Cells and Their Housekeeping Functions Nucleus and Other Organelles

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1 Cells and Their Housekeeping Functions Nucleus and Other Organelles Shu-Ping Lin, Ph.D. Institute of Biomedical Engineering Website: Date:

2 Chloroplasts, Cell Wall, and Vacuole Chloroplasts: green organelles that make food, found only in green plant cells Convert energy of light into chemical energy Chlorophyl: green pigment that gives leaves & stems their color Captures sunlight energy that is used to produce food called glucose (Glucose is a type of sugar) Cell wall: restrict shape change and mobility Vacuole: collect and store nutrient molecules and waste products Chloroplasts Cell wall

3 From Cell to Organism Cell The basic unit of life Tissue Group of cells working together Organ Group of tissues working together Organ System Group of organs working together Organism Any living thing made of 1 or more cells

4 1- Nucleus 2- Chromosomes 3- Mitochondria 4- Ribosomes 5- Chloroplasts 6- Vacuoles 7- ER 8- Cell Membrane

5 Nucleus Largest organelle within the cell, containing DNA DNA replication: necessary for cell division (so that both daughter cells have identical copies of DNA) Transcription: formation of an RNA template of a gene, essential for protein synthesis Library of genetic information Bounded by nuclear membrane, 2 lipid bilayers of double membrane are separated by a gap (20~ 40nm) with many openings or pores for trafficking small molecules and proteins

6 Chromosome Chromatin Identical sets of chromosomes (long and thin DNA molecules, store genetic information) in each eukaryote Histone: beadlike protein structure, Ex: each human cell has about 1.8 meters of DNA, but wound on the histones it has about 90 millimeters of chromatin Before cell division, DNA is replicated and tightly coiled and bound in identical pairs called chromatids Chromosomes can be seen by light microscopy.

7 DNA molecules are packed around and attached to beadlike protein structure (histon) Nucleosomes

8 Chromatin Chromosome Different levels of DNA condensation. (1) Single DNA strand. (2) Chromatin strand (DNA with histones). (3) Chromatin during interphase with centromere. (4) Condensed chromatin during prophase. (Two copies of the DNA molecule are now present) (5) Chromosome during metaphase.

9 Protein Synthesis-1 RNA polymerase: transcribe RNA from DNA template at average rate 30 nucleotides/sec, an enzyme consisting of 12 different polypeptides with mass of 500kDa, arrange in 10 subunits RNA polymerase II (pol II): central machine for synthesis protein of mrna in eukaryotes Polymerase I: ribosomal RNA, polymerase III: transfer RNA 9 out of 10 subunits are identical in these 3 enzymes Begin in nucleus with binding of transcription factors to regulatory sequence (protein-coding gene) Transcription: energyconsuming process and driven by energy-released by ATP hydrolysis

10 Protein Synthesis-2 Binding of transcription factors to regulatory sequence (protein-coding gene) on DNA Activate Pol II unwind DNA double helix Polymerize mrna and proofread the resulting transcript Pol II recognize promoter region of genes if DNA interact with transcription factors Subunits 1, 5, and 9 of pol II grip DNA downstream of active center Subunits 1, 2, and 6 clamp on DNA near active center Growing mrna strand locks this clamp, thus stabilizing transcribing complexes Complete RNA molecule (primary RNA transcript, pre mrna): 20,000 nucleotides because of noncoding introns RNA splicing mrna Transport out of nucleus and interact with ribosomes in cytoplasm to begin translation Spliceosome: recognize exon/intron interface, mediate excision, and anneal ends of exons Excised intron transcripts rapidly degrade in nucleus to provide raw material for new transcripts

11 Ribosomes Small and darkly staining spherical structures: ~20 nm in diameter that are made of 50 proteins and several long RNAs intricately bound together Ribosomes are made in the nucleolus, compartment in nucleus. Once constructed, ribosomes leave nucleus through nuclear pores. Float freely in the cytoplasm to synthesize cytoplasmic proteins without any further modification or attach to the endoplasmic reticulum (ER) to manufacture membrane proteins Make proteins Translate sequence information on mrna into polypeptides No membrane and disassemble into 2 subunits when not actively synthesizing protein Protein synthesis is extremely important, so eukaryotic cells contain million of ribosomes. Take 30 sec to synthesize a protein containing 400 amino acids, and human cell synthesize proteins in 24 hr

12 Translation of mrna by Ribosomes 1. Attachment of mrna to a ribosome 2. Begin knitting together amino acids, according to template encoded mrna 3. Translation is initiated by trna, acting as adapter and matching each codon on mrna with amino acid the codon prescribes. trnas: amino-acid-specific adapter molecules Ribosome has binding sites for 2 different trna molecules so that 2 amino acids joined to the growing polypeptide chain at one time.