FUNBIO-6.pptx

Royal College of Surgeons in Ireland - Medical University of Bahrain Cell ultrastructure: the nucleus Class Course Code Lecturer Date Foundation Year Fundamentals of Human Biology FUNBIO.6 Dr Ebrahim Rajab 26th September 2022 Learning Objectives • Describe the structure of the nucleus including...

Royal College of Surgeons in Ireland - Medical University of Bahrain Cell ultrastructure: the nucleus Class Course Code Lecturer Date Foundation Year Fundamentals of Human Biology FUNBIO.6 Dr Ebrahim Rajab 26th September 2022 Learning Objectives • Describe the structure of the nucleus including nuclear envelope, pores and karyoplasm. • Differentiate between euchromatic and heterochromatic nuclei. • Explain how each organism has a characteristic number of chromosomes • Describe the nucleolus and explain its function in ribosomal production. • Briefly summarise the production of messenger RNA. The Nucleus • All cells, with a few specialized exceptions, have nuclei • Exceptions: ­ Mammalian red blood cells ­ Platelets • “Cells without nuclei have limited futures” The Nucleus The Nucleus • One nucleus per cell Rounded body in the cell cytoplasm • The nucleus is composed of: – – – – Nuclear envelope Chromatin (Genome) Nucleolus Nucleoplasm/karyoplasm (=cytoplasm of nucleus) 1. Nuclear Envelope Structure • Nucleus is enclosed in a double-layered nuclear envelope, consisting of an outer membrane (OM) & inner membrane (IM). – Each membrane is composed of phospholipids arranged in a bilayer. This means that the entire nuclear membrane has four rows of phospholipids. • A ‘perinuclear’ space (20-40nm wide) is found between the two membranes. • Compartmentalizes the nucleus, i.e. it isolates nuclear material from the cytoplasm. 1. Nuclear Envelope • Outer membrane is continuous with the rough endoplasmic reticulum in the cytoplasm. • Inner membrane is anchored to the nuclear lamina. Perinuclear space 2. Nuclear Lamina • A nuclear lamina is found adjacent to the inner membrane of the nuclear envelope – Provides structural support Perinuclear space 2. Nuclear Lamina • This lamina is a meshwork of interconnected protein filaments. – Intermediate filaments. – The main component is laminins. • Function: •Provide support and shape to nuclear envelope. •Aid in positioning of nuclear pores. 3. Nuclear Pore There may be thousands of pores occupying 10 - 30% of the surface of the nuclear envelope 3. Nuclear Pore Structure • Ring-like arrangement called a pore complex. • Octagonal structure over 50nm diameter. 1nm = 1.0 × 10-9 meters 3. Nuclear Pore - Pore complex • Face-on - eight subunits. Channel protein subunit • Surround a central zone which contains a channel about 9nm in diameter. Central zone 3. Nuclear Pore - Pore complex 3. Nuclear Pore - Pore complex • Function: – Allows the transport of water-soluble molecules across the nuclear envelope. – Molecules which can pass though include: • RNA • Ribosomes • Proteins • Carbohydrates • Signal molecules • Lipids https://www.ncbi.nlm.nih.gov/books/NBK9927/ Nucleoplasm • The cytoplasm in the nucleus is called the nucleoplasm or karyoplasm. • highly viscous liquid that surrounds the chromosomes and nucleoli. • Many substances such as nucleotides* and enzymes are dissolved in the nucleoplasm. *Nucleic acids – a series of nucleosides joined together by a phosphate group by ester linkage to the sugar. DNA and RNA are polymers of nucleotides. Nucleoside = Sugar + Base Nucleotide = Sugar + Base + Phosphate Nucleoplasm A network of fibers known as the nuclear matrix can also be found in the nucleoplasm: • Similar to cell cytoskeleton • Precise composition and arrangement in the nucleus not known. Nucleus Nuclear matrix Chromatin Fibers • When the cell is not dividing (Interphase) the nucleoplasm contains a collection of threads and fibres called chromatin fibres which can be seen with the light microscope. The finest chromatin are fibrils about 2nm in diameter - this is about the dimension of a DNA molecule Types of Chromatin There are 2 types of chromatin: – Euchromatin – Heterochromatin Euchromatin • Lightly packed (or relatively dispersed) form of chromatin. • Typically found in metabolicallyactive cells (but not dividing!). • DNA is transcribed from Euchromatin. Euchromatin Types of Chromatin • Heterochromatin • Tightly packed form of DNA. (Heavier, more condensed masses of chromatin). • Usually located at the periphery of the nucleus. • The DNA in heterochromatin is not transcribed. • Typically found in relatively inactive cells. • The functions of heterochromatin are attributed to the dense packing of the DNA: •Gene regulation •Protection of the integrity of chromosomes Heterochromatin When cell is about to divide the chromatin fibres condense and form the chromosomes visible with the Chromatin, Chromatids and Chromosomes https://www.creative-diagnostics.com/blog/index.php/the-structure-and-funct ion-of-chromatin/ Chromatin Packing Chromatin undergoes several levels of coiling to form a chromosome: – Basic unit: Nucleosome –Consists of the DNA double helix coiled around a protein unit called a histone. –Organized nucleosomes found along the length of the chromatin. Chromosome Numbers Ploidy - Each multi-cellular organism has a characteristic number of chromosomes in its cells Haploid (n) – Haploid cells have half the number of chromosomes (n) as diploid - i.e. a haploid cell contains only one complete set of chromosomes (23) e.g. gamete. Diploid (2n) – Diploid cells contain two complete sets (2n) of chromosomes.(46) i.e the basic set. Polyploid – Multiple pairs of chromosomes beyond the basic set Aneuploidy – change in the number of chromosomes • Trisomy – Down’s syndrome (trisomy 21) • Monosomy – Turner Syndrome (X0) NUMBER OF CHROMOSOMES IN SPECIES Species Diploid - 2N Haploid - N Mosquito - Aedes 6 3 Fruit fly - Drosophila 8 4 Pike - Esox 18 9 Frog - Rana pipiens 26 13 Alligator - A. mississipiensis 32 16 Rat - Rattus spp. 42 21 Human - Homo sapiens 46 23 Chimpanzee - Pan troglodytes 48 24 Goat - Capra spp. 60 30 Duck - Anas spp. 80 40 Karyotype Describes the chromosome complement of an individual or species in terms of number, size, and morphology of its chromosomes. The largest pair of chromosomes is usually designated as number 1 and the rest are numbered in order of their relative diminishing size, to the smallest pair. trisomy Nucleolus Nucleolus (pl. nucleoli) • Typically, one to four per cell nucleus. • Dense basophilic material in nucleoplasm. • Not enclosed by a membrane. • Disappear during cell division. • Re-appear post-cell division. • Re-appearance organized by particular chromosomes which have an area called the nucleolar organiser region. Basophilic = appearance of cells, after staining with a basic dye e.g. haematoxylin Nucleolus Function Within the nucleolus, there is a specific region (i.e. nuclear organizer) consisting of several chromosome regions which contain instructions to make (or ‘transcribe’) ribosomal RNA (rRNA). Within the nucleolus, chromosomal DNA transcribes rRNA which then associates with proteins to form ribosomal particles that leave the nucleus through the nuclear pore. – Ribosomes contain roughly equal amounts of ribosomal RNA (rRNA) and proteins – Ribosomes are closely connected with protein synthesis Ribosomes - Formation *Recall that within the nucleolus, there is a specific region (i.e. nuclear organizer) consisting of several chromosome regions which contain instructions to make (or ‘transcribe’) ribosomal RNA (rRNA) • In the nucleolus, the loop of nucleolar organizer* DNA codes for the production of ribosomal RNA • The nucleolar organiser DNA transcribes a large 45S Precursor rRNA molecule (13,000 nucleotides) –This contains 28S, 5.8S and 18S rRNA molecules • The 5S rRNA is transcribed separately in the nucleoplasm S=Unit of measurement known as the Ribosomes - Formation • These complex with proteins made out in the cytoplasm • Forming a 80S pre-ribosomal particle • Which then breaks down to 40S and 60S subunits • The large 60S subunit takes about one hour to assemble • Final assembly of the two ribosomal subunits only takes place in the cytoplasm Nucleus : Site of mRNA Production Transcription • Transcription = copying. • The information stored in a gene’s DNA is transferred to a similar molecule called RNA (ribonucleic acid) in the cell nucleus. • The type of RNA that contains the information for making a protein is called messenger RNA (mRNA) because it carries the information, or message, from the DNA out of the nucleus into the cytoplasm. Genes, which are made up of DNA, act as instructions to make molecules called proteins. Recommended Reading Nucleus: • Solomon 9th Edition, pp86-89 • Alberts 4th Edition, pp 226-229, 669671, 676