13 Meiosis Flashcards by Jodie Buan

When does meiosis I begin?

Meiosis begins after chromosomes are replicated during S phase and the cell moves through G2 phase

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Early Prophase I

nuclear envelope begins to break down, chromosomes condense, the spindle apparatus begins to form, then synapsis occurs

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Synapsis

homologous chromosome pairs come together, usually requires breaking and connecting DNA of 2 homology at many spots along their length, creating a bivalent

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Bivalent

consists of paired homologous replicated chromosomes, where each homolog consists of two sister chromatids

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Late Prophase I

nuclear envelope breaks down, and the two homologs within each bivalent become attached to microtubules coming from opposing poles of the spindle apparatus

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Chiasmata/Chiasma

X-shaped structures that help hold homologous chromosomes together during Meiosis I and are a way in which Meiosis creates genetic diversity

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Crossing Over

process of exchange in the parts of genetic material between non-sister chromatids of maternal and paternal homologs at each chiasma after synapsis

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Metaphase I

kinetochore microtubules move the pairs of homologous chromosomes (bivalents) to a region called the metaphase plate where they are independently aligned

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Metaphase plate

an imaginary plane midway between the poles of the spindle apparatus

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Anaphase I

homologous chromosomes in each bivalent separate from each other as they are moved to opposite poles of the spindle apparatus

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Telophase I

homologs finish moving to opposite sides of the spindle, cytokinesis occurs, forming two haploid daughter cells

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What happens to the chromosome number of daughter cells after Meiosis I?

reduces and the cells become haploid, having only one copy of each type of chromosome

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What does Meiosis II do?

separate the sister chromatids of the replicated chromosomes into separate cells so each will contain unreplicated daughter chromosomes

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Prophase II

a spindle apparatus forms in both daughter cells, nuclear envelope breaks apart, chromosomes move toward the middle of each cell

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Metaphase II

each chromosome is attached by microtubules to both poles and are lined up at the metaphase plate

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Anaphase II

sister chromatids of each chromosome are separated and become independent daughter chromosomes

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Telophase II

chromosomes move to different daughter cells forming 4 haploid cells each with 1 daughter chromosome of each type in the chromosome set

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What are the stages in Prophase I?

(1) Condensation of Chromosomes as sister chromatids are held by cohesins (2) Synapsis where bivalent is held by synaptonemal complex (3) Breaks in DNA lead to crossover between non-sister chromatids (4) Synaptonemal complex disassembles as 2 homologs are held at the chiasmata until it breaks and leaves 2 unconnected chromosomes

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How does Meiosis promote genetic variation?

crossing over during meiosis I and the shuffling of maternal and paternal chromosomes into haploid cells

Asexual reproduction

any mechanism of producing offspring that involves a single parent

Sexual reproduction

production of offspring that involves two parents

What type of reproduction occurs in plants?

Both asexual (sporophytes) and sexual (gametophytes)

Principle of independent assortment

Each pair of hereditary elements (alleles of the same gene) behave independently of other genes during meiosis

Genetic recombination

change in the combination of alleles in a chromosome or in an individual

Self-fertilization

two gametes produced by the same individual fuse to form a diploid offspring that's genetically different from the parent

Outcrossing

gametes from different individuals combine to form offspring, increasing genetic diversity

Nondisjunction

meiotic error where either both homologs in meiosis I or both sister chromatids in meiosis II move to the same daughter cell; too many or too few chromosomes in daughter cell

Aneuploid

cells that have too many or too few chromosomes of a particular type

Purifying selection

natural selection against deleterious alleles

Why is sexual reproduction helpful?

(1) Offspring are not doomed to inherit harmful alleles (2) the production of genetically varied offspring means that at least some may be able to resist rapidly evolving pathogens and parasites

Meiosis

nuclear division that leads to a halving of chromosome number and ultimately to the production of sperm and egg in animals

What are cells produced by meiosis in plants and other eukaryotes called?

Spores, which often reproduce by mitosis rather than fuse

Autosomes

Chromosomes found in two copies in both males and females

Sex chromosomes

Chromosomes that are NOT found in two copies in both males and females

How many chromosomes do humans have?

24 chromosomes: 2 sex chromosomes (X and Y) and 22 autosomes labeled 1-22

What is the karyotype of females?

46XX; females have 2 of each autosome and 2 X chromosomes, a total of 46 chromosomes

What is the karyotype of males?

46XY; males have 2 of each autosomes and one of each sex chromosome

Karyotype

summaries of a species, number and types of chromosomes

Homologous chromosomes or homologs

Chromosome pairs that are the same size and shape, and carry the same genes

Gene

a section of DNA that influences some hereditary trait in an individual

Allele

Different versions of the same gene found on homologous chromosomes

Diploid

"double form," having two copies of each autosome; diploid cells have two alleles of each gene, one on each of the homologous pairs of chromosomes

Haploid

"single form," cells that have only one of each chromosome, produced by plants (called spores) and animals (called gametes) during reproduction; have one allele of each gene

Haploid number (n)

number of chromosomes present in a gamete

What is the haploid number for human sperm and egg cells?

n=23; eggs have one of each autosome and an X chromosome, sperm have one of each autosome and either an X or a Y chromosome

Polyploid

"many form," have more than 2 of each chromosome

Ploidy

number of chromosomes in a cell's nucleus