Each of your body cells has 2 sets of chromosomes – one from mum, one from dad. Any cell that has 2 complete sets of chromosomes is said to be DIPLOID OR 2n When 2 cells come together in fertilization to make a zygote, each can only have 1 set of chromosomes. These cells ( = egg/sperm, gametes) are said to be MONOPLOID or n.
Why do meiosis? Meiosis is “ REDUCTION DIVISION”. Reduces chromosome number, parent cell is 2n while daughter cells are n. Why do ovary and testes cells go through meiosis?
CHROMOSOME NUMBER In humans, 2n=46 This means we’re referring to a normal body cell that has a total of 46 chromosomes, but they are of 2 sets - 23 from mom, 23 from dad) In humans, n = 23 This means we’re talking about a cell that has only half the normal chromosome number (n) and that is only one set. This cell must therefore be a gamete.
If a horse retina cell has 36 chromosomes, how many does a horse sperm cell have?
If the n = 4 for fruit flies, how many chromosomes does a wing cell have?
3. If the 2n number of a pine tree is 86, how many chromosomes would be found in a pine ovum?
Homologous pairs. In each body cell you have 23 homologous pairs. 2 chromosomes are homologous if: - one is from mum, one is from dad - they have the same types of genes on them (eg. Circled chromosomes may contain genes for blood type hair colour etc.)
The chromosomes on the previous page are homologous because they have the same gene sequence. You can tell which chromosomes are homologous as they have: 1. the same type of genes. 2. The same banding pattern 3. The same position of the centromere.
CVS (chorionic villus sampling) and amniocentesis.
Karyotypes of adults can be obtained from any growing cells.
Karyotypes can not tell if there are mutations of a gene.
Products of MITOSIS: 2 x 2n cells. 2 diploid cells Products of MEIOSIS: 4 x n cells (4 monoploid cells) 2n 2n 2n 2n n n n n
MEIOSIS is two mitotic divisions in a row, (meiosis I and II) except you will see the homologous chromosomes coming together . - -- in prophase I the double homologous chromosomes come together to form a tetrad - Crossing-over (synapsis ) occurs in the tetrad in prophase to increase the variability in gametes. - There is no dna replication in the interphase between the first and second divisions
Reduction Division occurs during Meiosis II because it is defined as the point at which mum’s genetic information is separated from dad’s genetic information whereas a normal body cell has both mom’s and dad’s chromosomes.
Cytokinesis: cytoplasm divides. Total product is 4 cells, each with only a mom or Dad’s chromosome , in other words, half the normal chromosome number , or 4 monoploid (n) cells, or 4 gametes. Uneven cytokinesis in females: In oogenesis, get one big egg and 3 non-functional polar bodies: ( who die off) and 1 big functional gamete
First division: results in one big primary oocyte and a little polar body. Second division: One secondary oocyte and 3 polar bodies. ( These polar bodies die.) OOGENESIS Oocyte Polar Bodies Oogonium Primary Oocyte Mitosis Polar Body Secondary Oocyte Meiosis I Meiosis II
Spermatogenesis There are equal divisions producing four equal sized sperm .
COMPARISONS: Include similarities and differences.
Compare Mitosis and Meiosis
Compare spermatogenesis and oogenesis
Compare Meiosis I and Meiosis II
Compare haploid and diploid cells.
Compare a somatic cell to a gamete.
Compared to the number of chromosomes contained in a body cell of a parent, how many chromosomes would normally be contained in a gamete? F. the same number G. twice as many H. one-fourth as many J. half as many During mitosis, the chromosomes A. are located at the cell equator during prophase B. are located at the cell equator during telophase C. move toward the poles of the cell during anaphase D. move toward the poles of the cell during metaphase Some questions on mitosis and meiosis!
1. Meiosis occurs in this life cycle when A. an egg and a sperm fuse to form a zygote B. a mature polyp produces a medusa C. a planula produces a young polyp D. a medusa produces gametes . Which structures normally have a haploid number of chromosomes? A. Seed plant spores and animal zygotes B. Seed plant spores and animal gametes C. Seed plant zygotes and animal zygotes D. Seed plant gametes and animal zygotes
Processes II and IV illustrate budding. The information in the diagram indicates that budding is a type of A. asexual reproduction that produces haploid or diploid cells B. asexual reproduction that produces diploid cells C. sexual reproduction that produces haploid or diploid cells D. sexual reproduction that produces haploid cells
= Non-separation of chromosomes resulting in diploid or empty gametes. Non-disjunction
Downs syndrome = Trisomy 21 - 3 copies of chromosomes 21 equalling a 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. (monosomy X) = 2n-1 Klinefelter’s syndrome = XXY - male which has an extra X chromosome = 2n + 1 Conditions caused by non-disjunction
A simple check for the presence of a bone in the nose could more accurately test unborn babies for Down's syndrome, scientists say. Researchers say combining the nose test with existing screening methods (amniocentesis, CVS) could lead to a five-fold reduction in the number of miscarriages linked to an invasive procedure used to confirm Down's syndrome. Down’s syndrome – trisomy 21
Normal boy who develops some female secondary sex character-istics at puberty. -slightly lower IQ, infertile, delayed motor, speech, maturation -treated with testosterone. Klinefelter’s syndrome XXY