DNA, Genes and Cell Division

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Chapter 5

• DNA, Genes and Cell Division

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Genetics

• Began by Gregor Mendel while he was studying peas
• expressed ideas of traits/genes which he called
  elemente
• round vs wrinkled pea seeds not determined
  until 1990 that this was caused by an enzyme that
  altered starch structure lead to the roundness
• wrinkled peas do not have enzyme more sucrose
  and less starch seeds absorb H2O and as mature,
  they dry and become wrinkled
• more starch in round seeds so not as much
  absorption and seeds stay round

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Genes

• Influence phenotype appearance of organism
• DNA in nucleus is organized into chromosomes
• Initially thought they code for 1 protein but now
  we know that they also code for functional RNA
  molecules
• Basic unit of heredity

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2 Roles of Genes in Nature

• Genes pass genetic information to next generation
  of cells when parent cell divides
• In cell, genes are a molecular blueprint for what
  happens in the cell
• proteins made, structures, development, enzymes
  that control chemical reactions
• DNA ? RNA ? protein

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Chromosomes

• DNA is associated with proteins to form the
  chromosomes
• Can see only in dividing cells in the light
  microscope
• In non-dividing cells the DNA is not visible in
  the light microscope and is called chromatin

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(No Transcript)
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Nucleosomes

• In TEM the DNA/protein complex looks like a bead
  on a string called a nucleosome
• made of DNA and histone proteins 5 unique kinds
  of proteins
• DNA wraps around a histone made up of 4 histone
  proteins, each in 2 copies
• 5th histone protein is used to coil the DNA into
  chromatin
• Chromatin continues to loop and fold until it
  folds into a chromosome

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DNA Structure

• Described in the 1950s
• Helical structure with the nitrogenous bases
  acting as steps in the staircase
• Nucleotides linked by a phosphodiester bond in
  the phosphate-sugar backbone

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• A pairs with T by 2 H-bonds
• G pairs with C by 3 H-bonds

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3 Postulates of DNA Structure

• DNA is a double helix with a phosphate-sugar
  backbone
• Pair of bases form link between opposite sugar
• A with T and G with C
• Basepairs may be in any sequence along double
  strand but pairing must be maintained as above

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Diversity

• Order of basepairs is what generates the large
  diversity that is seen in genes and in organisms
  in nature
• Different genes will have different numbers of
  bases

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How Genes Work

• Genes are in the nucleus but proteins are made in
  the cytoplasm linked by RNA
• DNA is the template for RNA and subsequently
  proteins
• Transcription process of RNA synthesis from DNA
  template
• forms 1 transcript or pre-RNA
• RNA is modified in the nucleus before export to
  the cytosol
• Translation process of protein synthesis that
  mRNA template by the ribosome

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3 Types of RNA

• All are made from genes in the DNA
• mRNA message RNA used for protein synthesis
• tRNA transfer RNA binds the amino acid and
  brings it to the growing protein chain dictated
  by the mRNA
• rRNA ribosomal RNA combines with ribosomal
  proteins to make the ribosome to perform proteins
  synthesis
• tRNA and rRNA do not have a protein counterpart
• All three are present in all cells for protein
  synthesis

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Genes to Proteins
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Transcription

• Occurs in the nucleus
• Pre-RNA is the same length as the gene
• Different pairing in RNA to DNA
• A with U
• G with C
• RNA has ribose as its sugar

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3 Continuous Steps

• Initiation complex of proteins including RNA
  polymerase to recognize and start at the
  beginning of a gene
• Elongation RNA polymerase moves along the DNA
  from 3 to 5 adding nucleotides from 5 to 3
• Termination RNA polymerase reaches the end of
  the gene and stops transcription and releases a
  new pre-RNA

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Ribozymes

• An RNA molecule that functions as an enzyme
• probably played a very important role in
  evolution of life

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4 Components of mRNA

• CAP beginning of message to protect it
• Leader sequence location to start the
  replication machinery, also called promoter
• Coding sequence actual gene that will code the
  protein
• Trailer sequence end/poly A tail that also
  protects the message

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Transcription
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Working Unit of mRNA

• mRNA is set up in 3 nucleotide units called
  codons
• Each codon will dictate the amino acid sequence
  of the protein

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tRNA

• Brings in the amino acid to the ribosome during
  protein synthesis
• 3-D structure clover leaf structure
• Loop 2 has the anticodon region which recognizes
  the codon and at the 3 end that attaches to the
  amino acid
• 2 functions carry right amino acid to ribosome
  and to activate the amino acid so that it can
  join the growing protein chain

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Ribosomes

• Site of protein synthesis
• Complex of rRNA and protein made in the nucleolus
  and transported to the cytoplasm
• 2 subunits that join up once find a mRNA
• large is responsible for forming the peptide bond
  
• small holds the message for translation
• May be several ribosomes on the same message
  polyribosome or polysome making many copies of
  the protein

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Translation

• Protein synthesis uses a complex of enzymes to
  make the protein
• Use tRNA to bring in the AA using ATP energy
• Occurs in 3 continuous steps similar to
  transcription with initiation, elongation and
  termination

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Initiation

• mRNA, initiation factors and initiator-tRNA binds
  the small subunit at the initiator codon and then
  recruits the large subunit functional ribosome

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Elongation

• 2nd tRNA-AA comes in and joins with the codon
• Peptide bond is made by the transferase in the
  large subunit (rRNA molecule) by linking the NH2
  group to the COOH group attached to the tRNA
• Ribosome slides down to the next codon and the
  process repeats
• empty tRNA is kicked out

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Termination

• Arrive at the STOP codon (UGA, UAG or UAA)
• Incorporate H2O at this site to release the
  peptide chain and disassemble the ribosome
• lt1 minute to make a 400 AA protein

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Translation
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Genetic Code

• Is redundant or degenerate
• 3 nucleotides per codon and 1 of 4 options for
  each would indicate that there are 64 amino acids
  (43) if each one is specific
• now know that some amino acids have more than 1
  coding sequence
• 3 of the codons are stop codons to indicate the
  end of the process
• Links codons to amino acids

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Overview

• Transcription and Translation

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Gene Expression

• There are regulatory mechanisms at each step in
  the process of transcription and translation
• Plants need to have a fine tuned balance between
  the genes that are on and those that are off
• growth dictated by what genes are on and off

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Eukaryotic Chromosomes

• Contain interrupted genes non-coding regions
  (introns) that are among coding regions (exons)
• function of introns is unsure
• may be junk or help regulate plant development
• Introns are removed during 1 transcript, exons
  are spliced together to make mRNA
• See editing in tRNA and rRNA as well

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Genetic Engineering

• Major focus of modern biology
• Artificial manipulations of genes and the
  transfer of genes from one plant to another
  recombinant DNA technology
• organisms are transgenic genes from other
  organisms
• blue rose, potatoes that make plastic and glowing
  tobacco

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Pest Resistance

• Tobacco has been engineered to be resistant to
  tobacco mosaic virus by placing a gene for a TMV
  gene into the tobacco genome TMV will not
  infect cells already infected cells
• Use Bacillus thuringiensis in tomatoes, potatoes
  and other plants to be resistant to catepillars
  that feed on them
• eliminated the boll weevil this way

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Plant Cell Division

• Start as a single cell and undergoes growth and
  cell division
• Eventually the plant initiates gene transcription
  that causes the cell to mature and age
• growth and cell division is in the meristematic
  tissue

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Cell Cycle

• Cell growth and division
• Deals with DNA synthesis and chromosomal behavior
  during mitosis
• Cells spend most time in interphase

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Cell Cycle

• Occurs only in cells that divide
• 1st dividing cell is the zygote that is formed
  from the fusion of an egg and sperm
• grows into an embryo and divides until it forms
  specific tissues
• Meristematic tissues is where cell division
  occurs at root and shoot tips
• can get cells to divide in specialized tissues if
  plant is damaged
• cells are totipotent can revert back to
  meristematic tissue

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Cell Cycle

• Growth followed by cell division
• 2 main stages
• cell growth collectively called interphase
• cell division mitosis and cytokinesis
• mitosis division of the nucleus
• cytokinesis division of the cell into 2 cells

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4 Phases of Cell Cycle

• Process is continuous
• G1 phase 1st growth phase, end of mitosis to
  beginning of DNA synthesis
• S phase DNA replicated
• G2 phase 2nd growth phase, end of S phase to
  beginning of mitosis
• M phase mitosis that has 4 continuous phases

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Interphase

• G1, S and G2 phase
• Meristematic tissue spend 90 in interphase
• No clear start/stop to each phase

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G1 Phase

• Microtubules reassemble, cell enlarges,
  organelles multiply, mitochondrial and plastids
  increase DNA, enzymes/proteins made, nucleotides
  made
• very variable in length that it spends in this
  phase
• some cells never leave G1 and are said to be in
  G0 (nerve cells)

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S Phase

• DNA replication
• Semi-conservative as one old strand act as the
  template for the other and stays with the new
  strand
• complimentary base pairing

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DNA Replication

• Completed by DNA polymerase that catalyzes the
  bond between growing DNA strand and next
  nucleotide
• Makes a direct copy of DNA and all the genes

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G2 Phase

• Relatively short in plant cells and is the end of
  interphase 3 to 5 hours long
• Chromatin begins to coil and condense
• Make tubulins for mitotic spindles
• Make enzymes to break down nuclear envelope

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Mitosis/Cytokinesis

• Separation of chromosomes and make 2 identical
  nuclei for new cells

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Prophase

• Chromatin condenses rapidly to chromosomes
• Chromosome consists of 2 chromatids that are
  joined at the centromere
• By the end of this portion, no nuclear membrane
  or nucleoli visible

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Metaphase

• Spindle fibers (mircotubules) form near
  chromosomes and create the spindle apparatus
• Chromosomes become ordered and line up at the
  metaphase plate of the cell

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Anaphase

• Shortest phase
• Chromatids separate and move to the opposite
  poles and now is considered a chromosome
• Each new cell have identical set of chromosomes
• Ready to package in a new nucleus

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Telophase and Cytokinesis

• In telophase
• spindle apparatus disappears
• nuclear envelop reappears
• chromosomes decondense to chromatin
• In cytokinesis
• starts in late anaphase
• short microtubules form at division plane
• cell wall forms as the cell plate
• grows outward forming new cell wall

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Control of Cell Cycle

• Proteins called cyclins activate the cell cycle
  enzymes
• gene is called cell division cycle (cdc)
• Different ones function at the transition from G1
  to S and G2 to M
• may be influenced by plant hormones or external
  signals