Exploring Genetics: Gene Mapping and Chromosomal Analysis

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 • Prokaryotes • 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 • 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 homologouschromosomes (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?

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 • 4 phases • 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 cellsSee 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. 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.

Exploring Genetics: Gene Mapping and Chromosomal Analysis