Chromosomes

Learning Targets 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 Homologous chromosomes carry the same sequence of genes but not necessarily the same alleles of those genes Diploid nuclei have pairs of homologous chromosomes Haploid nuclei have one chromosome of each pair The number of chromosomes is a characteristic feature of members of a species A karyogram shows the chromosomes of an organism in homologous pairs of decreasing length. Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex Cairns’ techniques for measuring the length of DNA molecules by autoradiography. Use of karyograms to deduce sex and diagnose Down syndrome in humans Use of databases to identify the locus of a human gene and its polypeptide product. Description of methods used to obtain cells for karyotype analysis e.g. chorionic villus sampling and amniocentesis and the associated risks.

Bacterial Chromosomes and plasmids Bacteria have one chromosome consisting of a circular loop of DNA Bacteria often have extra small loops of DNA called plasmids. Plasmids often contain a few genes that are useful but not essential to the cell’s lifecycle. The plasmids replicate independently from the chromosome

More on plasmids For example, plasmids often contain genes for antibiotic resistance Plasmids can be transferred from one cell to another, or even one species to another Plasmids are used by biologists to transfer genes between species artificially

Cairns’s technique for measuring the length of DNA molecules by autoradiography Cells were grown for two generations in a culture of radioactive thymidine This radioactively labeled the cell’s DNA The cells were then put on a film and the film was developed This image shows that E. coli chromosome was a single circular DNA molecule with a length of 1,100 µm. E. coli is only 2 µm long.

Eukaryotic chromosomes are different Chromosomes are linear and associated with histone proteins Different chromosomes all carry different genes. For example a certain gene may be found specifically on the upper part of the 12th chromosome.

DBQ- Comparing the chromosomes of mice and humans

Loci nomenclature- thanks Wikipedia!

Online database activity! Complete the gene location worksheet with the database found at: www.ncbi.nlm.nih.gov/gene

DBQ: Differences in Chromosome number

Biological sex determination A human diploid cell has 46 chromosomes 44 of these are called autosomes, which are chromosomes that do not determine sex. The first 22 pairs of homologous chromosome in a karyogram are the autosomal chromosomes The last two chromosomes are called sex chromosomes because they determine biological sex People with two X chromosomes have a biological sex of female People with one X chromosome and one Y chromosome have a biological sex of male

What gender is a person with the following karyogram?

Down’s syndrome/ trisomy 21 Down syndrome is a genetic disorder caused by an extra 21st chromosome. It affects physical growth and intellectual abilities and is associated with characteristic facial features.

DBQ: A human karyotype