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MEIOSIS

MEIOSIS. GAMETE PRODUCTION IN SEXUAL REPRODUCTION. What are chromosomes?. Human cell = >6 billion nucleotide base pairs (~2 meters) Wrapped around protein = chromatin DNA/protein = chromosome . How many chromosomes do humans have?. Strait hair allele. Curly hair allele.

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MEIOSIS

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  1. MEIOSIS GAMETE PRODUCTION IN SEXUAL REPRODUCTION

  2. What are chromosomes? • Human cell = >6 billion nucleotide base pairs (~2 meters) • Wrapped around protein = chromatin • DNA/protein = chromosome

  3. How many chromosomes do humans have? Strait hair allele Curly hair allele • Humans are diploid (2n) • Two of each chromosome, one from each parent. • n = 23 unique chromosomes (haploid #) • 2(n) = 46 total chromosomes • The two copies of each chromosome in human cells are homologous

  4. Families: similar yet different. Why? Asexual Reproduction • Single parent • Offspring identical to each other and parent • Sexual Reproduction • Two parents • Offspring are unique • Offspring are similar to each other and parents • Combine DNA from two individuals • Combines characteristics of both individuals

  5. MEIOSIS • While mitosis conserves chromosome number, meiosis reduces the chromosome number by half • Meiosis has a few stages: • Interphase, • Meiosis I, and • Prophase I • Metaphase I • Anaphase I • TelophaseI • Meiosis II • Prophase II • Metaphase II • Anaphase II • Telophase II

  6. MEIOSIS INTERPHASE • Interphase is similar to the process in mitosis • The process results in two genetically identical sister chromatids • The chromatids are connected at their centromeres • Centrosomes also replicate

  7. PROPHASE I • Chromosomes begin to condense • Homologues (consisting of two sister chromatids) pair up • When the chromosome pairs become visible, they show up as a tetrad, a cluster of four chromatids • Homologous chromosomes can criss-cross, allowing the chromatids to switch segments, called crossing-over

  8. PROPHASE I continued • Centrosomes move away from each other • Microtubules form between the centrosomes • Nuclear envelope and nucleoli disperse • Prophase I can last for days or longer, and takes up 90% of the time required for meiosis

  9. METAPHASE I • Chromosomes arrange on the metaphase plate (equator), still in homologous pairs • Microtubules from one pole attach to one chromosome from each pair • The other chromosome (homologous pair) is attached to a microtubule from the other pole

  10. ANAPHASE I • The spindle apparatus guides the chromosomes towards the poles • Sister chromatids remain attached at their centromeres • The sister chromatids move together towards the same pole

  11. TELOPHASE I • The homologous chromosome pairs continue to move apart to each pole • Each pole contains a haploid (n) number of a chromosome set • Cleavage furrows form in animal cells, and cell plates appear in plant cells

  12. TELOPHASE I AND CYTOKINESIS • Cytokinesis usually occurs simultaneously with telophase I, forming two daughter cells • There is no further replication of genetic material • Depending on the species, the chromosomes de-condense and the nuclear membranes and nucleoli re-form.

  13. PROPHASE II • A spindle apparatus forms • The chromosomes progress towards the metaphase II plate (equator of the cell)

  14. METAPHASE II • The chromosomes are positioned on the metaphase plate in mitosis-like fashion • The microtubules attach to the centromeres of the sister chromatids

  15. ANAPHASE II • The centromeres of the sister chromatids finally separate • The sister chromatids of each pair, now individual chromosomes, move toward opposite poles of the cell

  16. TELOPHASE II AND CYTOKINESIS • Nuclei form at opposite poles of the cell • Cytokinesis occurs • In the end, there are four daughter cells, each with the haploid number of un-replicated chromosomes

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