Mitosis and Meiosis

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Prophase
Chromosomes condense
Mitotic spindle begins to form Composed of microtubules
Nuclear envelope starts to break down
Chromosomes attach to the spindle via kinetochores
2n
first stage of Mitosis
Metaphase
Nuclear envelope is completely
gone
Formation of the mitotic spindle is completed
Chromosomes line up along the metaphase plate.
2n
second stage of Mitosis
Anaphase
Centromeres separate
Sister chromatids separate and migrate
towards opposite poles of the cell
After separation they are called daughter chromosomes
Two forces pull chromosomes apart: Kinetochore microtubules shrink
Motor proteins of the polar microtubules push the two poles of the cell away from each other
2n
third stage of mitosis
telophase
Chromosome migration is complete
Each pole receives a complete set of chromosomes
Chromosomes decondense
A new nuclear envelope forms around each set of chromosomes
Spindle breaks down
Mitosis is complete when two independent nuclei have formed
2n
fourth stage in mitosis
Prophase 1
Nuclear envelope begins to break down
Chromosomes condense
The spindle apparatus begins to
form
Synapsis and Crossing over
partial separation of homologs
Metaphase 1
Tetrads remain paired
Line up on the metaphase plate
Different homologous pairs align
independently of each other
2N-1 number of chromosomal
Anaphase 1
Homologous chromosomes move to opposite poles
Chromatids are still connected (dyads)
Telophase 1
Nuclear envelope reforms
Daughter cells are now haploid
Cytokinesis occurs
cytokinesis
the division of the cytoplasm into two daughter cells
chromatin
the material that makes up eukaryotic chromosomes
chromatid
one double- stranded DNA copy of a replicated chromosome with its associated proteins
sister chromatids
the two attached, double-stranded DNA copies of a replicated chromosome
genetic material are identical
when replication is complete each chromosome has 2 of these
autosomes
all chromosomes other than the sex chromosomes
present in the same copy numbers in the 2 sexes
homologs
Chromosomes that have the same genes in the same position and are the same size and shape.
(homologous chromosomes)
non- sister chromatids
Chromatids on different members of a homologous chromosome pair.
unreplicated chromosome
A chromosome that consists of one double- helical molecule of DNA packaged with proteins for compactness.
replicated chromosome
A chromosome after DNA replication. Consists of two identical chromatids, each containing one double-helical DNA molecule packaged with proteins
Mitosis II
mitosis for the two haploid daughter cells created by mitosis I
Prophase II
Chromosomes condense
No synapsis
The spindle apparatus forms
Nuclear envelope disintegrates
Chromosomes are loaded onto the spindle apparatus by kinetochores
Metaphase II
Dyads line up along the metaphase plate
Anaphase II
Sister chromatids separate
The resulting daughter chromosomes begin moving to opposite sides of the cell
Telophase II
Chromosomes arrive at opposite sides of the cell
Nuclear envelope forms around each haploid set of chromosomes
Spindle apparatus breaks down
Chromosomes decondense
Cytokinesis occurs
Four haploid cells that are genetically different from each other and from the parental cell
fertilization
restores a diploid set of chromosomes
Haploid cells may mature to form gametes.
May lead to a sexual form of reproduction
Two different types of gametes (e.g. sperm and egg)
can fuse to form a diploid offspring
asexual reproduction
produces clones that are genetically identical to one another and to the parent
sexual reproduction
produces offspring with unique chromosome complements
results in novel combination of alleles different from those of the parents into the offspring
karyotype
refers to the full set of chromosomes that are visible during metaphase
nondisjunction
leads to gametes with abnormal chromosome numbers
occurs in metaphase I when homologs fail to separate
all gametes have too many or too few chromosomes after meiosis II
cause of Down syndrome