UNIT ____:  Chromosomes                                  Name:  _____________________

Essential Idea(s):

• Chromosomes carry genes in a linear sequence that is shared by members of a species.

IB Assessment Statements and Class Objectives

3.2.U5:  Homologous chromosomes carry the same sequence of genes but not necessarily the same alleles of those genes.

• Define homologous chromosome.
• State a similarity and a difference found between pairs of homologous chromosomes.

3.2.U6:  Diploid nuclei have pairs of homologous chromosomes.

• Define diploid.
• State the human cell diploid number.
• Outline the formation of a diploid cell from two haploid gametes.

3.2.U7:  Haploid nuclei have one chromosomes of each pair.

• Define haploid.
• State the human cell haploid number.
• List example haploid cells.

3.2.U10:  Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex.

• Outline the structure and function of the two human sex chromosomes.
• Outline biological sex determination by sex chromosomes.

6.6.U5:  A gene on the Y chromosomes causes embryonic gonads to develop as testes and secrete testosterone.

• Describe the mechanism by which the SRY gene regulates embryonic gonad development.

3.2.U8:  The number of chromosomes is a characteristic feature of members of a species.

• State that chromosome number and type is a distinguishing characteristic of a species.
• List mechanisms by which a species chromosome number can change.

3.2.A3:  Comparison of diploid chromosome numbers of Homo sapiens, Pan troglodytes, Canis familiaris, Oryza sativa, Parascaris equorum.

• State the minimum chromosome number in eukaryotes.
• Explain why the typical number of chromosomes in a species is always an even number.
• Explain why the chromosome number of a species does not indicate the number of genes in the species.
• Explain the relationship between the number of human and chimpanzee chromosomes

3.2.A1:  Cairns’ technique for measuring the length of DNA by autoradiography.

• Describe Cairn’s technique for producing images of DNA molecules from E. coli.
• Outline conclusions drawn from the images produced using Cairn’s autoradiography technique.  

3.2.NOS:  Developments in research follow improvements in techniques- autoradiography was used to establish the length of DNA molecules in chromosomes.

• Outline the advancement in knowledge gained from the development of autoradiography techniques.

3.2.U9:  A karyogram shows the chromosomes of an organisms in homologous pairs of decreasing length.

• Describe the process of creating a karyogram.

• List the characteristics by which chromosomes are arranged on the karyogram.

3.3.A2:  Description of methods used to obtain cells for karyotype analysis e.g. chorionic villus sampling and amniocentesis and the associated risks.

• Describe the two procedures for obtaining fetal cells for production of a karyotype.

3.2.A4:  Use karyograms to deduce sex and diagnose Down Syndrome in humans.

• Deduce the sex of an individual given a karyogram.
• Describe the use of a karyogram to diagnose Down syndrome.  

HOMOLOGOUS CHROMOSOMES

Sex determination in mammals is a process, with variation possible at each step of the way:

Chromosomes 🡪 genes 🡪  gonads 🡪 hormones 🡪 genital phenotype

 🡪 brain 🡪 behavior

Chromosome Number in Species

• Species have different numbers of chromosomes than other species

• All eukaryotic species have at least …

• Chromosome numbers are even numbers because….

• The number of chromosomes in a species has no specific significance nor does it indicate any relationship between two species which may have the same chromosome number.
• The size of chromosomes varies from species to species.
• The number of chromosomes does NOT correlate to the number of genes
• Members of the same species have the same number of chromosomes
• The chromosome number can change, for example:

The length of chromosomes can be measured using autoradiography

• John Cairns experiment (1962):

• Incubate cells with radioactive thymine

• The cells incorporate the radioactive thymine into their DNA – so the DNA becomes *radioactive*

• Place a radioactive sensitive film over the sample.  The radioactive emissions from the DNA will expose the film, creating an image of the DNA.

Karyotyping         (creating a karyogram)                                                

How?

1. Sample collection and cell culture

• White cells from a blood sample or amniotic cells from a developing fetus are ideal
• Amniocentesis:

• Chorionic villus sampling:

2. Inhibition of Mitosis with Drugs

• The cells are treated with drugs which arrest the ___________________________________________.

• Certain drugs (like colchicines) interfere with the ________________________and the chromosomes will remain paired at the ________________________; hence, these drugs are used to `freeze' the chromosomes in place at a time when they are the most visible (chromosomes become condensed and microscopically visible during mitosis metaphase)

3. Separate the Chromosomes

• A _____________________________is added to the cultured cells which causes the cells to take in water and in ________________________

• This process causes the chromosomes which are tightly paired at the metaphase plate to separate from one another (hence each chromosome can be individually seen).

4. Staining

• Chemicals are used to stain the DNA which makes up most of the chromosome composition; in this way the chromosomes are highly visible under the microscope.

5. Photography

• With a camera fitted to a microscope, photographs are taken of several `mitotic plates' in which the chromosomes are visible and individually separated.

6. Karyotype Preparation

• The individual chromosomes are paired up according to ______ and _____________________.  

• The _______________________________(# 1-5) are arranged on the top row, and in order of descending size, four more rows are constructed with the paired chromosomes.

• Once the _____ pairs of ________________________ have been paired up, the remaining sex chromosomes (XX or XY) are positioned at the bottom right of the karyotype.