Prokaryote vs. Eukaryote Chromosomes Prokaryotes have one chromosome consisting of a circular DNA molecule. Some prokaryotes also have plasmids but eukaryotes do not. Eukaryote chromosomes are linear DNA molecules associated with histone proteins. In a eukaryote species there are different chromosomes that carry different genes.

DNA and Chromosomes Eukaryote chromosomes are linear DNA molecules associated with histone proteins. Eukaryotic chromosomes contain DNA wrapped around proteins called histones. The strands of nucleosomes are tightly coiled and supercoiled to form chromosomes.

Structure of Nucleosomes Of eukaryote chromosomes The double helix has major and minor groves on its outer diameter. These groves expose chemical groups that can form hydrogen bonds. These chemical groups within DNA are bonded to by proteins. DNA is bonded to proteins called HISTONES. The diagram to the left is of a nucleosome: DNA is wound around and hydrogen bonded to eight histones. 146 DNA bases or 1.65 turns of the helix are associated with the 8 histones The combination of DNA and histones is secured by the 'H1 linker' protein.

Supercoiling of Eukaryote DNA Supercoiling condenses the DNA molecule by a factor of X 15,000 Histones are responsible for the packaging of DNA at the different levels (diagram left). The metaphase chromosome is an adaptation for mitosis and meiosis. . A little extra info:- The histones interact with the DNA structure to neutralize the acidity of this molecules. The DNA raps around the histones x 1.65 ( 146 base pairs)which means that although condensed in wrapping the DNA it is still accessible to polymerase molecules. This preserves the functionality of the DNA. This level of winding around the histones brings about a X 6.8 fold reduction in length. This is the so called 30nm fibre. However there is still a long way to go before achieving the 1600 (250 nm fibre) or the 10 000 reduction in the metaphase chromosome. Beads of histones still remain in regions being transcribed and also where there is polymerisation. It would appear that the histones of the nucleosome are just slightly displaces locally.

Prokaryote chromosomes Prokaryotes have one chromosome consisting of a circular DNA molecule. Some prokaryotes also have plasmids but eukaryotes do not.

Karyotypes

IB ASSESSMENT STATEMENT State that, in karyotyping, chromosomes are arranged in pairs according to their size and structure.

Gender Determination

Karyotyping is a standardized arrangement of all the chromosomes of a cell. Karyotypes are created by Medical Technicians using special biochemical techniques. In a karyotype, the chromosomes are arranged and displayed in a standard format where identical pairs are ordered by Size and position of centromere

Two things that can be deduced from Karyotypes Gender of the person Chromosomes abnormalities

IB Assessment Statement Explain that non-disjunction can lead to changes in chromosome number, illustrated by reference to Down syndrome (trisomy 21).

Chromosome disorder A disorder caused by an individual having more or less chromosomes than normal Chromosome abnormalities would occur during anaphase I and II of meiosis, when chromosomes failed to separate. This is called non-disjunction. In non-disjunction a gamete will have too many or too few chromosomes. If this gamete becomes fertilized the zygote will have a chromosome disorder.

Animation of Non-disjunction http://www.biostudio.com/d_%20Meiotic%20Nondisjunction%20Meiosis%20I.htm http://www.biostudio.com/d_%20Meiotic%20Nondisjunction%20Meiosis%20II.htm

3 chromosomes of chromosome 21 is down syndrome

Chromosome Abnormality Disorder Chromosome Abnormality Characteristics Down Syndrome (trisomy 21) 47 chromosomes 3 chromosomes of number 21 Age of mother influences the risk At maternal age 20 to 24, the risk is 1/1490; At age 40 the risk is 1/60, and at age 49 the risk is 1/11 poor muscle tone flat nasal bridge Most individuals with down syndrome have mental retardation The incidence of Down syndrome is estimated at 1 per 800 births an enlarged tongue Turner’s 45 chromosomes Normal female has XX. Turners syndrome only has one X. Short stature Low hairline Low-set ears Reproductive sterility The absence of a menstrual period 1 out of every 2500 female births

Klinefelter’s Syndrome 47 or more chromosomes A male w/ XXY or XXXY Affected males are almost always sterile, and some degree of language impairment may be present. possible characteristics vary widely and include little to no signs of affectedness, a lanky, youthful build and facial appearance, or a rounded body type with some degree 1 in 500 live male births Metafemale 47 Chromosomes XXX Triple X syndrome most often causes no unusual physical features or medical problems. Females with the condition are usually taller than average, and their weight may be low in comparison to their height 1/1000 female births

IB ASSESSMENT STATEMENT Analyse a human karyotype to determine gender and whether nondisjunction has occurred. Karyotyping can be done by using enlarged photographs of chromosomes.

IB Assessment Statement State that karyotyping is performed using cells collected by chorionic villus sampling or amniocentesis, for pre-natal diagnosis of chromosome abnormalities.

How is karyotypes done on unborn babies? Cells are obtained from unborn babies using two methods. Amniocentesis Chorionic villus sampling

Aminiocentesis A sample of fluid is removed from the amniotic sac around the fetus. To do this a hypodermic needle is inserted through the wall of the mother abdomen and the wall of the uterus. Amniotic fluid is removed, which contain fetus cells.

Once cells are obtained Chromosomes are visible in the metaphase stage of mitosis. A picture is taken of these chromosomes. The chromosomes are cut out and arrange into pairs according to their size and structure.

ttp://video.about.com/pregnancy/Amniocentesis.htm

Chorionic Villus Sampling Cells are removed from the fetal tissue in the placenta called chorionic villi. A hypodermic needle is inserted into the mother abdomen and uterus wall to obtain cells.

ttp://video.about.com/pregnancy/Chorionic-Villus-Sampling.htm

Once cells are obtained Chromosomes are visible in the metaphase stage of mitosis. A picture is taken of these chromosomes. The chromosomes are cut out and arrange into pairs according to their size and structure.