13 Meiosis Flashcards
When does meiosis I begin?
Meiosis begins after chromosomes are replicated during S phase and the cell moves through G2 phase
Early Prophase I
nuclear envelope begins to break down, chromosomes condense, the spindle apparatus begins to form, then synapsis occurs
Synapsis
homologous chromosome pairs come together, usually requires breaking and connecting DNA of 2 homology at many spots along their length, creating a bivalent
Bivalent
consists of paired homologous replicated chromosomes, where each homolog consists of two sister chromatids
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
Chiasmata/Chiasma
X-shaped structures that help hold homologous chromosomes together during Meiosis I and are a way in which Meiosis creates genetic diversity
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
Metaphase I
kinetochore microtubules move the pairs of homologous chromosomes (bivalents) to a region called the metaphase plate where they are independently aligned
Metaphase plate
an imaginary plane midway between the poles of the spindle apparatus
Anaphase I
homologous chromosomes in each bivalent separate from each other as they are moved to opposite poles of the spindle apparatus
Telophase I
homologs finish moving to opposite sides of the spindle, cytokinesis occurs, forming two haploid daughter cells
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
What does Meiosis II do?
separate the sister chromatids of the replicated chromosomes into separate cells so each will contain unreplicated daughter chromosomes
Prophase II
a spindle apparatus forms in both daughter cells, nuclear envelope breaks apart, chromosomes move toward the middle of each cell
Metaphase II
each chromosome is attached by microtubules to both poles and are lined up at the metaphase plate
Anaphase II
sister chromatids of each chromosome are separated and become independent daughter chromosomes
Telophase II
chromosomes move to different daughter cells forming 4 haploid cells each with 1 daughter chromosome of each type in the chromosome set
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
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
