Contributing to Future Generations

Where Do My Chromosomes Come From?  Every somatic cell (body cell) has 2 sets of chromosomes  1 set from mom & 1 from dad  Any cell that has 2 complete sets of chromosomes = diploid or 2n

Each pair carries genetic information for the same trait These chromosomes are said to be HOMOLOGOUS

When 2 cells come together during fertilization, each gamete (sperm or egg) can only have 1 set of chromosomes Egg & sperm cells are said to be haploid (1n) = 1 set of chromosomes Gametes – sex cells

Why is There a Need for Meiosis? Why do the ovary & testes cells go through meiosis?  Meiosis is the formation of SEX CELLS  Reduces the chromosome number in the cells (gametes) from 2 sets to 1 (diploid to haploid)

In humans: Parent ParentDiploid (2N) Ovum 2323 Sperm Haploid (N) Haploid (N) Zygote 46 Diploid (2N) Diploid to haploid c’some # cut in half fertilization

Chromosome Number Questions 1.If a horse retina cell has 36 chromosomes, how many does a horse sperm cell have? 2.If the n = 4 for fruit flies, how many chromosomes does a wing cell have?

A Homologous Pair is…  The 23 pairs of chromosomes in every body cell are also know as homologous pairs  1 from mom, 1 from dad  Same banding pattern  Centromere is in the same position on both How do you know if 2 chromosomes are homologous?

The Chromosomes- Revisited

The Making of a Karyotype 1.Photograph cell during metaphase 2.Cut out chromosomes from the picture 3.Arrange chromosomes in homologous pairs by comparing size, banding pattern, & centromere position 4.Lay pairs out from largest to smallest with sex chromosomes at the end

What’s a Karyotype Used For?  Look for any abnormalities- either in structure or number (e.g. Down’s Syndrome)  Determine sex  Last pair = sex chromosomes  XX = female, XY = male

How Do We Get the Chromosomes for a Karyotype?  Need a cell  For an embryo- obtain cell through amniocentesis or chorionic villus sampling  For an adult- obtain any growing cell

Quiz Time 1.In Streptomycin fungus n = 11. What is the diploid number for this species? 2.If n = 16 for goldfish how many chromosome in a fin cell? 3.If 2n = 108 for black spruce trees then what is the haploid number?

The Results Products of Mitosis: 2 - 2n cells Products of Meiosis: 4 - n cells 2n2n2n 2n nnnn Diploid Haploid

Overview Meiosis I – homologous chromosomes separate Meiosis II – chromatid pairs separate

Meiosis: Reduction Division  Reducing the number of chromosomes

Where are we going??  Males –4 sperm cells  Females – 1 ovum and 3 polar bodies

Meiosis I  4 stages:  Prophase I  Metaphase I  Anaphase I  Telophase I

Prophase I 1. Sister Chromatids condense –c’somes 2. Nucleolus disappears 3.Homologous chromosomes pair up 4.Spindle forms

Metaphase I 1.Spindle fibers attach to centromeres 2.Homologous chromosomes line up in pairs tetrad

Anaphase I  Spindle fibers pull homologous chromosomes apart  Separates mom’s chromosomes from dad’s chromosomes  Sister chromatids stay together

Telophase I  Nuclear membranes form around the 2 new nuclei  Cell pinches into 2 daughter cells

Summary of Meiosis I

Between Meiosis I and II there IS Interphase, but NO replication. Two cells from cytokinesis in Meiosis I are the same as the cells in Prophase II

Meiosis II  Meiosis II is like mitosis & consists of 4 stages:  Prophase II  Metaphase II  Anaphase II  Telophase II

Propahse II  Each cell has only mom’s or dad’s chromosomes with its copy (sister chromatid)  Nuclear membrane dissolves

Metaphase II  Sister chromatids line up across the metaphase plate

Anaphase II & Telophase II  Anaphase II  sister chromatids are pulled apart & migrate to opposite poles  Telophase II  Nuclear membranes reform

Summary of Meiosis II

The Final Product  Cytokinesis  The result is of meiosis is 4 haploid gamete cells  Uneven cytokinesis in females  The result is 1 large egg cell & 3 non-functioning polar bodies

Ooogenesis  One ovarian cell divides into:  one viable egg cell  three polar bodies.

Spermatogenesis  Equal division of cytoplasm resulting in 4 equal size sperm

Comparison of Spermatogenesis & Oogenesis

Crossing Over  Crossing over (synapsis) occurs when genetic information is exchanged between the chromatids  Result = greater variability

Summary of Meiosis

Non-disjunction  Failure of chromosomes to separate properly during meiosis

Conditions Caused by Non-disjunction  Downs syndrome = Trisomy copies of chromosomes 21  total of 47 chromosomes = 2n + 1 (chance of occurring in oogenesis increases with maternal age)  Turners syndrome = Monosomy X - has only one X chromosomes totalling only 45 chromosomes in her body cells = 2n-1  Klinefelter’s syndrome = XXY- male which has an extra X chromosome = 2n + 1

Downs Syndrome  Trisomy 21  Wide variety of problems  Developmental delays  Heart defects  Immune problems  Vision, hearing respiratory issues  Can also be very talented

Turner’s Syndrome  Monosomy X  Problems include  Short stature  Lack of ovarian development  Prone to kidney, cardiovascular, and thyroid problems  Difficulties with spatial orientation & awareness

Klinefelter’s Syndrome  XXYc= males  Problems include  Infertility  Language impairment  Less masculine body 3 Sex Chromosomes