Biology 3.2 Chromosomes

• Chromosomes carry genes in a linear sequence that is shared by members of a species.
• The asian rice ( Oryza sativa ) genome can be seen illustrated above.
• A karyogram can be used to determine sex and whether a person has Down Syndrome.
• The Biology Project activity "Diagnosing genetic disorders" can be used to practice skills and understanding of karyotyping.
• Down Syndrome is a genetic disorder that can be diagnosed through karyotyping.
• Rice possesses up 63,000 genes divided up between 12 chromosomes.
• A map of part of the first chromosome showing the gene loci present on it is illustrated below.
• Although different varieties (estimated 40,000 worldwide) will possess different alleles for genes, all individuals will share the same twelve chromosomes and the alleles of each variety will occur at the same position on same chromosome, i.e at the same gene loci.
• 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.
• There is only one copy of each gene on a single chromosome.
• A copy of the chromosome is made just before cell division (by binary fission).
• Prokaryotes have two types of DNA: a single chromosome and plasmids.
• Some prokaryotes also have plasmids but eukaryotes do not.
• Plasmids are naked DNA that is not associated with histone proteins and are small circular rings of DNA.
• Plasmids are useful in genetic engineering as they can be used to transfer genes into bacteria.
• Plasmids have been found in archea and eukaryota, but are very rarely responsible for normal life processes.
• The nucleoid chromosome controls or influences normal life processes.
• Plasmids commonly contain survival characteristics, such as antibiotic resistance.
• Plasmids can be passed between prokaryotes and can be incorporated into the nucleoid chromosome.
• A gene is a heritable factor that controls or influences a specific characteristic, consisting of a length of DNA occupying a particular position on a chromosome (locus).
• John Cairns used autoradiography to measure the length of DNA molecules in chromosomes.
• Eukaryote chromosomes are linear DNA molecules associated with histone proteins.
• Eukaryotic chromosomes may be up to 85mm in length.
• Nucleosomes are formed by wrapping DNA around histone proteins.
• The two DNA molecules formed by DNA replication prior to cell division are considered to be sister chromatids until the splitting of the centromere at the start of anaphase.
• After this, they are individual chromosomes.
• The number of chromosomes is a characteristic feature of members of a species.
• The genome size of T2 phage, Escherichia coli, Drosophila melanogaster, Homo sapiens and Paris japonica are 150 billion base pairs, 130 million base pairs, 4.6 million base pairs, 164 thousand base pairs, and 4.6 million base pairs respectively.
• A karyogram shows the chromosomes of an organism in homologous pairs of decreasing length.
• Karyotype is a property of the cell described by the number and type of chromosomes present in the nucleus (of a eukaryote cell).
• A karyogram is a diagram that shows, or can be used to determine, the karyotype.
• Methods used to obtain cells for karyotype analysis include chorionic villus sampling and amniocentesis and carry associated risks.
• The risk of a child having a trisomy such as Down Syndrome increases greatly in older mothers.
• Amniocentesis or chorionic villus samples can be taken and from them a karyotype can be constructed.