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What is meiosis?
Meiosis is a process where a single cell divides twice to produce four cells containing half the original amount of genetic information. These cells are our s*x cells – s***m in males, eggs in females.

During meiosis one cell? divides twice to form four daughter cells.
These four daughter cells only have half the number of chromosomes? of the parent cell – they are haploid.
Meiosis produces our s*x cells or gametes? (eggs in females and s***m in males).
Meiosis can be divided into nine stages. These are divided between the first time the cell divides (meiosis I) and the second time it divides (meiosis II):
Meiosis I
1. Interphase:
The DNA in the cell is copied resulting in two identical full sets of chromosomes.
Outside of the nucleus? are two centrosomes, each containing a pair of centrioles, these structures are critical for the process of cell division?.
During interphase, microtubules extend from these centrosomes.
2. Prophase I:
The copied chromosomes condense into X-shaped structures that can be easily seen under a microscope.
Each chromosome is composed of two sister chromatids containing identical genetic information.
The chromosomes pair up so that both copies of chromosome 1 are together, both copies of chromosome 2 are together, and so on.
The pairs of chromosomes may then exchange bits of DNA in a process called recombination or crossing over.
At the end of Prophase I the membrane around the nucleus in the cell dissolves away, releasing the chromosomes.
The meiotic spindle, consisting of microtubules and other proteins, extends across the cell between the centrioles.
3. Metaphase I:
The chromosome pairs line up next to each other along the centre (equator) of the cell.
The centrioles are now at opposites poles of the cell with the meiotic spindles extending from them.
The meiotic spindle fibres attach to one chromosome of each pair.
4. Anaphase I:
The pair of chromosomes are then pulled apart by the meiotic spindle, which pulls one chromosome to one pole of the cell and the other chromosome to the opposite pole.
In meiosis I the sister chromatids stay together. This is different to what happens in mitosis and meiosis II.
5. Telophase I and cytokinesis:
The chromosomes complete their move to the opposite poles of the cell.
At each pole of the cell a full set of chromosomes gather together.
A membrane forms around each set of chromosomes to create two new nuclei.
The single cell then pinches in the middle to form two separate daughter cells each containing a full set of chromosomes within a nucleus. This process is known as cytokinesis.
Meiosis II
6. Prophase II:
Now there are two daughter cells, each with 23 chromosomes (23 pairs of chromatids).
In each of the two daughter cells the chromosomes condense again into visible X-shaped structures that can be easily seen under a microscope.
The membrane around the nucleus in each daughter cell dissolves away releasing the chromosomes.
The centrioles duplicate.
The meiotic spindle forms again.
7. Metaphase II:
In each of the two daughter cells the chromosomes (pair of sister chromatids) line up end-to-end along the equator of the cell.
The centrioles are now at opposites poles in each of the daughter cells.
Meiotic spindle fibres at each pole of the cell attach to each of the sister chromatids.
8. Anaphase II:
The sister chromatids are then pulled to opposite poles due to the action of the meiotic spindle.
The separated chromatids are now individual chromosomes.
9. Telophase II and cytokinesis:
The chromosomes complete their move to the opposite poles of the cell.
At each pole of the cell a full set of chromosomes gather together.
A membrane forms around each set of chromosomes to create two new cell nuclei.
This is the last phase of meiosis, however cell division is not complete without another round of cytokinesis.
Once cytokinesis is complete there are four granddaughter cells, each with half a set of chromosomes (haploid):
in males, these four cells are all s***m cells
in females, one of the cells is an egg cell while the other three are polar bodies (small cells that do not develop into eggs).

100 Facts About DNA

1.DNA stands for deoxyribonucleic acid.
2.DNA is part of our definition of a living organism.
3.DNA is found in all living things.
4.DNA was first isolated in 1869 by Friedrich Miescher.
5.James Watson and Francis Crick figured out the structure of DNA.
6.DNA is a double helix.
7.The structure of DNA can be likened to a twisted ladder.
8.The rungs of the ladder are made up of “bases”
9.Adenine (A) is a base.
10.Thymine (T) is a base.
11.Cytosine (C) is a base
12.Guanine (G) is a base.
13.A always pairs with T in DNA.
14.C also pairs with G in DNA.
15.The amount of A is equal to the amount of T, same for C and G.
16.A+C = T+G
17.Hydrogen bonds hold the bases together.
18.The sides of the DNA ladder is made of sugars and phosphate atoms.
19.Bases attached to a sugar; this complex is called a nucleoside.
20.Sugar + phosphate + base = nucleotide.
21.The DNA ladder usually twists to the right.
22.There are many conformations of DNA: A-DNA, B-DNA, and Z-DNA are the only ones found in nature.
23.Almost all the cells in our body have DNA with the exception of red blood cells.
24.DNA is the “blueprint” of life.
25.Chromosomal or nuclear DNA is DNA found in the nucleus of cells.
26.Humans have 46 chromosomes.
27.Autosomal DNA is part of chromosomal DNA but does not include the two s*x chromsomes – X and Y.
28.One chromosome can have as little as 50 million base pairs or as much as 250 million base pairs.
29.Mitochondrial DNA (mtDNA) is found in the mitochondria.
30.mtDNA is only passed from the mother to the child because only eggs have mitochondria, not s***m.
31.There’s a copy of our entire DNA sequence in every cell of our body with one exception.
32.Our entire DNA sequence is called a genome.
33.There’s an estimated 3 billion DNA bases in our genome.
34.One million bases (called a megabase and abbreviated Mb) of DNA sequence data is roughly equivalent to 1 megabyte of computer data storage space.
35.Our entire DNA sequence would fill 200 1,000-page New York City telephone directories.
36.A complete 3 billion base genome would take 3 gigabytes of storage space.
37.If unwound and tied together, the strands of DNA in one cell would stretch almost six feet but would be only 50 trillionths of an inch wide.
38.In humans, the DNA molecule in a non-s*x cell would have a total length of 1.7 metres.
39.If you unwrap all the DNA you have in all your cells, you could reach the moon 6000 times!
40.Our s*x cells–eggs and s***m–have only half of our total DNA.
41.Over 99% of our DNA sequence is the same as other humans’.
42.DNA can self-replicate using cellular machinery made of proteins.
43.Genes are made of DNA.
44.Genes are pieces of DNA passed from parent to offspring that contain hereditary information.
45.The average gene is 10,000 to 15,000 bases long.
46.The segment of DNA designated a gene is made up of exons and introns.
47.Exons have the code for making proteins.
48.Introns are intervening sequences sometimes called “junk DNA.”
49.Junk DNA’s function or lack thereof is a source of debate.
50.Part of “junk DNA” help to regulate the genomic activity.
51.There are an estimated 20,000 to 25,000 genes in our genome.
52.In 2000, a rough draft of the human genome (complete DNA sequence) was completed.
53.In 2003, the final draft of the human genome was completed.
54.The human genome sequence generated by the private genomics company Celera was based on DNA samples collected from five donors who identified themselves only by race and s*x.
55.If all the DNA in your body was put end to end, it would reach to the sun and back over 600 times (100 trillion times six feet divided by 92 million miles).
56.It would take a person typing 60 words per minute, eight hours a day, around 50 years to type the human genome.
57.If all three billion letters in the human genome were stacked one millimeter apart, they would reach a height 7,000 times the height of the Empire State Building.
58.DNA is translated via cellular mechanisms into proteins.
59.DNA in sets of 3 bases, called a codon, code for amino acids, the building blocks of protein.
60.Changes in the DNA sequence are called mutations.
61.Many thing can cause mutations, including UV irradiation from the sun, chemicals like drugs, etc.
62.Mutations can be changes in just one DNA base.
63.Mutations can involve more than one DNA base.
64.Mutations can involve entire segments of chromosomes.
65.Single nucleotide polymorpshisms (SNPs) are single base changes in DNA.
66.Short tandem repeats (STRs) are short sequences of DNA repeated consecutively.
67.Some parts of the DNA sequence do not make proteins.
68.Genes make up only about 2-3% of our genome.
69.DNA is affected by the environment; environmental factors can turn genes on and off.
70.There are many ways you can analyze your DNA using commercially available tests.
71.Paternity tests compare segments of DNA between the potential father and child.
72.There are other types of relationship testing that compares DNA between siblings, grandparents and grandchild, etc.
73.DNA tests can help you understand your risk of disease.
74.A DNA mutation or variation may be associated with a higher risk of a number of diseases, including breast cancer.
75.DNA tests can help you understand your family history aka genetic genealogy.
76.DNA tests can help you understand your ethnic make-up.
77.DNA can be extracted from many different types of samples: blood, cheek cells, urine.
78.DNA can be stored either as cells on a cotton swab, buccal brush, or frozen blood or in extracted form.
79.In forensics, DNA analysis usually looks at 13 specific DNA markers (segments of DNA).
80.The odds that two individuals will have the same 13-loci DNA profile is about one in one billion.
81.A DNA fingerprint is a set of DNA markers that is unique for each individual except identical twins.
82.Identical twins share 100% of their genes.
83.Siblings share 50% of their genes.
84.A parent and child share 50% of their genes.
85.You can extract DNA at home from fruit and even your own cheek cells.
86.DNA is used to determine the pedigree for livestock or pets.
87.DNA is used in wildlife forensics to identify endangered species and people who hunt them (poachers).
88.DNA is used in identify victims of accidents or crime.
89.DNA is used to exonerate innocent people who’ve been wrongly convicted.
90.Many countries, including the US and UK, maintain a DNA database of convicted criminals.
91.The CODIS databank (COmbined DNA Index System) is maintained by the BI and has DNA profiles of convicted criminals.
92.Polymerase chain reaction (PCR) is used to amplify a sample of DNA so that there are more copies to analyze.
93.We eat DNA every day.
94.DNA testing is used to authenticate food like caviar and fine wine.
95.DNA is used to determine the purity of crops.
96.Genetically modified crops have DNA from another organism inserted to give the crops properties like pest resistance.
97.Dolly the cloned sheep had the same nuclear DNA as its donor mom but its mitochondrial DNA came from from the egg mom. (Does that make any sense?)
98.People like to talk about DNA even if it bears no relation to science or reality.
99.A group of bloggers who write regularly about DNA and genetics have banded together to form The DNA Network.
100.Lists about DNA can get a little boring :D

Which cell organell look like this....??