AP Bio Notes on Meiosis- Ch. 13 wps.prenhall.com homepage1.nifty.com

_Asexual_ reproduction produces offspring by mitosis that are _identical_ to each other and original parent. These identical offspring are called _clones_.

_Sexual_ reproduction provides for genetic _variation_ - the offspring have combined traits of the parents that make them _different_ from their parents and their siblings. This is very important because these new trait combinations may make them _better_ or worse off in their environment than others in their family or species.

Chromosome Numbers _Somatic Cell – any _body_ cell except egg or sperm cells (Ex. skin cell, blood cell, bone cell, etc) Each somatic cell has its chromosomes in pairs called _homologous_ chromosomes or _homologues_. Each chromosome has thousands of _genes_ (gene = part of a DNA strand that causes a certain _trait_ such as eye color, hair type, lactose intolerant, etc)

Homologous chromosomes are the same _size_, shape and have information for the same _traits_. The place on a chromosome where a particular gene is found is called its _locus_ (loci is plural).

Diagram:

Special chromosomes called _sex _chromosomes_ determine your sex. _X chromosome_ – a large sex chromosome _Y chromosome_ – a small sex chromosome In humans, a _female__ has two X chromosomes. XX = female A _male_ has one X and one Y chromosome. XY = male

Different _organisms_ have different numbers of _chromosomes_. _Autosomes_ – all other chromosomes except sex chromosomes Different _organisms_ have different numbers of _chromosomes_. Ex. Humans have 46 (23 pairs), a fly has 12 (6 pairs), a dog has 78 (39 pairs) A _diploid_ cell has its chromosomes in _pairs_. All __somatic__ cells are diploid cells.

_Gametes_ are reproductive cells such as _egg_ or _sperm_ cells. Gametes have only _1_ chromosome from each homologous pair of chromosomes. This is so that when they join, the new cell will have the right number of chromosomes.

Diagram:

Fertilization_ – when an egg and sperm join to make a new cell _Zygote_ – new cell made when egg and sperm join (fertilized egg) A _Haploid_ cell has only _1_ copy of each kind of chromosome. Eggs and sperm are _haploid_. The zygote is _diploid_.

If the haploid number of chromosomes is n, the diploid number is 2n. Ex. In humans, n = 23 so the diploid number is 2n = 2x23 = 46

Eggs and sperm can not be produced by mitosis or they would not be _haploid_. Instead, they are made by _meiosis_. In meiosis new cells are made – each with only _1_ copy of each chromosome type. No homologous pairs.

Overview of Meiosis

Stages of Meiosis The time between meiotic divisions is still called _interphase_ and the DNA is still copied by replication during the _synthesis_ stage as usual.

The cell that begins the process of meiosis is always a _diploid_ _somatic_ cell.  Remember, meiosis _makes_ gametes. It does _not_ occur _in_ gametes!

1. Prophase I A. Chromatin coils into doubled __chromosomes__ B. Nuclear envelope and nucleolus __disintegrate__ C. _spindle_ forms

Prophase I continued **D. _Synapsis_ occurs - Homologous chromosomes _line_ _up_ next to each other to form a group of 4 chromatids called a _tetrad_.

Tetrad

Prophase I continued **_Crossing_ _over_ can occur– part of one _chromatid_ breaks off and   is exchanged with the same part of a chromatid of its _homologous_chromosome)

Crossing over

Prophase is the longest stage of meiosis. Prophase I continued The place where the 2 chromatids overlap is called _chiasma__ (chiasmata = plural) Prophase is the longest stage of meiosis.

2. Metaphase I Pairs of the _homologous_ chromosomes line up on the equator

Metaphase I

3. Anaphase I Homologous chromosomes _move_ to opposite ends of the cell.

Anaphase I

Telophase I __Cytokinesis_ occurs. (Some cells uncoil the chromosomes and form new nuclear membranes and nucleoli, but most cells skip this since the division process is going to continue.)

Telophase I

5. Prophase II New _spindle_ forms

Metaphase II Chromosomes line up on _metaphase_ _plate_in single file.

Metaphase II

Anaphase II Centromeres divide and _chromatids_(single chromosomes) go to opposite poles.

Anaphase II

Telephase II New nuclear _membranes_ and __nucleoli_ form. __Spindle_ breaks up Chromosomes uncoil into __chromatin__ _Cytokinesis_ occurs

Telophase II

Now have _4_ _haploid_ cells!  __Spermatogenesis__ – meiosis in males to produce sperm. Makes four sperm as each haploid cell grows a tail.

Watch meiosis video clip

Diagram of spermatogenesis

_Oogenesis_ – meiosis to produce an egg cell in females. Cytokinesis is __unequal_ each time and only one large cell becomes an egg cell or _ovum_. The other three small cells are called _polar_ _bodies_. They die in animal cells.

Oogenesis

3 sources of genetic variability (Make offspring different from each other) 1. Independent assortment 2. Random fertilization 3. Crossing over

_Independent_ _assortment_ – homologous chromosomes are distributed randomly during meiosis – you can get either chromosome of each pair. This creates many possible combinations of chromosomes in the gametes.

Ex. Humans have 23 pairs, so 223 (about _8_ _million_) combinations are possible in an egg or sperm!

2. Random Fertilization Because two gametes join, there are 223 x 223 = 64 _trillion_ possible chromosome combinations in the children of two certain parents.

3. Crossing over Crossing over creates even more possibilities so the different possible gametes is almost _unlimited_.

Chromosome problems A person with the wrong number of chromosomes may or may not be _normal_. Ex. _Trisomy_ _21_ or Down’s syndrome – this person has three copies of chromosome number 21. Will have varying degrees of mental retardation

Down’s syndrome http://www.thespeciallife.com/images/downs-syndrome-boy.jpg

Down’s syndrome http://www.downssupport.org.uk/images/down'schild2.jpg

Can be caused by _nondisjunction__ – when chromosomes do not separate properly during cell division.

Diagram of nondisjunction during meiosis I

Diagram of nondisjunction during meiosis II

__Karyotype_ – a photo of chromosomes during division in which chromosomes are arranged in pairs to see if they are correct.  

Karyotype

Karyotype

Risk of nondisjunction and Down’s syndrome _increases_ as a woman ages. _Amniocentesis_ – amniotic fluid is taken from around a baby in the uterus to make a karyotype to check for chromosome problems in the baby.

Other disorders caused by nondisjunction: Turner syndrome: A female with only 1 X chromosome. This girl will be sterile (unable to have children) because her sex organs will not mature.

Turner syndrome continued Estrogen replacement therapy can help with development of secondary sex characteristics. Most have normal intelligence. Occurs in 1 out of 5,000 births.

B. Kleinfelter syndrome: A male with XXY sex chromosomes. The testes are abnormally small and he will be sterile. He will also have subnormal intelligence. Occurs in 1 out of 2,000 births.