There are two different ways for cells to divide:
mitosis – process by which almost all cells reproduce.
makes exact copies.
meiosis – process by which sperm and egg cells are made.
makes changed copies: the genes are shuffled so that the results are different every time.
Used for making offspring/children.
The diagram shows mitosis on left, & meiosis on right.
Let’s follow the right-hand sequence for meiosis.
Note: These are not human cells. These are simple models, with fewer chromosomes.
Start at the top/middle. This is the parent cell.
Here, how many chromosomes are there? 4
How many are blue (from mom) 2
How many are red (from dad) 2
Why are there 2 of each? 1 chromosome from each parent
– – – –
In human cells, how many chromosomes are there? 46
Why are they in pairs? 1 from each parent
In humans, how many chromosomes are from the mom? 23
In humans, how many chromosomes are from the dad? 23
Instead of 4 single chromosomes, what do we have now? 4 pairs of chromosomes.
What must have happened? The originals were duplicated
What do we call the set of the original and copy? Tetrad (tetra is Greek for “4”)
What is the total number of chromosomes right now? 4 x 2 = 8
How is this different from prophase in mitosis? In mitosis the 4 sets floated apart. But here in meiosis they stick together in pairs.
– – – – –
In a human cell, what would the total number of chromosomes be? 46 x 2 = 92
A spindle apparatus forms.
One tetrad attaches to the spindle on one side.
Other tetrad attaches to spindle on the other side.
Here we don’t want exact copies of the parent – we want all daughter cells to be different!
So the chromosomes start crossing over – like shuffling a deck of cards.
Anaphase and Telophase (images in the mid-right)
Tetrads have split apart. Half moved to the left, half moved to the right.
Parent cell split apart into 2 daughter cells. Each is a slightly shuffled/mixed-up copy of the parent.
Each has same # of chromosomes as the parent.
But we’re not done. We’re trying to make gametes (sperm and eggs), right?
Do they have the same # of chromosomes as regular body cells? Nope. They only have half the #!
So we need one more series of cell divisions.
Meiosis II: the final round
So we end up with four daughter cells. Each has:
* shuffled genes (all different from the parent)
* half the normal number of genes
Graphic summary of making gametes
HS-LS3-1. Develop and use a model to show how DNA in the form of chromosomes is passed from parents to offspring through the processes of meiosis and fertilization in sexual reproduction.
HS-LS3-2. Make and defend a claim based on evidence that genetic variations (alleles) may result from (a) new genetic combinations via the processes of crossing over and random segregation of chromosomes during meiosis, (b) mutations that occur during replication, and/or (c) mutations caused by environmental factors. Recognize that mutations that occur in gametes can be passed to offspring.
Disciplinary Core Idea Progression Matrix – “Nearly every cell in an organism contains an identical set of genetic information on DNA but the genes expressed by cells can differ. In sexual reproduction, genetic material in chromosomes of DNA is passed from parents to offspring during meiosis and fertilization. “