1. Meiosis

2. Reproduction Asexual Asexual Results from cell division (mitosis). Results from cell division (mitosis). One “parent” cell divides to form two identical “daughter” cells. One “parent” cell divides to form two identical “daughter” cells. Ex. Budding & binary fission Ex. Budding & binary fission Sexual Sexual Results from fusion of 2 cells to form offspring. Offspring are genetically different from parents.

3. Meiosis : An Overview Meiosis is the process by which an organism creates gametes. Meiosis is the process by which an organism creates gametes. Gametes are known as sex cells. Gametes are known as sex cells. The sex cells are sperm and egg. The sex cells are sperm and egg. Meiosis reduces the number of chromosomes in these cells by half. Meiosis reduces the number of chromosomes in these cells by half. This has to happen so that when fertilization occurs, the child has the correct number. This has to happen so that when fertilization occurs, the child has the correct number.

4. Chromosome Number Body cells (called somatic cells) are diploid (2n) in chromosomes. Body cells (called somatic cells) are diploid (2n) in chromosomes. This means that there are two sets of chromosomes – one received maternally and the other paternally. This means that there are two sets of chromosomes – one received maternally and the other paternally. In humans this number is 46 or 23 pairs of chromosomes In humans this number is 46 or 23 pairs of chromosomes Sex cells (called gametes) are haploid (n). Sex cells (called gametes) are haploid (n). Meiosis halves chromosome number so that each cell has 1 of each chromosome. Meiosis halves chromosome number so that each cell has 1 of each chromosome. In humans this is 23 chromosomes. In humans this is 23 chromosomes.

5. Diploid vs. Haploid Diploid is the total number of chromosomes in body cells. Diploid is the total number of chromosomes in body cells. The diploid number can also be represented by pairs. The diploid number can also be represented by pairs. Haploid is the total number of chromosomes in sperm and egg cells only. Haploid is the total number of chromosomes in sperm and egg cells only. The haploid number is ½ of the diploid number. The haploid number is ½ of the diploid number. OrganismDiploid# pairsHaploid Human4623 pairs23 Frog6030 pairs30 Dog7839 pairs39 Carrot147 pairs7 Tulip3216 pairs16

6. Meiosis The process of meiosis consists of two consecutive divsions of the nucleus. The process of meiosis consists of two consecutive divsions of the nucleus. Meiosis I – creates 2 diploid nuclei Meiosis I – creates 2 diploid nuclei Meiosis II – creates 4 haploid nuclei Meiosis II – creates 4 haploid nuclei DNA is not duplicated between divisions DNA is not duplicated between divisions

7. Prophase I Matching chromosomes pair together. Matching chromosomes pair together. A pair of the same chromosomes is called a homologous pair. A pair of the same chromosomes is called a homologous pair. A homologous pair is also called a tetrad because it consists of 4 chromatids. A homologous pair is also called a tetrad because it consists of 4 chromatids. Crossing Over occurs. Crossing Over occurs.

8. Chromosome Structure pair of homologous chromosomes = tetrad Centromere Crossing Over Chromosome from mother Chromosome from father Chromatids

9. Crossing Over When chromosomes are close together in a homologous pair, parts of chromatids can become tangled. These segments of the chromosomes are exchanged to create more variability in chromosomes. Creates more variety in offspring.

10. Fertilization Male and female gametes unite during fertilization and nuclei fuse (combine). Male and female gametes unite during fertilization and nuclei fuse (combine). Fusion of two haploid nuclei produces diploid nucleus in the zygote – fertilized egg Fusion of two haploid nuclei produces diploid nucleus in the zygote – fertilized egg The genetic combination of gametes which unite is random The genetic combination of gametes which unite is random Adds to variation (differences) among offspring Adds to variation (differences) among offspring

11. Mistakes in Meiosis If an accident occurs during meiosis, gametes will have the wrong number of chromosomes. If an accident occurs during meiosis, gametes will have the wrong number of chromosomes. These “mistakes” are called nondisjunction. These “mistakes” are called nondisjunction. If it occurs in Meiosis I, a pair of homologous chromosomes doesn’t separate. If it occurs in Meiosis I, a pair of homologous chromosomes doesn’t separate.

12. Mistakes in Meiosis If nondisjunction occurs in Meiosis II, chromatids fail to separate on at least one chromosome. If nondisjunction occurs in Meiosis II, chromatids fail to separate on at least one chromosome.

13. What does this mean? An extra chromosome is carried along with the normal two copies of a chromosome and ends up in the fertilized egg (zygote) and then in the embryo. An extra chromosome is carried along with the normal two copies of a chromosome and ends up in the fertilized egg (zygote) and then in the embryo. Nature's way of dealing with this abnormal development is called spontaneous abortion (miscarriage). Nature's way of dealing with this abnormal development is called spontaneous abortion (miscarriage). It is thought that well over half of all pregnancies end in miscarriage (although it is usually at such an early stage that the pregnancy isn't even noticed). It is thought that well over half of all pregnancies end in miscarriage (although it is usually at such an early stage that the pregnancy isn't even noticed).

14. Some Cases of Survival Down Syndrome – Trisomy 21 Cause: three copies of chromosome 21. Cause: three copies of chromosome 21. Effect: Some impairment of mental ability, physical growth and differences in facial appearance. Effect: Some impairment of mental ability, physical growth and differences in facial appearance. Diagnosis: Down syndrome can be identified during pregnancy or at birth. Diagnosis: Down syndrome can be identified during pregnancy or at birth. Occurrence: 1 per 800 to 1,000 births, rate increases with age of mother. Occurrence: 1 per 800 to 1,000 births, rate increases with age of mother.

15. Some Cases of Survival

16. Turner Syndrome Cause: Female with only 1 X chromosome. Cause: Female with only 1 X chromosome. Effect: Characteristic physical abnormalities Effect: Characteristic physical abnormalities short height, broad chest, low hairline, low-set ears, and webbed neck. short height, broad chest, low hairline, low-set ears, and webbed neck. Effect: Usually experience lack of menstruation and infertility. Effect: Usually experience lack of menstruation and infertility. Effect: Often difficulties in spatial, mathematic, and memory areas. Effect: Often difficulties in spatial, mathematic, and memory areas. Occurrence: 1 out of every 2500 girls. Occurrence: 1 out of every 2500 girls.

17. Some Cases of Survival Klinefelter’s Syndrome Cause: Male with XXY chromosomes instead of normal XY arrangement. Cause: Male with XXY chromosomes instead of normal XY arrangement. Effect: Development of small testicles and reduced fertility – almost always sterile. Effect: Development of small testicles and reduced fertility – almost always sterile. Effect: Possible characteristics vary widely and most often include a lanky, youthful build and facial appearance, or a rounded body type with some more breast tissue). Effect: Possible characteristics vary widely and most often include a lanky, youthful build and facial appearance, or a rounded body type with some more breast tissue). Effect: Language problems are also common. Effect: Language problems are also common.