Meiosis and Sexual Reproduction Topic 3: Genetics Meiosis and Sexual Reproduction

3.1 Genes Gene= a heritable factor that consists of a length of DNA and influences a specific characteristic. (i.e., fur color) Each gene occupies a specific location or position on a chromosome called a locus (plural loci)

Skill: Use of databases to identify the locus of a human gene and its polypeptide product. Go Here: Http://omim.org/ Under Advanced Search, choose “Search Gene Map” Follow the instructions on page 154 of book… Chromosome Number Locus (in base pairs)

Alleles Allele= one specific form of a gene. Gives the organism’s trait (variant for a character, i.e., brown) Alleles often differ from each other by one or only a few bases. New alleles are formed by __________.

3.2 Chromosomes Essential idea: Chromosomes carry genes in a linear sequence that is shared by members of a species.

Chromosomes in Prokaryotes vs. Eukaryotes have one chromosome (a circular DNA molecule.) some also have plasmids Eukaryotes chromosomes are linear DNA molecules associated with histone proteins. Different chromosomes carry different genes.

Homologous Chromosomes • Homologous chromosomes carry the same sequence of genes but not necessarily the same alleles of those genes.

Diploid vs. Haploid fertilization results in a zygote Diploid nuclei have pairs of homologous chromosomes. somatic cells are diploid (2N); Haploid nuclei have one chromosome of each pair. gametes are haploid (1N) fertilization results in a zygote

Human Chromosomes Humans have 23 pairs of homologous chromosomes (46): 1 pair of sex chromosomes that determine sex (X,Y) 22 pairs of autosomes (do not determine sex);

Theory of knowledge In 1922 the number of chromosomes counted in a human cell was 48. This remained the established number for 30 years, even though a review of photographic evidence from the time clearly showed that there were 46. For what reasons do existing beliefs carry a certain inertia?

Karyotype Karyotype = the number and type of chromosomes present in the nucleus of a cell The number of chromosomes is a characteristic feature of members of a species.

Application: Comparison of diploid chromosome numbers of Homo sapiens, Pan troglodytes, Canis familiaris, Oryza sativa, Parascaris equorum.

Karyogram • A karyogram shows the chromosomes of an organism in homologous pairs of decreasing length. Activity: http://learn.genetics.utah.edu/content/begin/traits/karyotype/index.html

Utilization An understanding of karyotypes has allowed diagnoses to be made for the purposes of genetic counselling.

Application: Describe methods used to obtain cells for karyotype analysis e.g. chorionic villus sampling and amniocentesis and the associated risks. For pre-natal diagnosis: performed using cells collected by chorionic villus sampling or amniocentesis Small risk of miscarriage for both Ethics?

Karyotype Analysis Analyze these karyograms:

• Application: Use of karyograms to deduce sex and diagnose Down syndrome in humans. See previous slide Aim 8: Pre-natal screening for chromosome abnormalities gives an indication of the sex of the fetus and raises ethical issues over selective abortion of female fetuses in some countries.

Meiosis Essential idea: Alleles segregate during meiosis allowing new combinations to be formed by the fusion of gametes.

Nature of science Making careful observations—meiosis was discovered by microscope examination of dividing germ-line cells.

Meiosis (Overview) Meiosis makes GAMETES! Meiosis: reduction division of a diploid nucleus to produce haploid nuclei. (i.e. diploid cell to haploid gametes.) Preceded by chromosome replication, but is followed by 2 cell divisions (Meiosis I & Meiosis II) Results: 4 haploid daughter cells (1N); variation Note: The halving of the chromosome number allows a sexual life cycle with fusion of gametes.

Before Meiosis INTERPHASE Normal cell life and metabolism DNA is uncoiled chromatin. DNA is replicated so all chromosomes consist of 2 sister chromatids! Why?

Starts with one diploid cell MEIOSIS Meiosis: cell division which produces 4 haploid gametes Starts with one diploid cell Ends with four haploid cells

Meiosis has two divisions Meiosis I 4 phases Meiosis II https://www.youtube.com/watch?v=jjEcHra3484 McGraw Hill 3d https://www.youtube.com/watch?v=nMEyeKQClqI Another one (more simple) F:\OneDrive\Desktop\animations from bio powerpoints\Chapter 10 BDOL IC

Stages of Meiosis Study the diagrams on p. 240-241 (3 min.) . C:\Documents and Settings\BBAUGHMAN\Desktop\bio powerpoints\Chapter 10 BDOL IC

Meiosis I Prophase I Nuclear envelope/nucleolus disappear Spindle apparatus forms. Chromosomes become visible (chromatin condenses) Homologous Chromosomes Pair up. Tetrad Crossing Over (Recombination) occurs the exchange of genetic material between homologous chromosomes. Chiasmata = crossing over sites

Metaphase I Tetrads line up on the metaphase plate Alignment is random (Independent Assortment of Chromosomes)

Anaphase I Homologous Chromosomes separate and move to opposite poles

Telophase I and cytokinesis Nuclear envelope re-forms Cytokinesis occurs Two new HAPLOID cells are created

Meiosis II Prophase II Nuclear envelope disappears Spindle apparatus forms and attaches to chromosomes

Metaphase II Chromosomes line up down metaphase plate (middle)

Anaphase II Sister chromatids separate and move toward opposite poles

Telophase II and cytokinesis Nuclei reform Spindle breaks down Cytoplasm divides Four haploid gametes have been formed!! C:\Documents and Settings\BBAUGHMAN\Desktop\bio powerpoints\Chapter 10 BDOL IC

Skill: Drawing diagrams to show the stages of meiosis resulting in the formation of four haploid cells See worksheet

Highlights of Meiosis Underline or highlight the following in your notes: (know the definition of each) Homologous chromosomes: chromosomes that have the same genes at the same loci. (note: One from mom, one from dad.) Tetrads Crossing over Chiasmata

Skill: Drawing diagrams to show chiasmata formed by crossing over. Guidance: • Diagrams of chiasmata should show sister chromatids still closely aligned, except at the point where crossing over occurred and a chiasma was formed.

Origins of Genetic Variation, I Independent assortment: homologous pair of chromosomes position and orient randomly (metaphase I) and non-identical sister chromatids during meiosis II Combinations possible: 2n; with n the haploid number of the organism (for humans= approx. 8 million) http://highered.mcgraw-hill.com/olc/dl/120074/bio18.swf

Origins of Genetic Variation, II Crossing over (prophase I): • the reciprocal exchange of genetic material between non-sister homologous chromatids during synapsis of Prophase I (recombinant chromosomes) Random fertilization: • 1 sperm (1 of 8 million possible chromosome combinations) x 1 ovum (1 of 8 million different possibilities) = 64 trillion diploid combinations! (actually higher! Why?)

Nondisjunction Nondisjunction: -members of a pair of homologous chromosomes do not separate properly during meiosis I or sister chromatids fail to separate during meiosis II Results: abnormal chromosome # Monosomy~ missing chromosome Trisomy~ extra chromosome (Down syndrome- Trisomy 21) Of sex chromosomes XXY- Klinefelter XO – Turner XXX, XYY (no big deal) Polyploidy~ extra sets of chromosomes (can happen in plants)

Application: Studies showing age of parents influences chances of nondisjunction. Figure:  Estimated birth incidence of maternally derived trisomy 21. The statistics show that the maternal age effect involves nondisjunction in both first and second meiotic divisions. (After Yoon et al., 1996.) http://10e.devbio.com/article.php?ch=19&id=189

Assignment at Home: P. 167-168 in textbook: Parental age and non-disjunction Do the 4 questions and submit your answers to the following Google form: https://docs.google.com/forms/d/1dXz63s0qTubRdBYrjHVqpWt0-sfAm9kM-bZuCfaNUkM/viewform?usp=send_form

Meiosis vs. mitosis (p.234) Differences: # cells made Haploid vs. diploid Synapsis= pairing of homologous chromosmes into tetrads (Prophase I) chiasmata (prophase I)- crossing over sites. Meiosis I separates homologous pairs of chromosomes, not sister chromatids of individual chromosomes.

3-D Animation of Meiosis http://www.mhhe.com/biosci/bio_animations/08_MH_Meiosis_Web/index.html

End of IB stuff

Alternative life cycles Fungi/some algae •meiosis produces 1N cells that divide by mitosis to produce 1N adults (gametes by mitosis) Plants/some algae •Alternation of generations: 2N sporophyte, by meiosis, produces 1N spores; spore divides by mitosis to generate a 1N gametophyte; gametes then made by mitosis which then fertilize into 2N sporophyte. Note: gametophyte and sporophyte generations may look identical or different.