B MCAT - Mitosis, Meiosis, Embryogenesis Flashcards
Mitosis (M phase)
The start of mitosis marks the end of the G2 phase of interphase.
PROPHASE - nuclear envelope disappears. Chromatid condenses into chromosomes (sister chromatids). Centrosomes migrate to opposite ends of the cell. Spindle apparatus begins to form.
METAPHASE - Spindle fibers from centrosomes attach to the kinetochores on chromosomes; chromosomes to align vertically on the metaphase plate at midline of cell.
ANAPHASE - spindle fibers contract and pull sister chromatids apart = disjunction.
TELEPHASE - Nuclear membrane reforms. Cytokinesis gives 2 identical daughter cells.
Interphase (G1, S, and G2 phases)
The eukaryotic cell life cycle is divided into two phases: interphase and mitosis.
During interphase, the cell grows and makes a copy of its DNA. Represents the majority of its life.
3 parts to interphase:
G1 (first gap phase) - The cell grows and copies organelles
S - Cell makes a complete copy of its DNA in the nucleus; two sister chromatids connected at centromere (referred to as a single chromosome). Duplicates its centrosome.
G2 (second gap phase) - Cell grows more, makes proteins and organelles, reorganizes in preparation for mitosis (the M-phase).
- Same general steps to both meiosis and mitosis interphase. However, meiosis begins with a diploid cell with a homologous set of chromosomes, from mom and dad.
Meiosis
Following interphase, the diploid cell enters meiosis.
PROPHASE I - Nuclear envelope disappears.
Chromatin condenses into chromosome (sister chromatids). Crossing over can occur; cross-over between mom and dad is very important for genetic diversity. Centrosomes migrate to opposite ends of the cell. Spindle apparatus begins to form.
METAPHASE I - Spindle fibers from centrosomes attach to the kinetochores on chromosomes. Chromosomes arrange horizontally on metaphase plate–homologous pairs next to each other.
ANAPHASE I - Spindle fibers contract and pull sister chromatids apart= disjunction. The homologs separate (moms sister chromatids to one side of the cell, dads to the other) = disjunction.
TELEPHASE I - Nuclear membrane reforms. Cytokinesis gives two daughter cells–one with mom’s chromosomes and one with dad’s.
The process repeats.
PROPHASE II -
METAPHASE II -
ANAPHASE II -
TELEPHASE II -
The second phase looks like mitosis and results in four haploid daughter cells.
haploid cell
A cell with a single set of chromosomes.
(23 in humans)
diploid cell
A cell with two complete sets of chromosomes.
(46 in humans)
cytokinesis
The splitting of a cell. Cleavage furrow forms and cell separates.
Fertilization
1) Sperm comes into contact with zona pellucida of a secondary oocyte; zp binds it.
2) ACROSOMAL REACTION. Acrosome of sperm is released and acrosomal enzymes leak into zona pellucida and digest it, allowing deeper dive.
3) When sperm gets inside secondary oocyte, it causes a CORTICAL reaction, where cortical granule enzymes inside the secondary oocyte move to the surface and release their contents. This triggers a ZONA reaction. Zona pellucida surrounding cell hardens, so it captures any other sperm that tries to penetrate. BLOCK TO POLYSPERMY.
4) Plasma membranes of sperm and egg cell FUSE. Genetic material of sperm nucleus transfers from sperm head into egg cell. = FERTILIZATION. Cell now called a ZYGOTE.
Though the sperm cell has mitochondria, little to none make it into the egg. The egg has thousand’s of mitochondria, so the mom contributes ALL of the mitochondrial DNA.
Meiosis II happens after fertilization…
Early Embryogenesis
After fertilization, Meiosis II happens.
Except, instead of cell division, we get cell CLEVAGE of cells WITHIN the hardened zona pellucida, and the divided cells get smaller and smaller. Zygote now called MORULA.
Inside the MORULA, the cells get closer together = COMPACTION. Start to DIFFERENTIATE into TROPHOBLASTS (outer morula) and EMBRYOBLASTS (inner morula).
Embryoblasts clump to one side of zona pellucida into a INNER CELL MASS, leaving a space on the other half of the interior called the BLASTOCOEL. This ball of cells is now called a BLASTOCYST.
Zona pellucida disintegrates.
Inner cell mass separates to make an AMNITOTIC CAVITY. More DIFFERENTIATION into a BILAMINAR DISK (pancake-like structure) of HYPOBLASTS AND EPIBLASTS on one side, with the TROPHOBLASTS still forming a sphere.
GASTRULATION begins when PRIMATIVE STREAK begins to form as epiblasts migrate and form TRILAMINAR DISK. Now cells are in 3 germ layers:
1) ECTODERM (from eptiblasts)
2) MESODERM
3) ENDODERM
NEURULATION. Neural tube forms.
Oogenesis
Once a woman reaches her menarche (first menstrual cycle) one primary oocyte per month will complete meiosis I, producing a secondary oocyte and a polar body right before ovulation. This division is characterized by unequal cytokinesis, which doles ample cytoplasm to one daughter cell (the secondary oocyte) and nearly none to the other (the polar body). The polar body does not divide further and will never produce functional gametes. The secondary oocyte remains arrested in metaphase II and does not complete the remainder of meiosis II unless fertilization occurs.
Implantation
About a week after fertilization,
chiasmata
Chiasmata is the place where crossing over of mom’s chromosomes and dad’s chromosomes occurs during prophase I of meiosis.
genotype vs. phenotype
Genotype describes an individual’s combination of genes (ex. heterozygous vs. homozygous.
Phenotype describes an OBSERVABLE trait in an individual.
(Watch out for this distinction in complex questions! Also watch for CARRIER vs. expressed; unborn odds of a trait (x/4) vs. born so we know phenotype (likely x/3)
penetrance
The proportion of individuals whose phenotype actually matches their genotype and is expressed as we would expect based on Mendel’s Laws (proposed in 1865!).
expressivity
The degree to which a trait is expressed; the degree to which an individual is affected by a trait.
single chromosomal-level genetic mutations
Chromosomal-level mutations affect large segments of DNA.
Partial monosomy= deletion of a chunk of genes from a single chromosome.
Duplication=a chunk of genes is duplicated–two copies of all the genes within that segment. (partial trisomy)
Inversion=a segment of genes is flipped so the order of genes is changed. Many genes not affected, but the splicing sites can yield very different sequences.
