Mitosis Guided Notes - Biology
All cells come from cells
“Where a cell exists, there must have been a preexisting cell…” (Rudolf Virchow 1855)
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Repair and growth
Replacement of lost or damaged cells
Increasing in size
Reproduction
Asexual reproduction= process in which a single cell or group of cells each duplicates its genetic
material then splits into two new genetically identical cells
Sexual reproduction= the process in which, the genetic material from each of two parents combines,
producing offspring that differ genetically from either parent
Involves the union of sex cells (egg and sperm)
The cell cycle multiplies cells
Nucleus contains almost all of an organisms genes
Chromatin= long, thin fibers made up of DNA and proteins, located in the nucleus of eukaryotic
organisms
Chromosomes= condensed threads of genetic material formed from chromatin as a cell prepares to
divide
Before division a cell duplicates all of its chromosomes…each chromosome now consists of two identical
joined copies called sister chromatids
Centromere= the region where the two chromatids are joined together
Humans have 46 chromosomes
Eukaryotic cells that divide undergo an organized sequence of events called the cell cycle
Interphase= stage during which a cell carries out its metabolic processes and performs its functions
G1= growth phase
S= synthesis phase…duplication of DNA
G2= cell gets ready for division
M= mitotic phase
Mitotic phase (2 stages)
1. Mitosis= nucleus and duplicated chromosomes divide and are evenly distributed
2. Cytokinesis= cytoplasm divides
Cells divide during the mitotic phase
Movement of chromosomes is guided by a football shaped framework of microtubules called the
spindle
The spindle microtubules grow from two centrosomes= in animal cells, area of cytoplasmic material that
contain structures called centrioles
Centriole:
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Role unknown
Plant cells lack them
If a cell does not have them, the spindle will still form normally
Mitosis:
Prophase
Metaphase
Anaphase
Telophase
Prophase – 1st phase
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Chromosomes appear, seen as a pair of sister chromatids
Spindle forms
Chromatids attach to microtubules in spindle
Metaphase – 2nd phase
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Chromosomes line up in the center of the cell
Spindle is completely formed
Anaphase – 3rd phase
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Sister chromatids separate…each chromatid is called a daughter chromosome
Proteins in the centromere help the daughter chromosomes travel along the microtubules
toward the poles
At the same time the spindle fibers are shortening, bringing the chromosomes closer to the
poles
The spindle fibers that are not attached to the centromeres grow longer, pushing the poles
farther apart
Telophase – 4th phase
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Begins when the chromosomes reach the poles of the spindle
Spindle disappears
2 nuclear envelopes reform around each set of daughter chromosomes
Chromosomes uncoil and lengthen
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Nucleoli reappear
Mitosis is now finished
Cytokinesis completes the division process
Animal Cells: Indentation around the middle of the cell, caused by a ring of microfilaments in the
cytoplasm, underneath the plasma membrane (like a drawstring)
Plant cells: disk containing cell wall material, called a cell plate, forms inside the cell and grows outward
What would happen if cells did not have mechanisms that controlled cell division?
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Benign tumor= abnormal mass of “normal” cells
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Malignant tumor= masses of cells that result from the reproduction of cancer cells
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Cancer= a disease caused by the severe disruption of the mechanisms that control the cell
cycle…leading to uncontrolled cell division
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Metastasis= spread of cancer
Meiosis
Occurs in the sex organs
Ovaries
Testes
Typical human body cell contains 46 chromosomes…we get 23 chromosomes from our mother and 23
chromosomes from out father
Karyotyping matches up our chromosomes in pairs, according to size, location of centromere, and
staining bands
Each pair consists of one maternal chromosome and one paternal chromosome
Each pair of matching chromosomes are called homologous chromosomes
Each homologous chromosome in a pair carries the same sequence of genes controlling the same
inherited characteristics
Ex: eye color
How do homologous chromosomes differ from sister chromatids?
We have 23 pairs of chromosomes
1 pair is called the sex chromosomes
Females have 23 homologous pairs
The sex chromosomes look alike (X X)
Males have 22 homologous pairs
The sex chromosomes are different (X Y)
Most of the genes carried on the X chromosome do not have counterparts on the tiny Y chromosome
and vice versa
Diploid cell
Typical human body cell
Two sets of chromosomes
46 chromosomes...for humans (will differ depending on the species)
Haploid cell
One set of chromosomes
(Half the number of chromosomes)
Produced through the process of meiosis
Gametes= sex cells (haploid)…eggs and sperm
Fertilization= the fusion of the nuclei and cytoplasm of the sex cells
Zygote= fertilized egg (diploid)
Has two homologous sets of chromosomes
The zygote eventually develops into a sexually mature adult with trillions of cells produced by
mitosis
Interphase
Cell duplicates its DNA
Each chromosome consists of 2 identical sister chromatids
Meiosis I
Prophase I
Proteins cause the homologous chromosomes to stick together along their length
Paired chromosomes now consist of 4 chromatids, called a tetrad
Metaphase I
Tetrads line up in middle of cell
Anaphase I
Homologous chromosomes separate and move to opposite poles
Telophase I
Chromosomes arrive at the poles
Nucleus forms around chromosomes
Each pole now has a haploid daughter nucleus (it has only one set of chromosomes even though each
chromosome consists of two sister chromatids)
Cytokinesis
Meiosis II
Prophase II
Within each haploid cell, a spindle forms
Metaphase II
Chromosomes line up in the middle of the cell
Anaphase II
Sister chromatids separate and move to opposite poles
Telophase II
Chromatids are now considered individual chromosomes
Chromosomes arrive at the poles
Cytokinesis
Result is 4 daughter cells
Genetic Variation
Offspring that result from sexual reproduction are genetically different from their parents and from their
siblings
Genetic variation depends on
1. How the homologous chromosomes line up and separate at meiosis I
The assortment of chromosomes occurs randomly
For humans there are about 8 million possible chromosome combinations (223)
1. Crossing over= the exchange of genetic material between homologous chromosomes
Occurs during prophase I
Crossing Over: Homologous chromosomes are stuck together along their length
There is a precise gene-by-gene alignment between adjacent chromatids of the two chromosomes
Segments of chromosomes can be exchanged at one or more sites
Genetic recombination= a new combination of genetic material as a result of crossing over
A single chromosome contains many hundred genes
More than one crossing over event can occur in each tetrad
No wonder offspring can be so varied
A karyotype is a display of chromosomes paired according to their size, location of the centromere, and
staining patterns.
A karyotype reveals abnormalities in chromosome number or structure.
Humans have 23 pairs of chromosomes; 22 pairs of autosomes and one pair of sex chromosomes.
Autosome= a non-sex chromosome
Females are XX and males are XY.
Amniocentesis uses a needle to extract amniotic fluid from the uterus of a pregnant woman from the
14th to 17th week of pregnancy.
Up to 400 chromosome and biochemical problems can be detected by culturing fetal cells that are in the
amniotic fluid.
There is a slight risk of spontaneous abortion with this procedure.
Karyotyping: Sampled fetal cells are stimulated to divide in culture medium and another chemical stops
division during metaphase when chromosomes are highly condensed.
The stained cells are photographed and can be paired based on stained cross-bands, and size and shape.
Downs Syndrome