RVHS 9. Organisation of Genomes - Eukaryotes 9477

River Valley High School 1 2025 JC1 H2 Biology Lecture Topic 9: Organisation of Genomes - Eukaryotes River Valley High School 2025 JC1 H2 Biology Lecture Topic 9: Organisation of Genomes - Eukaryotes Name: ( ) Class: 25J____ Date: References Title Authors Biology (9th Edition) Campbell and Reece Biology: An Australian Focus (3rd Edition) Knox, Ladiges, Evans and Saint Molecular Cell Biology (6th Edition) Lodish, Berk, Kaiser, Krieger, Scott, Bretscher, Ploegh and Matsudaira Molecular Biology of the Cell (5th Edition) Alberts, Johnson, Lewis, Raff, Roberts and Walter Principles of Genetics (3rd Edition) Snustad and Simmons H2 Biology Syllabus 9477 (2025) Candidates should be able to use the knowledge gained in the following section(s) in new situations or to solve related problems. Related Topics Content Control of Gene Expression - Eukaryotes Organisation of Genomes - Prokaryotes Control of eukaryotic gene expression Prokaryotic genome Learning Outcomes 2B. Organisation of Genomes a. Describe the structure and organisation of viral, prokaryotic and eukaryotic genomes (including DNA/RNA, single-/double-stranded, number of nucleotides, packing of DNA, linearity/circularity and presence/absence of introns). e. Describe the structure and function of non -coding DNA in eukaryotes (i.e. portions that do not encode protein or RNA, including introns, cen tromeres, telomeres, promoters, enhancers a nd silencers) (knowledge of transposons, satellite DNA, pseudo -genes and duplication of segments is not required). Lecture Outline I. Structure and Organisation of Eukaryotic Chromosomes A. Relating Structure of Eukaryotic Chromosomes to Cell Cycle B. Packing of DNA in Eukaryotic Chromosomes C. Organisation of DNA in Eukaryotic Chromosomes i. Coding and Non-coding DNA ii. Relating Structure of Centromere to its Role iii. Relating Structure of Telomere to its Role D. Comparing the Structure/Organisation of Prokaryotic & Eukaryotic Chromosomes

River Valley High School 2 2025 JC1 H2 Biology Lecture Topic 9: Organisation of Genomes - Eukaryotes Websites URL Description https://www.dnalc.org/resources/3d/08-how- dna-is-packaged-advanced.html packing of DNA in eukaryotic chromosome. https://highered.mheducation.com/sites/983409 2339/student_view0/chapter14/telomerase_func tion.html description of telomerase action. *Note: you would need to use a laptop/desktop and download a chrome extension (e.g. “Flash Player for Chrome”) to view the tutorial.

River Valley High School 3 2025 JC1 H2 Biology Lecture Topic 9: Organisation of Genomes - Eukaryotes I. STRUCTURE AND ORGANISATION OF EUKARYOTIC CHROMOSOMES The genomes of most eukaryotes are larger and more complex than those of prokaryotes. In this chapter, we shall consider the characteristics of eukaryotic genome1 (in particular, the eukaryotic nuclear genome), i.e. the features of genes and the other DNA sequences that comprise the genome, and how these DNA is structured and organised by proteins within the cell. Since the bulk of the eukaryotic genome is packed into chromosomes found in the nucleus 2, let’s revisit key concepts related to the structure of eukaryotic chromosomes. A. Relating Structure of Eukaryotic Chromosomes to Cell Cycle 1. The structure of eukaryotic chromosomes varies greatly during the cell cycle. For non-dividing cells: Each “chromosome” comprises one DNA double helix , which exists as decondensed chromatin so that genes are accessible to the transcriptional machinery. From the electron micrograph, • Euchromatin is decondensed and is thus transcriptionally active. • Heterochromatin does not fully decondensed following cell division and is thus transcriptionally inactive. For dividing cells: During that part of interphase before S phase, each “chromosome” comprises one DNA double helix, which exists as decondensed chromatin. After S phase (i.e. semi -conservative DNA replication) , each chromosome comprises two DNA double helices , which exists as • decondensed chromatin after S phase of interphase and before prophase; and • two identical sister chromatids (i.e. condensed form ) during nuclear division (i.e. from prophase to telophase). 1 Definition of genome: the genetic material of an organism or virus, i.e. the complete complement of an organism’s or virus’s genes along with its non-coding nucleic acid sequence. 2 Recall that chloroplasts and mitochondria contain their own genome, which are not stably inherited.

River Valley High School 4 2025 JC1 H2 Biology Lecture Topic 9: Organisation of Genomes - Eukaryotes 2. Eukaryotic chromosomes are characteristically linear structures that contain a centromere, which is responsible for attaching chromosomes to the kinetochore microtubules during cell division. 3. Consider mitotic metapha se chromosome s, which are large enough to be seen under a light microscope. Using the above example of the human male karyotype, we see that i. the human genome is packaged into 23 pairs of chromosomes. ii. the karyotype above shows a diploid genome. i.e. two sets of 23 chromosomes. iii. the haploid genome (i.e. one set of 23 chromosomes) consists of 3 x 10 9 base-pairs (bp) – this stretch of DNA is approx. 1000 mm long from end to end. • this metre of DNA is subdivided amongst 23 chromosomes of variable size and shape, with each chromosome containing 15 to 85 mm of DNA. iv. each chromosome has two sister chromatids, indicating that it is taking part in cell division.

River Valley High School 5 2025 JC1 H2 Biology Lecture Topic 9: Organisation of Genomes - Eukaryotes B. Packing of DNA in Eukaryotic Chromosomes First level of condensation: 1. DNA is packed into nucleosomes to produce the 10-nm interphase chromatin fibre a.k.a. nucleosome fibre (“beads on a string”). 2. A nucleosome consists of DNA wound twice around a protein core composed of an octamer of 4 histone molecules, two each of histones H2A, H2B, H3 and H4. • Histones are small proteins (Mr = 11 000) with a high concentration of basic residues, e.g. lysine and arginine (positively-charged R groups) , which interact with the negatively-charged sugar - phosphate backbone of DNA. • Histones assemble in groups of eight (i.e. an octamer) to form a core upon which DNA is bound. • dsDNA, which is 146 bp in length, is coiled 1 ¾ turns around the histone core, forming a nucleosome core. This gives the chromatin a ‘beads-on-a-string’ look. • The 10-nm chromatin fibre consists of the nucleosomes and the linker DNA . Such chromatin resembles beads on a string. Each “bead” is a nucleosome, the “string” between the beads is called linker DNA. 3. In the cell cycle, the histones leave the DNA only briefly during DNA replication and gene expression, processes that require access to the DNA by the cell’s molecular machinery. Source: Molecular Biology of the Cell. 4th Edition. pp 20 Second level of condensation: 1. DNA is further folded or coiled to produce the 30-nm chromatin fibre, a.k.a. solenoid. 2. Histone H1 and linker DNA is involved in this coiling of the 10 -nm nucleosome fibre to produce the 30-nm chromatin fibre.

River Valley High School 6 2025 JC1 H2 Biology Lecture Topic 9: Organisation of Genomes - Eukaryotes Third level of condensation: 1. Finally, non-histone chromosomal proteins form a scaffold that is involved in condensing the 30 -nm chromatin fibre into loops called looped domains, forming the 300-nm fibre. 2. In mitotic and meiotic chromosomes, the looped domains themselves coil and fold, further compacting all the chromatin to produce the characteristic metaphase chromosome. The width of one chromatid is 700 nm. 3. Particular genes always end up located at the same places in mitotic and meiot