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An organism whose cells lack organelles (specialized
segregated structures such as nuclei, mitochondria, and chloroplasts).
Prokaryote DNA is not arranged in chromosomes but forms a coiled structure
called a nucleoid. The prokaryotes comprise only the bacteria and cyanobacteria
(blue-green
algae); all other organisms are eukaryotes.
Plant Cell
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| Cell Notes | |
| Studying the Cell |
Active & Passive
Meiosis-
(my-oh’sis) Process in which a 2n cell undergoes two successive nuclear
divisions (meiosis I and meiosis II), potentially producing 4n nuclei;
leads to the formation of gametes in animals and spores in plants; division
of cells during sexual reproduction.
Meiosis I
Meiosis I reduces the number of chromosomes by half,
but each chromosome still contains two sister chromatids.
Meiosis II
Mitosis-
(my-toh’sis) Division of the cell nucleus, resulting in two daughter nuclei,
each with the same number of chromosomes as parent nucleus. Mitosis consists
of four phases: prophase, metaphase, anaphase, and telophase. Cytokinesis
(division of the cytoplasm to form two separate cells) usually overlaps
the telophase stage. This process is the division of body (somatic) cells
during reproduction.
Major differences between mitosis and meiosis:
1. Number of cell divisions and products.
mitosis - one cell division resulting in
two daughter cells.
meiosis - two cell divisions resulting in
four products of meiosis.
2. Ploidy (# chromosome sets) per nucleus.
mitosis - 2N to 2N
meiosis - 2N to N to N
3. Synapsis of homologous chromosomes.
mitosis - no pairing
meiosis - pairing at zygotene of prophase
I
4. Exchange of genetic material between synapsed homologous chromosomes.
mitosis - does not occur
meiosis - occurs at pachytene of prophase
I
(first visible at diplotene)
5. Timing of division of centromeres.
mitosis - occurs at anaphase
meiosis - occurs at anaphase II but not
at anaphase I
6. Genetic variation.
mitosis - conservative process; does not
lead to genetic variation
meiosis - leads to increased genetic variation
following recombination (crossing-over)
DNA- Deoxyribonucleic acid.
The genetic material of all organisms; composed of two complementary chains
of nucleotides wound in a double helix. Chromosomes contain the genetic
information that dictates what characteristics the daughter cells will
possess. It is useful to visualize a chromosome as a continuous strand
of DNA. Arrayed along the DNA strand are the genes, specific regions
whose sequences carry the genetic code for making specific proteins.
All species have a characteristic number
of homologous pairs of
chromosomes in their cells called the diploid
(or 2n) number.
Below are examples of various species.
Homo sapiens (human) 46
Mus musculus (house mouse) 40
Zea mays (corn or maize) 20
Drosophila melanogaster (fruit fly) 8
Xenopus laevis (South African clawed frog) 36
Caenorhabditis elegans (microscopic roundworm) 12
Equisetum arvense (field horsetail, a plant) 216
Saccharomyces cerevisiae (budding yeast) 32
Canis familiaris (domestic dog) 78
Arabidopsis thaliana (plant in the mustard family) 10
Myrmecia pilosula (an ant) 2
Parascaris equorum var. univalens (parasitic roundworm) 2
Cambarus clarkii (a crayfish) 200
The complete set of chromosomes in the cells of an organism is its karyotype.
Human Karyotype
The image above shows one of each chromosome, a normal cell will have homologous pairs of chromosomes 1 -22
The karyotype of the human female contains
23 pairs of homologous chromosomes:
22 pairs of autosomes
1 pair of X chromosomes
The karyotype of the human male contains:
the same 22 pairs of autosomes
one X chromosome
one Y chromosome
| Stem Cells | |
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