MGen: Mitosis & Meiosis

Prokaryotic Cells

• respiration is performed in cytoplasm & plasma membrane

• unicellular

• typically smaller than eukaryotes

• no cell division (no nucleus & organelles)

Eukaryotic Cells

• multicellular

• intracellular organelles

• Plant cells have cell walls but Animal cells don’t

Prokaryote Genetic Structures

• Prokaryotic DNA is not highly ordered and packed

• Circular chromosomes

• Less DNA than eukaryotes

Eukaryotic Genetic Structures

• DNA is closely associated with histones to form tightly packed chromosomes

• linear chromosomes

• multiple DNA

Q: Genetically speaking, prokaryotes are different from eukaryotes because

A. prokaryotes usually have one main circular chromosome, whereas eukaryotes have multiple linear chromosomes

B. many prokaryotes do not have histone proteins to package their DNA

C. prokaryotes do not have a nucleus to house their DNA as eukaryotes do

D. prokaryotes have less total DNA than do eukaryotes

E. All of the Above

E

Prokaryotic Cell Division

• binary fission

• simple divison: separation of a replicated circular chromosome and cytoplasm

• fast rate of division

Eukaryotic Cell Replication

• much more complex than prokaryotic

• takes more time to complete

• chromosomes are linear and organized

• multiple chromosomes typically present

• Ploidy: chromosomes organized into “sets” of chromosomes

• yeast are unicellular - exception of multicellular eukaryotes

Q: Humans have how many pairs of diploid cells?

23 pairs

Haploid vs Diploid

• Haploid (N): each chromosome (and gene) represented once

• Diploid (2N): each chromosome (and gene) represented twice

Homologous chromosomes

pairs of chromosomes in diploid cells that have the same genes in the same locations

Eukaryotic Chromosome Structure

• Linear Structure

• Ends stabilized by repetitive sequences called telomeres

• Bind spindle microtubules during mitosis through repetitive sequences called centromeres

when are sister chromatids duplicated?

During S Phase of the cell cycle

Q: Most body (nonreproductive) cells of humans and other multicellular eukaryotes have two sets of each chromosome. Such cells are ___, and the matching pairs of chromosomes are called _____

diploid, homologous chromosomes

Q: If an organism is diploid and a certain gene found in the organism has 18 known alleles (gene variants), then any individual organism of that species can/must have which of the following?

at most, two alleles for that gene

How are chromosomes replicated and transmitted?

• Eukaryotic cells undergo two forms of cell division:

  • Mitosis: produces 2 identical daughter cells, each with the same number of chromosomes as parent cell (2n)

  • Meiosis: reduces genetic content and leads to production of sex gametes, which contain half as many chromosomes (n) as the parent cell

Chromosomes must be ____ before cells divide (think cell cycle)

replicated

Q: A cell that has acquired damage to some of its DNA will be stalled at _____ of the cell cycle

G2/M checkpoint

Mitosis Cycle

• Prophase: Chromosomes condense and mitotic spindle forms

• Prometaphase: Nuclear envelope disintegrates, and spindle microtubules anchor to kinetochores

• Metaphase: Chromosomes align on the metaphase plate; spindle-assembly checkpoint

• Anaphase: Sister chromatids separate, becoming individual chromosomes that migrate toward spindle poles

• Telophase: Chromosomes arrive at spindle poles, the nuclear envelope re-forms, and the condensed chromosomes relax

• Cytokinesis: Cytoplasm divides; cell wall forms in plant cells

Q: After S phase, a chromosome consists of 2 sister chromatids. Does the genetic information on the 2 sister chromatids come from only one parent or from both?

one due to DNA replication. During S phase, a single chromosome makes an exact copy of its own DNA molecule.

Q: Sister chromatids separate and move toward opposite poles during which phase of mitosis?

Anaphase

Q: In the example below, how many chromosomes are in each cell in prophase and in anaphase? (include pic)

4 and 8

Q: If an organism has a diploid number of 16, there will be __ chromatids visible at the end of mitotic prophase and __ chromosomes moving to each pole during anaphase of mitosis

32; 16. 2n = 16, n=32

Mitosis produces 2 daughter cells that are genetically _______ to each other

identical

______ holds sister chromatids together during mitosis and meiosis

cohesin

Overview of Meiosis (not cell cycle)

• Interphase: DNA synthesis and chromosome replication phase

• Meiosis I: separation of homologous chromosome pairs, and reduction of the chromosome number by half

• Meiosis II: separation of sister chromatids, also known as equational division

define synapsis, tetrad, and crossing over

• synapsis: close pairing of homologous chromosomes

• tetrad: closely associated four-sister chromatids of two homologous chromosomes

• crossing over: exchange of genetic information between chromosomes segments from the sister chromatid of one chromosome to the sister chromatid of the other synapsed chromosome

Q: If a cell has completed meiosis I and is just beginning meiosis II, which of the following is an appropriate description of its contents?

it is haploid due to homologous chromosome separation

meiosis causes genetic variation through

independent assortment

Q: this is a picture of a cell from an organism whose somatic cells are 2n. what stage of mitosis or meiosis is this cell in? (include pic)

metaphase 1 of meiosis 1

Q: this picture depicts a cell that started out as 2n=4. what stage of mitosis or meiosis is it in? (include pic)

metaphase

Q: if this cell were isolated from a diploid organism, it is probably in (include pic)

prophase II

Meiosis I Cell Cycle

• prophase I: chromosomes condense, homologous chromosomes synapse, crossing over takes place, the nuclear envelope breaks down, and the mitotic spindle forms

• metaphase I: homologous pairs of chromosomes line up on the metaphase plate

• anaphase I: the two chromosomes (each w/ two chromatids) of a homologous pair separate and move toward opposite poles

• telophase I: chromosomes arrive at the spindle poles

• cytokinesis: the cytoplasm divides to produce two cells, each having half the original number of chromosomes

• interkinesis: in some types of cells, the spindle breaks down, chromosomes relax, and a nuclear envelope re-forms, but no DNA synthesis takes place

Meiosis II Cell Cycle

• prophase II: chromosomes condense, the spindle forms, and the nuclear membrane disintegrates

• metaphase II: individual chromosomes line up on the metaphase plate

• anaphase II: sister chromatids separate and move as individual chromosomes toward the spindle poles

• telophase II: chromosomes arrive at the spindle poles; the spindle breaks down and a nuclear envelope re-forms

• cytokinesis: the cytoplasm divides

_______ facilitate homologue separation in meiosis I

crossovers. cohesin between sisters does not break homologues.

______ facilitates sister separation in meiosis II

cohesin

Q: are homologous pairs of chromosomes present in mitosis? ****

yes, but they only pair up in meiosis. they are present in mitosis when a cell is diploid because a diploid cell (2n) inherently contains 2 complete sets of chromosomes

Meiosis in humans

Q: how many chromosomes would have been in an oogonium that produced an ovum with 18 chromosomes?

36

Q: the horse has 32 pairs of chromosomes, whereas the donkey has 31 pairs of chromosomes. how many chromosomes would be expected in the somatic tissue of a mule?

63

nondisjunction

failure of homologous chromosomes or sister chromatids to separate properly during cell division

autosomal aneuploidy consequences for:

• Nullisomy (misisng a pair of homologs)

• Monosomy (one chromosome missing)

• Trisomy (one extra chromosome)

• Nullisomy: pre-implantation lethal

• Monosomy: embryonic lethal

• Trisomy: usually lethal, trisomy 21 viable, trisomy 18 may survive to term