sets of chromosomes?
– One set from mom and one set from dad
Chromosome numbers of various organisms
Organism
Penicillium
Saccharomyces (yeast)
Mosquito
Housefly
Garden Pea
Corn
Adder’s tongue fern
Frog
Human
Orangutan
Dog
Number of Chromosomes
1-4
18
6
12
14
20
1,262
26
46
48
78
Adders Tongue Fern
Reducing chromosome
numbers
• There is a name for cells that have 2 sets
of chromosomes
– Diploid
– Humans are a diploid species
Reducing chromosome
numbers
• Why do we need to reduce the
chromosome number?
• Basic math!
• Fertilization requires 2 cells with only 1 set
of chromosomes
• 1 set + 1 set = 2 sets
• 23 + 23 = 46
Reducing chromosome
numbers
• There is a name for cells that have 1 set of
chromosomes
– Haploid
– Human gametes are haploid
Reducing chromosome
numbers
• Haploid gametes fuse during fertilization
and form a zygote
Creating genetic variation
• How can this happen?
• Does it create new genes or new DNA?
– No it just results in new combinations of both
parents genes
– New combinations = diversity
• Why is genetic diversity important?
Creating genetic variation
• 1. Crossing over
• 2. Independent assortment
• 3. Fertilization of random cells
Creating genetic variation
• Crossing over: Homologous chromosomes
come together and trade segments
Creating genetic variation
• What is a homologous chromosome?
– Chromosomes that carry the same genes
• 1 from each set
Creating genetic variation
• Are they exactly the same?
– NO! They do not have the exact same DNA
sequence.
– Where did they come from?
Creating genetic variation
• Trading segments mixes the DNA, creating
new combinations
Creating genetic variation
• Independent assortment
• When homologous chromosomes line up,
they line up independently (randomly)
– They do not have a preset pattern of how they
line up
What do you mean random?
• Lots of possible combinations!
Creating genetic variation
• Random alignment of chromosomes
creates new combinations
Creating genetic variation
• Fertilization of random cells
• Sexually reproducing organisms do not get
to choose which cells get to fuse during
fertilization
• Random cells fuse together creating new
combinations
Time for Meiosis!
Meiosis 1
•
•
•
•
•
•
•
•
DNA condenses into chromosomes
Nucleus dissolves and disappears
Centrioles produce spindle fibers
Crossing over occurs between homologous
chromosomes
Chromosomes line up in pairs, randomly arranged
Homologous chromosomes are separated into 2 sets
2 nuclei come back
Cell divides, forming 2 haploid daughter cells
Time for Meiosis!
Meiosis 2
• Nucleus dissolves and disappears (in both
cells)
• Chromosomes line up
• Sister chromatids are separated
• 2 nuclei come back
• Cells divide (both cells)
• 4 total daughter cells are produced
Meiosis 1: First step
• DNA coils up into chromosomes
• Nucleus disappears
Crossing Over
Homologous chromosomes come together
and trade segments
Crossing Over Cont.
• Homologous pairs of chromosomes line up
randomly in the middle of the cell
• Homologous chromosomes separate into
2 sets
• 2 nuclei come back
• Cell splits in half forming 2 haploid
daughter cells
Meiosis 2
• We do it all again but just slightly different
• Don’t forget all the steps of meiosis 2 are
happening in 2 cells at the same time!
Meiosis 2
• Nucleus disappears
• Chromosomes line up
• How is this different from the first time?
What happens next?
• Sister chromatids are separated
• 2 nuclei come back
• Cells split in half
• 4 total daughter cells are produced
Then what?
• Fertilization
Lets review
• How is meiosis different than mitosis?
Comparing sexual and asexual
reproduction
• Advantages and disadvantages of both