Cell Division – Knit of Identity, Source of Distinction

Question: How do cells conserve genetic information?

Corollary: Why care?

Question: How do cells reduce and rearrange genetic information?

Corollary: Why care?

Mitosis and Meiosis

Mitosis retains genetic information because it maintains chromosome number through nuclear division.

Meiosis reduces and rearranges genetic information because it rearranges chromosomes and reduces their number through nuclear division.

Mitosis and Meiosis

Meiosis is used to produce sex cells (gametes), the thread of life that connects generations.

Mitosis is used for all other cell divisions.

And there is plenty of cell division – about 4,000,000 per second in each of us.

So, What’s a Chromosome? (and why care?)

A human chromosome at metaphase

Chromosomes are structures that organize the genetic material (DNA).

Each chromosome contains one (G1 phase) or two (G2 phase) immensely long molecules of DNA wrapped with proteins.

The DNA molecule is twisted and turned, but if it were stretched out, one would find a linear array of genes extending from one end to the other.

So, What’s a Chromosome (and why care)?

So, What’s a Chromosome (and why care)?

If you take care of your chromosomes, you’ll take care of your DNA (i.e. your genetic blueprint). Not a bad idea.

Packing Up – DNA into Chromatin into Chromosomes

The Link Between DNA and Chromosome Replication

Chromosome AnatomyA few key points -

The chromosome shown on the right is seen for a fleeting period during the cell division cycle. The rest of the time the DNA is much less tightly packed (condensed).

The most confusing point - a chromosome can have one chromatid or two, but in either case its still one chromosome.

telomere

Chromosomes come in matched pairs (homologous pairs).

Cell division requires coordinated division of chromosomes (mitosis) …..

…… and division of the cytoplasm (cytokinesis).

Cytokinesis in animals

Coordination of DNA Replication, Mitosis and Cytokinesis in Cell Division

Cell Division is Cyclic

The cell cycle

Mitosis – a Knit of Identity

Mitosis in Action

Blue shows DNA, green shows spindle fibers.

Meiosis – A Source of Distinction

Why do you share some but not all characters of each parent?

What are the rules of this sharing game?

At one level, the answers lie in meiosis.

Meiosis – A Source of Distinction

Meiosis does two things -

1) Meiosis takes a cell with two copies of every gene (a diploid; 2n) and makes cells with a single copy of every gene (a haploid; 1n). This is a good idea if you’re going to combine two cells to make a new organism. This trick is accomplished by halving chromosome number (2n 1n). 2) Meiosis scrambles the specific forms of each gene that the sex cells receive. This makes for a lot of genetic diversity. This trick is accomplished through independent assortment and crossing-over.

Meiosis With a Single Chromosome Pair

Diploid

Haploid

Reduction of Chromosome Number

Meiosis I – the first and most significant cell division of meiosis.

Reduction of Chromosome Number

Meiosis II – finishing business in the second cell division of meiosis.

2 chromatids/chromosome 1 chromatid/chromosome

One way meiosis makes lots of different sex cells (gametes) – Independent Assortment.

Independent assortment produces 2n different possible gametes, where n = the number of unique chromosomes. In humans, n = 23 and 223 = 6,000,0000.

That’s a lot of diversity by this mechanism alone.

Another way meiosis makes lots of different sex cells (gametes) – Crossing-Over.

Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.Crossing over scrambles genes because each one of the paired chromosomes often contain different forms of the same gene.

The Fundamental Distinction Between Mitosis and Meiosis is in Chromosome Pairing and Alignment

Mitosis Meiosis

Boy or Girl? The Y Chromosome Decides.

The sex chromosomes are paired in females (XX) and unpaired in males (XY).

Spermatogenesis – One path of gametogenesis following meiosis

Men are busy – meiosis produces roughly 250,000,000 sperm per day.

Oogenesis – Another path of gametogenesis following meiosis

Woman are less busy in meiosis (but at least as important) than men– meiosis produces only a few hundred mature eggs over a lifetime.

What Meiosis is AboutA way to create a new and unique individual through sexual reproduction.