Meiosis and Life Cycles

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Lecture 22

Last updated 10:17 PM on 4/21/26
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32 Terms

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Meiosis

the cell division of gametes in sexually-reproducing organisms

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2 rounds & 4 haploid cells

How many rounds are in meiosis and how many haploid cells does it result in?

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Heredity

the transmission of traits from one generation to the next

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Variation

differences in appearance from parents to offspring

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Genetics

the scientific study of heredity and variation

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Alleles

different forms of a gene that can be represented as dominant (A) or recessive (a)

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Genotype

the genetic makeup (alleles) of an organism

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Phenotype

the outward expression of alleles in an organism

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Gene locus

the specific position on a gene in the genome

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Gametes

reproductive cells (sperm & egg in humans); passes down genetic information

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Sexual Reproduction

two parents, gametes fuse together; only in multicellular organisms

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Asexual Reproduction

one parent, no gametes; occurs in multicellular and unicellular organisms

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Clone

a group of genetically identical individuals produced from asexual reproduction

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Life Cycle

the generation-to-generation sequence of stages in the reproductive history of an organism

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46 chromosomes stored in 23 pairs

How many chromosomes does a human have?

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Karyotype

the visualization of all chromosomes and their corresponding pairs from the cell

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Homologous chromosomes / homologs

the same size and shape; express genes for the same traits, but may have differing alleles

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XX & XY

female & male sex chromosomes

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Autosomes

the other 22 pairs of non-sex chromosomes

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Diploid cell (2n)

has two sets of chromosomes (2n=46 for humans)

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Gametes

sex cells that contain a single set of chromosomes (haploid cells); divided by meiosis, not mitosis

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Zygote

a fertilized egg

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Meiosis I

replication of each pair of homologous chromosomes and split equally between the two daughter cells

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Meiosis II

separates the homologous chromosomes into individual sister chromatids

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Prophase I

  • homologous chromosomes pair up, aligned gene by gene

  • exchange of DNA segments (crossing-over) at the chiasmata (x-shaped region)

<ul><li><p>homologous chromosomes pair up, aligned gene by gene</p></li><li><p>exchange of DNA segments (crossing-over) at the chiasmata (x-shaped region)</p></li></ul><p></p>
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Metaphase I

  • homologous pairs line up at the metaphase plate

  • tetrads have two chromosomes (one from each parent) and four chromatids; microtubules are attached to the kinetochores of the chromosomes

<ul><li><p>homologous pairs line up at the metaphase plate</p></li><li><p>tetrads have two chromosomes (one from each parent) and four chromatids; microtubules are attached to the kinetochores of the chromosomes</p></li></ul><p></p>
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Anaphase I

  • homologous pairs separate

  • chromosomes move to opposite poles

  • sister chromatins remain attached at the centromere and move to opposite poles

<ul><li><p>homologous pairs separate</p></li><li><p>chromosomes move to opposite poles</p></li><li><p>sister chromatins remain attached at the centromere and move to opposite poles</p></li></ul><p></p>
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Telophase I & Cytokinesis

  • splitting of haploid set of chromosomes

  • each cell has half of the chromosomes which is then replicated to create two haploid daughter cells

  • animal cells form a cleavage furrow; plant cells form a cell plate

  • cytokinesis (splitting of the cytoplasm) occurs simultaneously

<ul><li><p>splitting of haploid set of chromosomes</p></li><li><p>each cell has half of the chromosomes which is then replicated to create two haploid daughter cells</p></li><li><p>animal cells form a cleavage furrow; plant cells form a cell plate</p></li><li><p>cytokinesis (splitting of the cytoplasm) occurs simultaneously</p></li></ul><p></p>
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Prophase II

  • spindle apparatus forms

  • sister chromatids migrate towards the metaphase plate

<ul><li><p>spindle apparatus forms</p></li><li><p>sister chromatids migrate towards the metaphase plate</p></li></ul><p></p>
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Metaphase II

  • sister chromatids line up at the metaphase plate (no longer identical due to crossing-over)

  • microtubules attach to the kinetochores

<ul><li><p>sister chromatids line up at the metaphase plate (no longer identical due to crossing-over)</p></li><li><p>microtubules attach to the kinetochores</p></li></ul><p></p>
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Anaphase II

  • each sister chromatid is separated and moves toward opposite poles

  • two new individual chromosomes at each pole

<ul><li><p>each sister chromatid is separated and moves toward opposite poles</p></li><li><p>two new individual chromosomes at each pole</p></li></ul><p></p>
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Telophase II & Cytokinesis

  • nuclei form, chromosomes decondense

  • results in 4 daughter gamete cells that contain a haploid (unduplicated) set of chromosomes; each daughter is genetically unique

<ul><li><p>nuclei form, chromosomes decondense</p></li><li><p>results in 4 daughter gamete cells that contain a haploid (unduplicated) set of chromosomes; each daughter is genetically unique</p></li></ul><p></p>