Meiosis

Vocab

Heredity – the passing of traits/genes from one generation to the next

Genetics – the study of heredity

Genes – segments of DNA that code for a specific trait

Cell Division / Asexual Reproduction

Asexual reproduction – 1 parent ….identical offspring (clones)*single celled eukaryotes…..yeast, paramecium, amoeba*simple multicellular eukaryotes...hydra, jellyfish, *binary fission……bacteria *mitosis ……identical daughter cells (clones), same # of chromosomes, same DNA

buddingbudding

Sexual reproduction – 2 parents = diff offspring

+ =

What are the disadvantages of asexual reproduction?What are the advantages to asexual reproduction?

Karyotype – picture of 23 pairs of chromosomes (46)

Homologous chromosomes – identical chromosomes…same length, centromere, same staining pattern…exception…males sex chromosome

1-22 autosomes 23 sex chromsomes

Human female karyotype

46 chromosomes23 pairs

Human male karyotype

46 chromosomes23 pairs

Homologous chromosomes• Paired chromosomes– both chromosomes of a pair carry “matching” genes

• control same inherited characters• homologous = same information

diploid2n2n = 4

single stranded homologouschromosomes

double strandedhomologous chromosomes

How do we make sperm & eggs?• Must reduce 46 chromosomes 23

23

2346

egg

sperm

46

meiosis 46

fertilization

23

23

gametes

zygote

Gametes – single set of 22 chromosomes 1 sex chromosome…(n)=23

Meiosis: production of gametes• Happens in gonads – Conserves chromosome #

• diploid haploid• 2n n

– humans: 46 23• meiosis reduces chromosome

number• makes gametes

– fertilization restores chromosome number• haploid diploid• n 2n

Meiosis I – Division of homologous pairs

Interphase I : -G1, S, G2 *replicates only once!

Prophase I : -chromosomes condense & form homologous chromosomes called tetrads or bivalents-crossing over occurs forming chiasmata-synapsis allows for maternal chromsomes to pair up w/ its paternal homolog-centrosomes move to poles, meiotic spindle forms-nuclear membrane disappears -90% of time

Meiosis I – Division of homologous pairs

Meiosis I – Division of homologous pairs

Metaphase I : -homologous pairs align at metaphase plate-kinetochore microtubules attach to one chromosome of each pair

Meiosis I – Division of homologous pairs

Anaphase I : -homologous chromosomes move toward opposite poles*separate the pair

Meiosis I – Division of homologous pairs

Telophase I & Cytokinesis : -two new nuclei form-homologous chromosomes form into chromatin -nuclear membrane reappears-divide cytoplasm

Meiosis I Animation

Meiosis II – Division of sister chromatids

Interphase II : -G1, G2 * No replication!!

Prophase II : -spindle apparatus forms-sister chromatids become visible -move toward metaphase plate -nuclear membrane disappears

Meiosis II – Division of sister chromatids

Metaphase II : -chromosomes on metaphase plate -kinetochore microtubules attach to sister chromatids of each chromosome

Meiosis II – Division of sister chromatids

Anaphase II : -sister chromatids move toward opp poles

Meiosis II – Division of sister chromatids

Telophase II : -nuclear membrane reforms -chromatin forms -cytokinesis produces 4 haploid (n) cells

Meiosis II Animation

Meiosis I and II

Meiosis 1

Meiosis 2

One way meiosis generates genetic variability is through the different ways in which maternal and paternal chromosomes are combined in the daughter cells.

The number of possible chromosome combinations in the haploid nuclei is potentially very large. In general, the number of possible chromosome combinations is 2n, where n is the number of chromosome pairs.

For example, in fruit flies, which have 4 chromosome pairs, the number of possible combinations is 2n, or 16. For humans, with 23 chromosome pairs, there are over 8 million metaphase arrangements. 2 parents = zygote : 223 x 223 = over 70 trillion

3 Sources of Genetic Variability

1. Independent Assortment of homologous chromosome in meiosis I 2. Crossing over during prophase I 3. Random fertilization of egg and sperm

http://www.pbs.org/wgbh/nova/body/how-cells-divide.html

Differences across kingdoms• Not all organisms use haploid & diploid stages in

same way– which one is dominant (2n or n) differs– but still alternate between haploid & diploid

• must for sexual reproduction

http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter12/animations.html#

http://www.youtube.com/watch?v=0gF8bCE4wqA&safe=activeHela Cell