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Meiosis is preceded by an interphase consisting of the G 1 , S, and G 2 phases, which are nearly identical to the phases preceding mitosis. The G 1 phase is the first phase of interphase and is focused on cell growth. In the S phase, the DNA of the chromosomes is replicated. Finally, in the G 2 phase, the cell undergoes the final preparations for meiosis.

During DNA duplication of the S phase, each chromosome becomes composed of two identical copies (called sister chromatids) that are held together at the centromere until they are pulled apart during meiosis II. Again, homologous chromosome pairs separate in meiosis I (i.e. reductional division) and sister chromatids separate during meiosis II (i.e. equational division).

Meiosis i

Early in prophase I, the chromosomes can be seen clearly microscopically. As the nuclear envelope begins to break down, the proteins associated with homologous chromosomes bring the pair close to each other. The tight pairing of the homologous chromosomes is called synapsis    . In synapsis, the genes on the chromatids of the homologous chromosomes are precisely aligned with each other. An exchange of chromosome segments between non-sister homologous chromatids occurs and is called crossing over    . This process is revealed visually after the exchange as chiasmata    (singular = chiasma ) ( [link] ). As will be discussed later, crossing over can result in genetic variability in the gametes.

As prophase I progresses, the close association between homologous chromosomes begins to break down, and the chromosomes continue to condense, although the homologous chromosomes remain attached to each other at chiasmata. The number of chiasmata varies with the species and the length of the chromosome. At the end of prophase I, the pairs are held together only at chiasmata ( [link] ) and are called tetrads because the four sister chromatids of each pair of homologous chromosomes are now visible.

The crossover events are the first source of genetic variation produced by meiosis. A single crossover event between homologous non-sister chromatids leads to a reciprocal exchange of equivalent DNA between a maternal chromosome and a paternal chromosome. Now, when that sister chromatid is moved into a gamete, it will carry some DNA from one parent of the individual and some DNA from the other parent. The recombinant    sister chromatid has a combination of maternal and paternal genes that did not exist before the crossover. It is important to note that crossing over will only produce genetic diversity if there was diversity between the maternal and paternal chromosomes.

This illustration shows a pair of homologous chromosomes that are aligned. the ends of two non-sister chromatids of the homologous chromosomes cross over, and genetic material is exchanged. the non-sister chromatids between which genetic material was exchanged are called recombinant chromosomes. the other pair of non-sister chromatids that did not exchange genetic material are called non-recombinant chromosomes.
In this illustration of the effects of crossing over, the blue chromosome came from the individual’s father and the red chromosome came from the individual’s mother. Crossover occurs between non-sister chromatids of homologous chromosomes. The result is an exchange of genetic material between homologous chromosomes. The chromosomes that have a mixture of maternal and paternal sequence that differ genetically are called recombinant and the chromosomes that are completely paternal or maternal are called non-recombinant. Note: Crossing over can occur several times between the same pair of homologous chromosomes.

Questions & Answers

what is the difference between a vaccine and a antiserum
Silver Reply
An antiserum contains antibodies already produced and is used to pass on immune responses. A vaccine contains a substance that stimulates the production of antibodies to create an immune response.
A vaccine a preparation of antigens for one (or more) diseases that is given to stimulate active immunity and protect against the disease (s). while an antiserum either neutralizes the "infection " or stimulates your immune system to attack an infection.
what is deoxyribonucleic acid
Carlene Reply
A negatively charged molecule; polymer of nucleotides found in the nucleus of a cell ... DNA.
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to maintain certain biological activities of cell
jeeni Reply
what are difference between maleriya and dengue
Shaili Reply
caused by two different agents.. malaria is caused by Culex mosquito in birds and Anopheles in humans.. while dengue fever is caused by Aedes mosquito
what is immune system
Shaili Reply
system that defends the body against invading microbes n pathogens... (white blood cells makes immune system)
what is interferon's ?
maya Reply
interferon is any group of glycoproteins, produce by the immune system.
what is immune system
the difference between human biology and principles of biology
Henry Reply
human biology is the studying of stucture and of function of the body and the principles of how humans interacts with men, animal and plants around them and the social activities that take place.
human biology is the studying of human body structures function in the way of interacting physical and internal. while biology is the study of living and non living things.
what is Mitosis/Meiosis? What are the similarities and differences?
Abayomi Reply
differences: occurence... one in somatic n other in germ cells no of daughter cells.. 2 and in meisis 4. genetic recombination... only occurs in meiosis during crossing over no. of chromosomes.. remains same in mitosis n reduced to half in meiosis major phases... similar karyokinesis n cytokinesis
karyokinesis... divided into prophase.. meta.. ana.. and telophase... in meiosis.. further division into meiosis 1 and meiosis11 so phases are named as pro1..meta1..ana1..and telo1
differences among phases.. prophase.. other changes similar.. but diff occur in meiosis where synapsis, crossing over,chiasmata formation and tetrad form metaphase... in mitosis chromosomes align at equatorial plate.. in meiosis homologouy chromosomes align at equatorial plate.
anaphase.. in mitosis chromatids gets separated in meiosis chromosomes gets separated.. meiosis 11 is similar to mitosis except that during interkinesis.. duplication in S phase doesnt occur
What is the difference between prokaryotic and eukaryotic organisms
AAlfred Reply
Prokaryotic cell are unicellular, lacks a membrane bound organelles ... e.g bacteria and archea. Eukaryotic cells are multicellular, membrane bound organelles ... e.g Animals, Plants, Fungi, Algae.
what are organelles
Lorella Reply
Organelles are specialized structures in the that perform a specific function. Examples: mitochondrial , Golgi apparatus, etc.
Organelles are specialized structures in the cell that perform a specific function. Examples: mitochondrial, ribosomes, etc.
what connect bone to bone
Rebecca Reply
what is the function of appendix?
Isaya Reply
it don't have any function
what is a joint
Rebecca Reply
A joint is a place where two bones meet or articulate.
what is the function of lysozyme in saliva
Isaya Reply
what is chile
Isaya Reply

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