MEIOSIS CREATING GAMETES SEX CELLS Page 97 EQ

MEIOSIS CREATING GAMETES (SEX CELLS) Page 97 EQ: Why is meiosis important for sexually reproducing organisms?

CHROMOSOMES

CHROMOSOMES • A human body cell (called a somatic cell) has 23 pairs of homologous chromosomes for a total of 46 chromosomes. • One chromosome of each homologous pair comes from the mother (called a maternal chromosome) and one comes from the father (paternal chromosome). • Homologous chromosomes are similar but not identical. • They carry the same genes in the same order, but the alleles for each trait may not be the same

DIPLOID AND HAPLOID • A cell that contains both sets of homologous chromosomes is said to be diploid • (2 sets; 2 N; 46 chromosomes) • Your somatic cells are diploid • A cell that only contains 1 set of chromosomes is said to be haploid. • (1 set; N; 23 chromosomes) • Your eggs/sperm cells are haploid

MITOSIS VS. MEIOSIS Mitosis • Results in 2 identical diploid cells • Involves asexual reproduction • 1 division • Function: growth & repair • • Meiosis Results in 4 genetically different haploid cells Involves sexual reproduction 2 divisions Function: create sex cells

MITOSIS VS. MEIOSIS

WHAT IS MEIOSIS? • Meiosis is a two-part cell division process occuring in organisms who sexually reproduce • The point of meiosis is to create gametes (egg & sperm cells) so that babies can be made • The random formation of gametes during meiosis creates genetic diversity in a population.

Chromosomes from parents Meiosis I Chromosomes from both parents are copied and paired so that crossing over can occur MEIOSIS 1 Chromosomes replicate Homologous chromosomes pair up Crossing over occurs Daughter nuclei have single chromosomes and a new mix of genetic material Nucleus divides into 2 nuclei Meiosis II Daughter nuclei divide again

PHASES OF MEIOSIS • Meiosis I: (P, M, A, T) 1 cell divides to create 2 diploid daughter cells • Meiosis II (P, M, A, T) 2 cells divide to create 4 haploid cells; this is when the gametes form

Meiosis I PHASES OF MEIOSIS • Your initial cell has 46 chromosomes – 23 from mom, and 23 from dad • These 46 chromosomes will duplicate into double stranded chromosomes – all 46 were initially single chromatids however they have replicated and become double stranded – There are still only 46 chromosomes! • In Meosis 1 the chromosomes will align in pairs – These are homologous pairs (ie code for same traits, one is from mom, the other from dad) – This is when crossing over occurs • The cell divides and you get two cells. Each cell now has 23 double stranded chromosomes. This is the end of Meiosis I

PHASES OF MEIOSIS Meiosis II • In both daughter cells, the chromosomes are aligned at the middle of the cell (NOT replicated) & then pulled apart A B

CROSSING OVER Occurs in Meiosis I 1. Homologous chromosomes form a tetrad 2. Chromatids cross over one another 3. The crossed sections of chromatids are exchanged ***This produces a new combination of genes which contributes to diversity within a population. ***