Meiosis
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Meiosis
Cell division that reduces the chromosome number Production of haploid cells
Zeus
Vocabulary • • • • • • • •
Homologous Diploid Haploid Meiosis autosomes Crossing-over Gametes Zygote
Background • Until the 19th century, scientists believed that traits from parents would blend in the offspring. • Gregor Mendel crossed different strands of pea plants and discovered that genetic traits or “factors” are not blended but passed onto the offspring • These “factors”, or genes are part of the chromosome and have specific locations.
Chromosomes • Humans have 23 homologous pairs • Two sets of chromosomes • Diploid: 2n= 46 total • Homologous: appear nearly identical and contain the same genes but are derived from different parents. • 22 are called autosomes • 1 pair is a sex chromosome • Sex chromosomes= ♂ male “XY” ♀ female “XX”
Review of chromosomes • Chromosomes are genetically the same in different types of somatic cells. (specialization) • Chromosome number is specific for the species. • Chimpanzees have 48 • Mendel’s pea plants have 14 • Some fern species have over 1,000
Meiosis Similar to mitosis but different • • • •
Mitosis is asexual cell reproduction Meiosis is a prelude to sexual reproduction Meiosis and Mitosis creates more cells Meiosis has an additional step which reduces the # of chromosomes in each daughter cell to ½ the original amount • Meiosis: DNA replication occurs only once • Meiosis: occurs in germ cells (gametes)
Meiosis vs. Mitosis
Gametes: a.k.a. germ cells • Cells produced in reproductive organs • Humans: testes & ovaries • Egg and sperm cells only contain 23 chromosomes • created by parent cells through meiosis • Haploid (n= 23)
Sexual reproduction • Sperm cell fuses with an egg to create a new organism • Genetic mix of both parents • In order for the offspring to have the correct number of chromosomes the parent cells (gametes) must only have half the usual number of chromosomes
Meiosis: two steps
Meiosis creates diversity • Meiosis I – Prophase I – Chromosomes form a tetrad *crossing over occurs which is an overlap b/n chromosomes – Metaphase I – Anaphase I – Telophase I
Meiosis II • Proceeds like mitotic division • Each daughter cell from meiosis I splits into two haploid gametes. • Result is 4 daughter cells with 23 chromosomes each.
Online resources • Meiosis tutorial • Cornell tutorial Excellent pictures and online quiz • Animation of meiosis • Meiosis tutorial step by step
Review 1. What do the terms haploid and diploid mean? 2. What are the principal differences between mitosis and meiosis? 3. What is crossing over? 4. In human cells, 2N=46. How many chromosomes would you expect to find in a sperm cell? In an egg cell? In a white blood cell? 5. Describe the main results of meiosis.
Cell division that reduces the chromosome number Production of haploid cells
Zeus
Vocabulary • • • • • • • •
Homologous Diploid Haploid Meiosis autosomes Crossing-over Gametes Zygote
Background • Until the 19th century, scientists believed that traits from parents would blend in the offspring. • Gregor Mendel crossed different strands of pea plants and discovered that genetic traits or “factors” are not blended but passed onto the offspring • These “factors”, or genes are part of the chromosome and have specific locations.
Chromosomes • Humans have 23 homologous pairs • Two sets of chromosomes • Diploid: 2n= 46 total • Homologous: appear nearly identical and contain the same genes but are derived from different parents. • 22 are called autosomes • 1 pair is a sex chromosome • Sex chromosomes= ♂ male “XY” ♀ female “XX”
Review of chromosomes • Chromosomes are genetically the same in different types of somatic cells. (specialization) • Chromosome number is specific for the species. • Chimpanzees have 48 • Mendel’s pea plants have 14 • Some fern species have over 1,000
Meiosis Similar to mitosis but different • • • •
Mitosis is asexual cell reproduction Meiosis is a prelude to sexual reproduction Meiosis and Mitosis creates more cells Meiosis has an additional step which reduces the # of chromosomes in each daughter cell to ½ the original amount • Meiosis: DNA replication occurs only once • Meiosis: occurs in germ cells (gametes)
Meiosis vs. Mitosis
Gametes: a.k.a. germ cells • Cells produced in reproductive organs • Humans: testes & ovaries • Egg and sperm cells only contain 23 chromosomes • created by parent cells through meiosis • Haploid (n= 23)
Sexual reproduction • Sperm cell fuses with an egg to create a new organism • Genetic mix of both parents • In order for the offspring to have the correct number of chromosomes the parent cells (gametes) must only have half the usual number of chromosomes
Meiosis: two steps
Meiosis creates diversity • Meiosis I – Prophase I – Chromosomes form a tetrad *crossing over occurs which is an overlap b/n chromosomes – Metaphase I – Anaphase I – Telophase I
Meiosis II • Proceeds like mitotic division • Each daughter cell from meiosis I splits into two haploid gametes. • Result is 4 daughter cells with 23 chromosomes each.
Online resources • Meiosis tutorial • Cornell tutorial Excellent pictures and online quiz • Animation of meiosis • Meiosis tutorial step by step
Review 1. What do the terms haploid and diploid mean? 2. What are the principal differences between mitosis and meiosis? 3. What is crossing over? 4. In human cells, 2N=46. How many chromosomes would you expect to find in a sperm cell? In an egg cell? In a white blood cell? 5. Describe the main results of meiosis.
