Meiosis: reductive division

- What is meiosis?- What are the stages?- Independent assortment- What can “go wrong”?- Karyotyping

Refer to chapter 10 in text

Gregor Mendel- 19th century Austrian monk, (?)

looking at pea plants…- Described patterns of trait inheritance (e.g. segregated- not-blended),

but offered no mechanism.- His work was not recognized until

half a century later,in the wake of Darwin.

One version of each gene (one allele) comes from each parent…

If all the parental genes were passed on offspring would have twice as much DNA as parents.

We have two versions of each chromosome in our somatic cells (diploid).Gametes have one version of each (haploid).

Meiosis is reduction division, forming haploid gametes.

- What is meiosis?

Homologous chromosomes are the two parental chromosome versions, found in diploid organisms.(This graphic shows a diploid number of 2: one pair of homologous chromosomes- In humans 2n= 46, or 23 pairs.)

Interphase I, as in mitosis, includes an S phase for the

replication of the DNA.Sister chromatids are joined at centromeres.

meiosis I (first round),results in two haploid cells,with sister chromatids still attached.

meiosis II (second round)results in four haploid cells, withone half of the original amount of DNA.

- What are the stages?

NB →

names of phases, synapse, to make tetrads (aka bivalents) chiasmata/crossover, kinetochore, metaphase plate, spindle, centrosome…

*

* Some interesting details in three slides

rest of phase names, cleavage, cytokinesis…

NB→

Interphase (like mitosis, ending with replicated DNA,sister chromatids bound at centromere).

Prophase IHomologous chromosomes together, forming tetrads.

ChiasmataMetaphase I

Tetrads line up at center.Anaphase I

Homologous pairs migrate apart.Centromeres do not split.

Telophase ITwo cells, each with two copies of ½ info.

Prophase II Spindles form in each cell.

Metaphase IIChromosomes line up, still joined to sisters.

Anaphase IIPairs split at centromere, move apart.

Telophase IINew cells form.

Result: four haploid cells—gametes.

RECAP.

ME

IOS

IS I

..

ME

IOS

IS II

.

About that prophase I,It can be divided into 5 sub-phases, with self explanatory names:

(b) Leptotene (thin threads): DNA condensing, synaptonemal complex forming for…

(c) Zygotene (double threads): pairing/zipping together of homologous chromosomes(d) Pachytene (thick threads): chiasmata form(e) Diplotene (2 threads): SC degrades, homologs back off except at chiasmata(e also) Dikinesis (move through): DNA packs tighter,

nucleus dissolves, spindles forming

Independent assortment

There is no control over how the homologs align in metaphase I …In meiosis, the possible outcomes equal 2n, where n = the number of chromosome pairs: In you, 223, or 8,288,608 possible combos of paternal and maternal chromosomes!

(Mendel just happened topick traits for his experimentsthat all sorted independently...)

What can “go wrong”?

Chiasmata, crossing over between non-sister chromatidsduring tetrad formation in prophase I.

If the parts miss-align, you get some of those chromosomal mutations.

If they line up, it is an important contributor toward genetic variability.

Generally notso tidy!

www.biologie.uni-hamburg.de/b-online/e11/4.htm

With this added in, the gamete genome possibilitiesskyrocket well beyond 8 million: “ Effectively infinite”.

Gene Mapping

Without chiasmata, genes on a given chromosome would always be inherited together:With chiasmata, distant genes on the same chromosome can recombine, resulting in non-parental mixes.Very distant genes may sort independently, as if on separate chromosomes.The closer the genes, the less likely they are to sort independently.This is used to estimate gene locations.

(Note- it is not really this simple:proximity to centromere has a role.Male Drosophila chromosomes don’t cross over…!

What can “go wrong”? cont.

Nondisjunction occurs when chromosomes fail to separate in meiosis:- tetrads do not split in meiosis I, OR- sister chromatids fail to split in meiosis II

If such a gamete is used, result is aneuploidy of the zygote.Note: trisomy shown. n-1 gamete results in monosomy.

(primary nondisjunction) (secondary nondisjunction)

A karyogram is a tool used to screen for abnormalities in karyotype.A sample can be taken from a fetus byamniocentesis or chorionic villus sampling.

Mitosis in a somatic cell is arrested,the chromosomes are stained, a micrograph is taken, and using cut-and-paste chromosomes are ordered by size, centromere location, and banding pattern.

In humans1 through 22 areautosomal chromosomes

The 23rd “pair” are the sex chromosomes. male = XY female= XX Female somatic cells have one X disabled as a Barr body. (In cats this results in calico and tortoiseshell colors)

You are to actually work with karyotyping…

What is meant by independent assortment?

Draw and label the stages of meiosis.

What are chiasmata?Significance? (“Good” and “bad”)

How is a karyotype done?Why is a karyotype done?

Describe how Patau syndrome happens (trisomy 13).

Compare meiosis and mitosis, thoroughly.

What is the purpose of meiosis?

Gregor Mendel telophase I independent assortment

somatic cell prophase II chromosomal mutations

diploid metaphase II genetic variability

gamete anaphase II gene mapping

haploid telophase II nondisjunction

meiosis synapse aneuploidy

homologous chromosome tetrad trisomy

Interphase I chiasma zygote

S phase crossover monosomy

sister chromatid kinetochore karyotype

meiosis I metaphase plate amniocentesis

meiosis II spindle chorionic villus sampling

prophase I centrosome autosomal chromosome

metaphase I cleavage sex chromosome

anaphase I cytokinesisBarr bodykaryogram