3.2 Chromosomes

3.2 Chromosomes Understandings: Prokaryotes have 1 chromo consisting of circular DNA Some prokaryotes also have plasmids but eukaryotes do not Eukaryote chromo’s are linear DNA molecules associated with histone proteins In a euk species there are diff chromo’s that carry diff genes Homo chromo’s carry the same sequence of genes but not necessarily the same alleles Diploid nuclei have pairs of homo chromo’s Haploid nuclei have 1 chromo of each pair The # of chromos 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 chromos and autosomes are chromos that do not determine sex

Understanding: Prokaryotes have ONE chromosome consisting of a circular DNA molecule

Understanding: Some prokaryotes also have plasmids but eukaryotes do not

Bacterial Plasmid

Multiple Resistance Plasmid

MRSA = multiple resistance Staphylococcus aureus

Understanding: Eukaryote chromosomes are linear DNA molecules associated with histone proteins

Nucleosome Subunit of eukaryotic chromosome Consists of 8 histone proteins DNA wraps around histones twice Function: Supercoils DNA around histones to pack it into a small nucleus

Understanding: In a eukaryote species there are different chromosomes that carry different genes

Understanding: Homologous chromosomes carry the same sequence of genes but not necessarily the same alleles of those genes

Understanding: Haploid nuclei have one chromosome of each pair

Understanding: Diploid nuclei have pairs of homologous chromosomes

Understanding: The number of chromosomes is a characteristic feature of members of a species

Sex is determined by sex chromosomes & autosomes are chromosomes that do not determine sex

A karyogram shows the chromosomes of an organism in homologous pairs of decreasing length Karyotype Karyogram

Karyogram of egg vs sperm: Sperm have 2 possible karyograms

Male or female? Normal or disorder?

Male or female? Normal or disorder?

Turner Syndrome

Klinefelter Syndrome

Superfemale!

Down’s Syndrome

Down’s Syndrome characteristics

Nature of Science: Developments in scientific research follow improvements in techniques: Autoradiography was used to establish the length of DNA molecules in chromosomes. Images, rather than quantitative data, provide evidence Autoradiography used since 1940s to discover where substances were located in cells or tissues John Cairns used autoradiography differently in 1960s – got images of whole DNA molecules from E. coli and discovered that bacteria have a SINGLE CHROMOSOME and showed replication forks Cairn’s technique was used by other scientists to discover structures in eukaryotes

Application: Cairns technique for measuring DNA length using autoradiography

John Cairns

Cairn’s autoradiography technique Radioactively labelled DNA (tritium – radioactive isotope of H) produced by E. coli DNA released onto surface of dialysis membrane Thin photographic film applied to membrane and left in darkness for 2 months, which some DNA decays and emits high energy electrons that react with film Film developed and examined with microscope. Where each tritium atom decayed there is a dark grain, showing DNA