Genetics - MEIOSIS

Background Information  During interphase, DNA is uncondensed and is called chromatin.  When cells get ready to divide, the chromosomes condense into chromosomes.

Chromosome vs. Chromatin

Structure of a Chromosome  Chromatids – the two identical “sister” parts of a chromosome  Centromere – where the two chromatids attach

You have 2 types of cells!  Somatic Cells – regular body cells (skin cells, hair cells, heart cells, brain cells, etc.) They are considered diploid (2n). They have 2 copies of each chromosome (one from mom and one from dad).  Gametes – reproductive cells (sperm & eggs) They are considered haploid (n). They only have one copy of each chromosome (mom OR dad’s).

Mitosis  Somatic cells are created through mitosis.  One cell replicates its DNA and then divides to create 2 identical daughter cells.

HUMAN CHROMOSOMES 46 total chromosomes 23 homologous pairs; two sets 1 set from mom 1 set from dad

Challenger Questions  What is our diploid number?  What is our haploid number?

Challenger Questions  What is our diploid number? 46  23 from dad  23 from mom  What is our haploid number? 23

Meiosis A type of cell division that produces gametes with half as many chromosomes as regular body cells.

Meiosis I Prophase I Metaphase I Anaphase I Telophase I Meiosis II Prophase II Metaphase II Anaphase II Telophase II The Two Stages of Meiosis

Chromosomes (DNA) are duplicated in the S-phase of interphase (prior to division). DNA is uncondensed, in the form of chromatin. Interphase

DNA forms chromosomes. Homologous chromosomes pair up in a tetrad Crossing over happens Prophase I

Homologous Chromosome  A pair of replicated chromosomes, one inherited from each parent, that have corresponding gene sequences and that pair during meiosis.  They are the same size, have the same genes and show the same banding pattern in a karyotype.

Synapsis  The pairing up of homologous chromosomes.

Tetrad Homologous chromosomes that are paired together, ready to divide. (4 chromatids together)

Crossing Over Exchange of genetic material between homologous chromosome tetrads during Prophase I

Crossing Over: the result is genetic recombination: an exchange of genes. Produces genetic variability; any gene can be found in any gamete

Centriole Metaphase I Tetrads are arranged in the middle of the cell. Spindle fibers attach

Spindle Fibers Attach to chromatids and pull them apart during meiosis and mitosis

Metaphase I

Anaphase I homologous chromosomes are pulled apart Sister chromatids remain together

disjunction  The separation of homologous chromosomes

Telophase I: chromatids move to opposite sides of the cell. Cells become haploid (one set in each cell).

Cytokinesis nuclear membrane forms and 2 new cells form

Prophase II: chromosomes get ready again

Metaphase II Chromosomes line up down the middle

Anaphase II chromosomes move apart

Telophase II & Cytokinesis 4 haploid sex cells form Each is unique

Meiotic Division Results Initial Cell Result of Meiosis I Result of Meiosis II

Meiosis reduces chromosome number

Mitosis vs Meiosis

Diploid Cell Has the full number of chromosomes; means “two sets” ; body cells

Haploid Has half of the normal number of chromosomes; means “one set”; sex cells

Chromatid Identical, replicated chromosomes joined by a centromere

From Parents to Offspring  Parents’ cells (diploid) undergo meiosis to produce sex cells (haploid)  Sex & Fertilization occurs  The fertilized egg is now diploid, with a complete set of chromosomes  The fertilized egg has genes from both maternal and paternal grandparents-the parents’ genes  The egg undergoes mitosis to grow

Q: Why is there meiosis? A: If gametes (sex cells) had a full set of chromosomes, the number of chromosomes in the adult would double with every generation!!

Quick “practice” quiz! Meiosis!

1. The gamete that contains genes contributed only by the mother is _______________. a. a sperm b. an egg c. a zygote d. syngamy e. ovary

2. The paternal gamete is _________. a. a sperm b. an egg c. a zygote d. syngamy e. gonad

3. Chromosomes that contain genes for the same trait in the same order are called ____________________. a. haploid b. gametes c. diploid d. homozygous e. homologous

4. If a fruit fly has 4 chromosomes, what is its diploid number? a. 16 b. 8 c. 6 d. 4 e. 2

5. If a fruit fly has 4 chromosomes, what is its haploid number? a. 16 b. 8 c. 6 d. 4 e. 2

6. If a fruit fly has 4 chromosomes and a fruit fly sperm fertilizes a fruit fly egg, how many chromosomes does the zygote have? a. 16 b. 8 c. 6 d. 4 e. 2

7. Why are the daughter cells formed during meiosis haploid?

a. Meiosisb. Mitosis For Q’s 8-12, use the key to match the term with the proper phrase. 8. Forms cells that are diploid.

9. Forms cells with half the amount of genetic material as a somatic (body) cell. a. Meiosisb. Mitosis For Q’s 8-12, use the key to match the term with the proper phrase.

10. Crossing over often occurs. a. Meiosisb. Mitosis For Q’s 8-12, use the key to match the term with the proper phrase.

11. Produces genetic variability. a. Meiosisb. Mitosis For Q’s 8-12 use the key to match the term with the proper phrase.

12. If a gorilla’s sperm cell has 24 chromosomes, what is the gorilla’s diploid number? a. 24 b. 12 c. 26 d. 96 e. 48

CLOSURE  DRAW A PICTURE COMPARING MITOSIS AND MEIOSIS.