Meiosis and Sexual Reproduction Notes

To be used with Meiosis and Sexual Reproduction Guided

Notes

Before Meiosis lets look back at Mitosis

Genes and Heredity

• Heredity - the passing of traits from parents to

offspring.

• Genetics - the study of heredity (the passing of traits)

• Each human cell contains 30 thousand different genes

Genes and Heredity

Genes: • Factors that control organism traits or characteristics.

• Part of a chromosome containing the genetic code

• Passed on from parent to offspring.

• All of your genes are located on 23 pairs of chromosomes

Alleles• Alleles: part of a gene that is located on a

chromosome.Example:

– Height(gene):– tallness or shortness(alleles)

• The alleles determinehow each gene is expressed.

At least two alleles for one gene.

Locus: Location of gene on a chromosome.

T

T t t

Chromosomes

Sister

ChromatidChromosome Homologous

Chromosome

Homologous Chromosomes: -a pair of chromosomes that contain genes for the

same traits.

-one chromosome is from mom the other from dad.

-same size & centromeres are in same location.

Amniocentesis

• Amniocentesis: a medical procedure used in prenatal diagnosis of chromosomal abnormalities.

• Chromosomes are taken from the amniotic fluid.

Karyotype• The chromosomes from the amniotic fluid are then placed on a karyotype.

• A karyotype is a chart of an organism’s chromosomes.

Karyotype

Next, the chromosomes are matched up according to:

1. if they came from mom or dad.2. how tall they are.

3. centromere location.

Karyotype

A karyotype can reveal if a person… 1. is, male or female2. has a normal number of chromosomes3. has an abnormal number of chromosomes.

Example: Down’s syndrome

Meiosis•Meiosis:

A type of cell division where the chromosome number is reduced to half.

The process in which cells undergo two dividing phases to form

gametes.

Meiosis only produces sex cells (gametes).

Meiosis

Gametes are your sex cells.Your egg and sperm cells.

Sexual reproduction (sperm + egg)produces a zygote.

Meiosis

Diploid vs. Haploid• Diploid Number (2n): the number of

chromosomes found in all body cells.

Humans 46.

-found in the somatic(body) cells

• Haploid Number (n): 1/2 the diploid number of chromosomes. Humans 23.– found in the egg and sperm cells of a species

DIPLOID HAPLOID

Diploid vs. Haploid

Organism Diploid # (body cells) Haploid # (sex cells)

Human 46

Chimpanzee 24

Dog 78

Fruit fly 4

Garden pea 14

Adder’s fern 1260

23

48

39

8

7

630

Terms• Gametes:

eggs or sperm • Gonads:

specialized organs in higher animals where the gametes are made

• Ovaries: female gonads

• Testes: male gonads

MeiosisMeiosis I• Has all the phases mitosis has. I P M A

T

Meiosis II• Has all the phases meiosis I has,

except Interphase. P M A T

Prophase I of Meiosis• During Prophase 1 crossing over takes place.

• Crossing over is : the exchange of chromosome pieces.

– Crossing over increases the variability of the offspring– This is why the offspring of sexual reproduction show many variations - Crossing over occurs only during Prophase I

Mitosis vs. Meiosis

Problems during Meiosis

Normally, homologous chromosomes separate and move toward opposite poles of the cell.

– Problems during anaphase: this disorder is called non Disjunction

– Nondisjunction: the failure of homologous chromosomes to separate

Nondisjunction• Down's syndrome: results from

the nondisjunction of chromosome 21– individual has an extra 21st chromosome

Down’s Syndrome Rates

Mistakes in Mitosis can be Good

• polyploidy: having an entire extra set of chromosomes

– Will result in the 3n or 4n number of chromosomes

- Many plants are a result of polyploidy; banana & day lily

Summary of Mitosis & Meiosis

1. Meiosis is very similar to two divisions of mitosis.

2. The diploid number (2n) of chromosomes is reduced by half to the haploid number (n)

3. Eggs and sperm are formed

4. Crossing Over causes lots of variations5. Variations rarely occur in MITOSIS6. Variations often occur in MEIOSIS

Summary7. Mitosis is associated with asexual reproduction.

8. Mitosis takes place in somatic (body) cells.

9. What is produced from mitosis?Two identical daughter cells How many? 4

10. Meiosis is associated with sexual reproduction.

11.Meiosis takes place in sex cells.

12.Meiosis produces gametes and three polar bodies.