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Interphase

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 genetic engineering
Ebot Reply
good day please what is a cell?
Colota Reply
cell is the basic unit of all living things/organism
Hassan
differenciate b/w cilia and flagellum
Abshira
Cilia: 1. They are smaller in number. 2. Cilia usually occur throughout or major part of the surface of a cell. 3. They beat oar like and in a co-ordinated rhythm 4. Cilia help in locomotion, feeding, circulation, etc.. 5. Example: Paramecium
iti
Flagella: 1. They are larger in size. ADVERTISEMENTS: 2. Flagella are commonly found at one end of the cell. 3. The flagella beat whip-like and independently. 4. Flagella help in locomotion. 5. Example: Euglena
iti
smallest structural and functional unit of an organism!
zidane
cell is the basic unit of all living things
Lukman
what is homeostasis
alex Reply
the ability of an organism to maintain stable internal environment, even when the external environment changes
Bee
what's the function of the hypothalamus
Agyekum
what is homeostasis
Edward Reply
constant maintenance of internal body temperature
Agyekum
What is the function of the Pituitary Gland?
WeNdlovu
how can homeostasis be maintained?
alex
pituitary gland also known as the master gland secretes hormones which triggers other gland to secretes their hormones
Agyekum
what's the full meaning of DNA and RNA
Deborah Reply
DNA is Deoxyribonucleic acid and RNA means Ribonucleic acid
Christian
sha is mitochondria?
Sparrow Reply
what is biology
Femi Reply
biology is the study which deals with the study of the biological components or animal and plants.
Juhi
biology is the study of life.
Roger
biology is the branch of science which deals with study of two parts 1.botany&2zoology these is known as biology....
Chetan
biology is the study of human beings ...
Chetan
Biology is a branch of science that deals with the study of nature
Agyekum
biology is the science of living organism
Abshira
biology is the branch of science that deals with study of living organisms
Lukman
what is neculer cell?
is it nuclear cell?
iti
asthma occur in which part of the body organ?
Festus Reply
what is hemoglobin
Lucky Reply
a protein that carries oxygen, it is found within red blood cells
EmmyOrangePeel
Apart from endocrinology, what other alternative should be done to a D/m client
Deborah Reply
good day please what is excretion
Blaise Reply
excretion is the process by which metabolic wastes and other none-useful materials are eliminated from an organism.
Petrene
its the process of eliminating or removal of metabolic waste and non useful materials from an organ.
eyong
Its the removal of metabolic waste product from the body
Boo
it is the removal from the body of living organisms, toxic waste products of metabolism which if allowed to accurate can cost harm to the body..
Renzo
how are protons formed?
Brian Reply
What is isotopes?
Brian
isotopes is a phenomenon in which atoms of the same element has the same atomic number but different mass number due to a difference in their neutron number or relative abundance in the earth crust
Efosa
give me more mcqs there r just two mcqs
Salma Reply
Define the term Disease in one paragraph.
Sutanya Reply
Disease is any disorders that counteract with the normal performance of the immune system.
Agyekum
what is cell?
Colota
Cell is the basic unit of life It is the building block of an organism
Agyekum
do agree with me that if someone needs to truely be a biologist he should be mathematician physicist & chemist in oreder to acheive the level #biologist
Assila
what is a protoplasm?
Colota
colourless living part of the cell
Jenny
what is protoplasm made up of?
Jack
what are the social differences between man and primates
flore Reply
Hy what is the meaning of DNA
Fahhama

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Source:  OpenStax, Human biology. OpenStax CNX. Dec 01, 2015 Download for free at http://legacy.cnx.org/content/col11903/1.3
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