Cytoskeleton

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Cytoskeleton

• Microfilaments
• Thinnest elements are composed of actin
• Most prevalent at periphery of cell
• Two general functions
• Providing mechanical support
• Basic strength and shape of cell
• Anchor integral proteins
• Support microvilli
• Helping generate movements
• Muscle contraction
• Cell division
• Cell locomotion

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Cytoskeleton

• Intermediate filaments
• Thicker than microfilaments but thinner than
  microtubules
• Several different proteins
• Exceptionally strong
• Found in parts of cells subject to mechanical
  stress
• Stabilize position of organelles such as nucleus
• Help attach cells to one another

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Cytoskeleton

• Microtubules
• Largest of cytoskeletal components
• Long, unbranched hollow tubes
• Composed mainly of tubulin
• Assembly begins in centrosome growing outward
  into cell
• Help determine cell shape
• Involved in the movement of vesicles,
  chromosomes, cilia, and flagella

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Centrosome

• Located near the nucleus
• Consists of
• Pair of centrioles
• Cylindrical structures with nine clusters of
  three microtubules arranged in circular pattern
• Pericentriolar material
• Hundreds of ring-shaped complexes composed of
  tubulin
• Organizing centers for growth of mitotic spindle
  in cell division
• Replicate during cell division

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Cilia and Flagella

• Cilia
• Numerous, short, hair like projections extending
  from surface of cell
• Function to move fluid along cell surface
• Flagella
• Similar in structure to cilia, but much longer
• Function to move entire cell

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Ribosomes

• Sites of production of protein synthesis
• Contain high content of RNA
• Consists of two subunits
• Made in nucleus
• Large subunit
• Small subunit
• Some attach to endoplasmic reticulum
• Free ribosomes produce proteins used in cytosol

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Endoplasmic Reticulum

• Network of membranes
• Rough ER
• Continuous with nuclear envelope
• Studded with ribosomes
• Proteins synthesized here enter spaces in ER for
  processing and sorting
• Smooth ER
• Lacks ribosomes
• Contains unique enzymes that synthesize fatty
  acids and steroids
• Specialized function depending on location

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Golgi Complex

• Most proteins synthesized on rough ER transported
  to other regions of cell
• All pass through the Golgi complex
• 3-20 cisternae
• Small flattened membranous sacs
• Two surfaces
• Entry face on rough ER surface
• Exit face on plasma membrane surface

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Lysosomes

• Membrane enclosed vesicles
• Form from Golgi complex
• Contain as many as 60 enzymes
• Break down molecules
• Help recycle worn out cell parts
• Autophagy digestion of other organelles
• Work best in acidic pH (5)
• Membrane pumps import hydrogen ions keeping pH low

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Peroxisomes

• Similar in structure to lysosomes but smaller
• Contain oxidases
• Enzymes that oxidize various organic substances
• Detoxifying some
• Normal metabolism for others
• One enzyme is catalase
• Decomposes toxic hydrogen peroxide into water and
  oxygen gas

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Proteasomes

• Destroy unneeded, damaged, or faulty proteins in
  cell
• Contain proteases
• Enzymes that break proteins into small peptides
• Leading step to recycling amino acids

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Mitochondria

• Site of most ATP production
• Numbers vary from hundreds to thousands per cell
• Double membraned organelle
• Contains its own DNA
• Can self replicate
• Inner membrane characteristics
• Inward folds are called cristae
• Inner space called matrix
• Cellular respiration reactions occur here

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Concept 3.5 Nucleus
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Nucleus

• Usually most prominent feature of cell
• A few cells have multiple nuclei but most have
  only one
• Have a double membrane
• Outer membrane contiguous with rough ER
• Openings in envelope called nuclear pores
• Control movement of substances

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Nucleus

• Contains DNA
• DNA has genes
• Genes arranged along chromosomes
• Humans have 46 chromosomes (23 pair)
• Entire genetic information is called genome
• Chromosomes are DNA coiled around specialized
  proteins
• Uncoiled DNA with its associated proteins
  (histones) and RNA is called chromatin

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Concept 3.6 Transcription and Translation
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Gene Expression

• DNA is the blue print for RNA
• RNA is the blue print for protein
• Proteins determine the physical and chemical
  characteristics of cells
• Three RNA nucleotides (codon) code for a
  particular tRNA (anticodon) which carries a
  particular amino acid
• Therefore, the sequence of DNA nucleotides
  determines sequence of RNA nucleotides which in
  turn determines the sequence of amino acids

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Transcription

• DNA is the blue print for RNA
• DNA stays in the nucleus, RNA is transcribed from
  DNA and is mobile
• mRNA strand is read by ribosomes and signal for
  particular tRNAs carrying specific amino acids
• RNA polymerase is the enzyme catalyzes
  transcription
• Promoter sequence before coding region

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Transcription

• Base pairing
• DNA nucleotide binds to RNA nucleotide
• Adenine Uracil
• Thymine Adenine
• Guanine Cytosine
• Cytosine Guanine
• Coding region of gene stops at terminator site
  which signals enzyme to release

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Translation

• Nucleotide sequence is read by ribosome
• Occurs in cytosol
• Codon on mRNA signals for tRNA carrying
  particular amino acid
• Small subunit has binding site for mRNA
• Large subunit has two binding sites for tRNA P
  site and A site.
• See figure 3.26

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Concept 3.7 Cell Division
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Cell Division

• Cytoplasmic division called cytokinesis
• Somatic cell division
• Produces two new identical cells
• Nuclear division called mitosis
• Reproductive cell division
• Produces gametes with half as many chromosomes as
  primary cell
• Nuclear division two-step division called meiosis

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Somatic Cell Division

• Cell cycle
• Sequence of events that cell undergoes until it
  divides
• Cells with pairs of chromosomes are diploid
  symbolized 2n
• One member of each pair is inherited from
  biological father and the other member is
  inherited from biological mother
• Pair are called homologues
• Each one called homologous chromosome

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Somatic Cell Division

• Cell cycle
• Interphase
• G1
• Cell is metabolically active replicates most
  everything BUT its DNA (8-10 hours)
• S phase
• Cell duplicates its DNA (8 hours)
• G2
• Growth continues
• Cell prepares for division
• Replication of centrosomes completed
• Cell now ready for mitotic phase

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Mitotic Phase of Cell Cycle

• Nuclear division followed by cytokinesis
• Prophase
• Metaphase
• Anaphase
• Telophase
• See figure 3.30a, b, c, d, e

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Cytokinesis

• Process begins during anaphase
• Formation of cleavage furrow
• When complete two new identical cells results
• See figure 3.32

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Reproductive Cell Division

• Meiosis occurs in two rounds of cell division
• Meiosis I and meiosis II
• The goal of meiosis I is to mix (during synapsis)
  and separate the homologous chromatids
• Cells now haploid
• Meiosis II looks just like mitosis but with half
  as many chromosomes

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Meiosis versus Mitosis

• Mitosis results in two new identical cells
• Meiosis results in four cells which are all
  genetically different from original starting cell

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End Chapter 3