Meiosis Section 8 3 Objectives 4 List and

Meiosis Section 8. 3

Objectives 4 List and describe the phases of meiosis 4 Compare the end products of mitosis with those of meiosis 4 Explain crossing-over and how it contributes to the production of unique individuals

Objectives 4 Summarize the major characteristics of spermatogenesis and oogenesis

Meiosis 4 Process of nuclear division that reduces the number of chromosomes in new cells to half the number in the original cell.

Meiosis 4 Process of cell division that results in the formation of gametes (egg and sperm)

Two Divisions 4 First Division – Reduction Division • Chromosome number is reduced by 1/2 to the haploid condition 4 Second Division – Sister chromatids separate

Meiosis I 4 Pairing of homologous chromosomes - synapsis 4 Each pair of homologous chromosomes is called a tetrad 4 Figure 8 -10 page 154

Stages of Meiosis 4 Prophase I – chromosomes shorten, coil, contract – Crossing-over can occur • Pieces of chromosomes from homologous pairs are exchanged • Results in genetic recombination

Stages of Meiosis 4 Metaphase I – Homologous chromosomes line up at the equator

Stages of Meiosis 4 Anaphase I – sister chromosomes remain connected – chromosome number is reduced by 1/2 – Homologous chromosomes separate – Random assortment of homologous chromosomes is called independent assortment

Stages of Meiosis 4 Telophase I – Cytoplasm divides – Each cell contains the haploid number of chromosomes

Meiosis II 4 Cells divide again 4 Form 4 new cells – Gametes (egg & sperm)

Stages of Meiosis 4 Prophase II – chromosomes move to equator

Stages of Meiosis 4 Metaphase II – Chromosomes line up at the equator

Stages of Meiosis 4 Anaphase II – Sister chromatids separate

Stages of Meiosis 4 Telophase II – gametes are formed – each cell contains n number of chromosomes – ex. humans - 4 sperm; 1 egg – In females - cytoplasm doesn’t divide equally • functional egg - get the most cytoplasm • polar bodies - smaller amount of cytoplasm

Formation of Gametes 4 Spermatogenisis – Making sperm cells 4 Oogenesis – Making egg cells

n 2 combinations 4 n = haploid number 4 223 = > 8 million combinations

Types of Reproduction 4 Asexual – Producing offspring from one parent – Offspring look identical to the parent 4 Sexual – Producing offspring through egg and sperm – Genetic variation

FYI 4 Human somatic cells – Contain 46 chromosomes – Diploid number – Gametes - 23 – Haploid number

More FYI 4 Within 15 generations, humans would have over 1 million chromosomes per cell 4 Usually zygotes with extra sets of chromosomes are spontaneously aborted by the mother

More FYI 4 In human males – One to several cells resume meiosis each month – Meiosis takes about 74 hours – Begins about age 12 -13 – Several hundred million sperm are produced daily – 20% are likely to be defective

More FYI 4 In human females – 5 th-7 th month of development, several million immature sex cells develop in the ovaries but stop after prophase I, resume meiosis when puberty is reached

More FYI 4 In human females – Resume meiosis for one to several cells each month – Completion of meiosis occurs when the ovum is fertilized – 99. 9% of sex cells never develop beyond prophase I – Less than 500 eggs are produced over a lifetime