Ch 3

• • A gene is a heritable factor that consists of a length of DNA and influences a specific characteristic.
• • A gene occupies a specific position on a chromosome.
• • The various specific forms of a gene are alleles
• • Alleles differ from each other by one or only a few bases
• • New alleles are formed by mutation
• • The genome is the whole of the genetic information of an organism
• • The entire base sequence of human genes was sequenced in the Human Genome Project
• • Application: The causes of sickle cell anemia, including a base substitution mutation, a change to the base sequence of mRNA transcribed from it and a change to the sequence of a polypeptide in hemoglobin.
• • Application: Comparison of the number of genes in humans with other species.
• • Students should be able to recall one specific base substitution that causes glutamic acid to be substituted by valine as the sixth amino acid in the hemoglobin polypeptide.
• • 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’ technique for measuring the length of DNA molecules by autoradiography.
• • Application: Comparison of genome size in T2 phage, Escherichia coli, Drosophila melanogaster, Homo sapiens and Paris japonica.
• • Application: Comparison of diploid chromosome numbers of Homo sapiens, Pan troglodytes, Canis familiaris, Oryza sativa, Parascaris equorum.
• • Application: Use of karyograms to deduce sex and diagnose Down syndrome in humans.
• • The terms karyotype and karyogram have different meanings. Karyotype is a property of a cell—the number and type of chromosomes present in the nucleus, not a photograph or diagram of them.
• • One diploid nucleus divides by meiosis to produce four haploid nuclei
• • The halving of the chromosome number allows a sexual life cycle with fusion of gametes.
• • DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids.
• • The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensation.
• • Orientation of pairs of homologous chromosomes prior to separation is random.
• • Separation of pairs of homologous chromosomes in the first division of meiosis halves the chromosome number.
• • Crossing over and random orientation promotes genetic variation.
• • Fusion of gametes from different parents promotes genetic variation.
• • Application: Non-disjunction can cause Down syndrome and other chromosome abnormalities.
• • Application: Studies showing age of parents influences chances of non-disjunction.
• • Application: Description of methods used to obtain cells for karyotype analysis e.g. chorionic villus sampling and amniocentesis and the associated risks. (amniocentesis : later less risk )
• • Skill: Drawing diagrams to show the stages of meiosis resulting in the formation of four haploid cells.
• • Mendel discovered the principles of inheritance with experiments in which large numbers of pea plants were crossed.
• • Gametes are haploid so contain only one allele of each gene.
• • The two alleles of each gene separate into different haploid daughter nuclei during meiosis.