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Sexual Reproduction and Meiosis
Sexual Reproduction
• Sexual reproduction – the process in which a new
individual is produced by the union of the nuclei of two
cells
• Fertilization – the union of two sex cells during
sexual
reproduction
• Zygote – the single cell formed from fertilization
• Gamete – a specialized haploid sex cell
• Haploid (1n) – a specialized cell that contains half the
chromosome number
• Diploid (2n) – the cells which contain two
homologous copies of each chromosome
• Organisms divide diploid cells to create
haploid cells
Biology 5.1 - Sexual Reproduction and Meiosis
Sexual Reproduction and Meiosis
Meiosis
• Meiosis – the specialized type of cell division in which haploid
gametes are formed
• DNA is duplicated only once, but the nucleus divides twice
• Meiosis I divides the homologous chromosomes – the
chromosomes that control the same hereditary traits
• Interphase – DNA of the parent cell is duplicated, creating
sister chromatids
• Meiosis II divides sister chromatids, duplicated strands of
the same chromosome
Stage
Prophase I
Metaphase I
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Anaphase I
Telophase I
and Cytokinesis
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Meiosis I
Description
Nuclear membrane begins to dissolve
Homologous chromosomes pair up
side by side
Paired homologous chromosomes line
up along the metaphase plate
Each pair of homologous
chromosomes separates and are pulled
to each side of the cell
New nuclear membranes form
Most organisms divide the cytoplasm
and create a new plasma membrane
during cytokinesis
Plant cells form a new cell wall plate
Meiosis II
Description
• No additional DNA replication
• Nuclear membrane begins to dissolve
Stage
Prophase II
• Chromosomes line up individually
along the metaphase plate
• Sister chromatids separate and are
pulled to each side of the cell
Metaphase II
Anaphase II
Telophase II
and Cytokinesis
• New nuclear membranes form
• Cytokinesis creates four daughter cells
• Each daughter cell contains only one
of two homologous chromosomes
• Total chromosome number halved
Biology 5.1 - Sexual Reproduction and Meiosis
Sexual Reproduction and Meiosis
• Comparison of Meiosis and Mitosis
• Both begin with diploid (2n) cells
• Both duplicate DNA to produce sister chromatids
• Prophase I of meiosis includes crossing over
• Synapsis – the process of pairing homologous chromosomes during meiosis
• Chiasmata – the point at which homologous chromosomes cross during synapsis
• Crossing over – the exchange of corresponding regions on chromosomes during meiosis
• Recombinant chromosomes – the chromosomes that have combined DNA from two or more sources
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Mitosis
One division
Chromosomes line up
individually along the
equator
Two diploid (2n)
daughter cells are
formed
Occurs in different
types of cells as a
means of growth or
repair
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Meiosis
Two separate divisions
In metaphase I
homologous
chromosomes line up
in pairs
In metaphase II
chromosomes line up
individually
Four haploid (n)
daughter cells are
formed
Occurs within
reproductive structures
Biology 5.1 - Sexual Reproduction and Meiosis
Sexual Reproduction and Meiosis
Significance of Sexual Reproduction and Meiosis
• Produces haploid gametes
• Asexual reproduction produces offspring that are genetically identical to the parent
• Sexual reproduction increases diversity
• Offspring have unique combinations of genetic material
• Random assortment creates different gametes
• Crossing over increases diversity
• Increases the possibility that some members will be better adapted
• Individuals are more likely to survive environmental changes and transmit helpful variations to offspring
• Increases species’ chances of long-term survival
Biology 5.1 - Sexual Reproduction and Meiosis