BHS016-1 Molecular Genetics Lecture 02 - Chromosome Structure PDF

Genetics (BHS016-1) Dr Taiwo Shittu 1 Chromosome Structure BHS016-1 Topic 2 2 Learning Outcomes ◼ After this topic you should be able to: ❑ Describe the structures and elements of chromosomes. ❑ Name and define key genetic te...

Genetics (BHS016-1) Dr Taiwo Shittu 1 Chromosome Structure BHS016-1 Topic 2 2 Learning Outcomes ◼ After this topic you should be able to: ❑ Describe the structures and elements of chromosomes. ❑ Name and define key genetic terms relating to chromosomes and genomes. ❑ Understand the key differences and similarities between prokaryotic and eukaryotic chromosomes. ❑ Explain the locations and roles of genomic vs. extrachromosomal DNA in cells. 3 What Are Chromosomes? ◼ Definition: CHROMOSOME A B C D E F G 4 5 Prokaryotic vs. Eukaryotic DNA Image taken from Pierce, BA. (2012). Genetics: A Conceptual Approach 4th Ed. 6 Bacterial Chromosomes 7 Bacterial DNA: Circular And Supercoiled Image taken from Pierce, BA. (2012). Genetics: A Conceptual Approach 4th Ed. 8 Bacterial DNA: Folded In Twisted Loops Image taken from Pierce, BA. (2012). Genetics: A Conceptual Approach 4th Ed. 9 Bacterial DNA: Clumped In Nucleoid Image taken from Pierce, BA. (2012). Genetics: A Conceptual Approach 4th Edn 10 Prokaryotic DNA ◼ Usually a single chromosome. ◼ Usually circular. ◼ Anchored to proteins, giving long loops of DNA. (Not usually wrapped around histones). ◼ Present in cytoplasm of bacterial cell. ◼ Definition: NUCLEOID. ◼ Contains single origin of replication. ◼ Definition: ORIGIN OF REPLICATION. 11 Viral Chromosomes 12 Viral Genome May Be DNA Or RNA 13 Eukaryotic Chromosomes 14 Eukaryotic Genomic DNA 15 Eukaryotic Genomic DNA ◼ Multiple chromosomes. ◼ Usually linear. ◼ Tightly packaged with histone proteins, creating chromatin. ◼ Present in nucleus of eukaryotic cell. ◼ Contains many origins of replication per chromosome. 16 Eukaryotic Chromosomes ◼ Different species contain different numbers of chromosomes. ◼ Most eukaryotes have a pair of each chromosome. ◼ Definition: DIPLOID. ◼ E.g. Humans have 46 chromosomes (23 pairs). ◼ Definition: GENOME. 17 Diploid Chromosome Numbers 18 Genome Size Image taken from Pierce, BA. (2012). Genetics: A Conceptual Approach 4th Ed. 19 Genome Size – extremes Image taken from Dodsworth et al. (2016). Evolution doi:10.1111/evo.13112 20 Genome Size – extremes Image taken from Dodsworth et al. (2015). Current Opinion in Genetics and Development 35:73-78. 21 Eukaryotic Chromosomes ◼ Definition: AUTOSOMAL CHROMOSOMES. ◼ E.g. Human chromosomes 1 to 22. ◼ All exist as pairs. (humans are diploid) ◼ Definition: SEX CHROMOSOMES. ◼ E.g. Human X chromosome and Y chromosome. ◼ Exist as paired XX in females. ◼ Exist as paired XY in males. 22 Homologous Chromosomes Definition: HOMOLOGOUS CHROMOSOME PAIRS 23 Karyotype ◼ Definition: KARYOTYPE. ◼ A karyotype is a physical description of the chromosomes inside a cell. ❑ (e.g. 46,XX or 46,XY). ◼ A full account of a karyotype may include: ❑ Number ❑ Type ❑ Shape ❑ Banding (pattern of staining) ❑ Other cytogenetic information. 24 How Is Karyotyping Done? ◼ Cells are arrested during cell division often using colchicine (tubulin inhibitor). ◼ In "classic" karyotyping a dye, often Giemsa (G-banding), less frequently Quinacrine, is used to stain bands on the chromosomes. ◼ Giemsa is specific for the phosphate groups of DNA. ◼ Quinacrine binds to the adenine-thymine-rich regions. 25 Karyotype – G-banding 26 Karyotype – G-banding 27 (Sister) Chromatids Definition: CHROMATID. A threadlike strand of DNA composed of one double helix. After DNA replication, two chromatids per chromosome. Image taken from Pierce, BA. (2012). Genetics: A Conceptual Approach 4th Ed. 28 Basic Structure Of A Eukaryotic Chromosome 29 Centromeres Definition: CENTROMERE. Region of heterochromatin. Site where sister chromatids joined. Required for correct separation of chromosomes during cell division Image taken from Pierce, BA. (2012). Genetics: A Conceptual Approach 4th Ed. 30 Centromeres Chromosomes can be classified by the location of the centromere. Image taken from Pierce, BA. (2012). Genetics: A Conceptual Approach 4th Ed. 31 Telomeres Definition: TELOMERES. Repetitive sequence at the ends of chromosomes. Prevents chromosomes joining together (fusing). Image taken from Pierce, BA. (2012). Genetics: A Conceptual Approach 4th Ed. 32 Complexity Of Eukaryotic Chromosomes & Genomes 33 Complexity In Eukaryotic Genomes ◼ Definition: DENATURATION. ◼ Double strand DNA separates when heated. ◼ Temperature at separation is melting temp. ◼ Definition: HYBRIDISATION. ◼ If you denature then reanneal complementary strands of genomic DNA from eukaryotes... ◼ …experiment shows regions with very different hybridisation rates. ◼ Suggests at least three types of regions within the DNA. 34 Complexity In Eukaryotic Genomes 1. Highly repetitive DNA: ❑ Rapidly re-natures (hybridises). ❑ Typically short sequences (