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During metaphase I, the homologous chromosomes are arranged in the center of the cell with the kinetochores facing opposite poles. The orientation of each pair of homologous chromosomes at the center of the cell is random. As is discussed later, this too can contribute to genetic variation in the gametes.

This randomness, called independent assortment, is the physical basis for the generation of the second form of genetic variation in offspring. Consider that the homologous chromosomes of a sexually reproducing organism are originally inherited as two separate sets, one from each parent. Using humans as an example, one set of 23 chromosomes is present in the egg donated by the mother. The father provides the other set of 23 chromosomes in the sperm that fertilizes the egg. In metaphase I, these pairs line up at the midway point between the two poles of the cell. Because there is an equal chance that a microtubule fiber will encounter a maternally or paternally inherited chromosome, the arrangement of the tetrads at the metaphase plate is random. Any maternally inherited chromosome may face either pole. Any paternally inherited chromosome may also face either pole. The orientation of each tetrad is independent of the orientation of the other 22 tetrads.

In each cell that undergoes meiosis, the arrangement of the tetrads is different. The number of variations depends on the number of chromosomes making up a set. There are two possibilities for orientation (for each tetrad); thus, the possible number of alignments equals 2 n where n is the number of chromosomes per set. Humans have 23 chromosome pairs, which results in over eight million (2 23 ) possibilities. This number does not include the variability previously created in the sister chromatids by crossover. Given these two mechanisms, it is highly unlikely that any two haploid cells resulting from meiosis will have the same genetic composition ( [link] ).

To summarize the genetic consequences of meiosis I: the maternal and paternal genes are recombined by crossover events occurring on each homologous pair during prophase I; in addition, the random assortment of tetrads at metaphase produces a unique combination of maternal and paternal chromosomes that will make their way into the gametes.

This illustration shows that, in a cell with a set of two chromosomes, four possible arrangements of chromosomes can give rise to eight different kinds of gamete. These are the eight possible arrangements of chromosomes that can occur during meiosis of two chromosomes.
To demonstrate random, independent assortment at metaphase I, consider a cell with n = 2. In this case, there are two possible arrangements at the equatorial plane in metaphase I, as shown in the upper cell of each panel. These two possible orientations lead to the production of genetically different gametes. With more chromosomes, the number of possible arrangements increases dramatically.

In anaphase I, the spindle fibers pull the linked chromosomes apart. The sister chromatids remain tightly bound together at the centromere. It is the chiasma connections that are broken in anaphase I as the fibers attached to the fused kinetochores pull the homologous chromosomes apart ( [link] ).

In telophase I, the separated chromosomes arrive at opposite poles. The remainder of the typical telophase events may or may not occur depending on the species. In some organisms, the chromosomes decondense and nuclear envelopes form around the chromatids in telophase I.

Questions & Answers

what is Actin? I don't understand this definition
Rhonda Reply
Actin is a family of globular multi-functional proteins that form microfilaments. 
Lauren
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.
Nana
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.
Oliver
what is deoxyribonucleic acid
Carlene Reply
A negatively charged molecule; polymer of nucleotides found in the nucleus of a cell ... DNA.
Abayomi
guys u needs your help i am in this conversation but i don't understand what you guys were talking about please i need your help.
Asali
Hi
Janelle
hi
Oliver
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
Rayyan
what is immune system
Shaili Reply
system that defends the body against invading microbes n pathogens... (white blood cells makes immune system)
Rayyan
what is interferon's ?
maya Reply
interferon is any group of glycoproteins, produce by the immune system.
Henry
what is immune system
Shaili
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.
Kay-Anna
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.
shanta
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
Rayyan
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
Rayyan
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.
Rayyan
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
Rayyan
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.
Abayomi
what are organelles
Lorella Reply
Organelles are specialized structures in the that perform a specific function. Examples: mitochondrial , Golgi apparatus, etc.
Francis
Organelles are specialized structures in the cell that perform a specific function. Examples: mitochondrial, ribosomes, etc.
Francis
what connect bone to bone
Rebecca Reply
ligaments
Abayomi
what is the function of appendix?
Isaya Reply
it don't have any function
Rebecca
what is a joint
Rebecca Reply
A joint is a place where two bones meet or articulate.
Ebube
what is the function of lysozyme in saliva
Isaya Reply

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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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