Chapter 15 Controls over Genes

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Chapter 15 Controls over Genes
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When DNA Cant Be Fixed?

• Changes in DNA are triggers for skin cancer, like
  the most deadly type malignant melanoma
• Cancers are malignant forms of tumors
• Neoplasm (tumors) are tissue masses that arise
  through mutations in the genes that govern growth
  and division
• Malignant tumors grow rapidly causing destructive
  effects on surrounding cells

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• Gene controls are molecular mechanisms that
  govern when an how fast specific genes will be
  transcribed and translated

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Types of Control Mechanisms

• Because all cells in your body have the same
  genetic instructions, only a relatively small
  amount number of genes are active at any given
  time in any given tissue
• Which genes are expressed depends on the type of
  cell, its responses to chemical signals and
  built-in control systems

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• Regulatory Proteins intervene before, during or
  after gene transcription or translation
• Regulatory proteins interact with DNA, RNA, or
  actual gene products

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Several controls system are used by cells

• Negative Control- a regulatory protein binds to
  the DNA to block transcription it can be removed
  by an inducer
• Slows down or curtail gene activity
• Positive Control- a regulatory protein binds to
  the DNA and promotes initiation of transcription
• Regulatory proteins promote or enhance gene
  activity

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• In chemical modification, regions of newly
  replicated DNA can be shut down by methylation
  and access to genes can be controlled by
  acetylation of histones proteins that organize DNA

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Negative Control of the Lactose Operon

• E. coli bacteria (commonly found in the human
  digestive system) can metabolize lactose because
  of a series of genes that code for
  lactose-digesting enzymes
• Three genes are preceded by a promoter and an
  operator all together called an operon
• Operator is a binding site for a repressor
  protein that can prevent gene transcription

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• A regulator gene nearby codes for a repressor
  protein that binds to the operator when lactose
  concentration are low and effectively block RNA
  polymerases access to the promoter
• When milk is consumed, the lactose binds to the
  repressor changing its shape and effectively
  removing its blockage of the promoter thus the
  RNA polymerases access can now initiate
  transcription of the genes

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Gene controls in eukaryotic cells

• Much less is known about gene controls in
  multicelled eukaryotes because patterns of gene
  expression vary within and between body tissue

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Cell Differentiation and Selective Gene Expression

• All body cells have the same genes, but the cells
  of different tissues are differentiated
  (specialized) because of selective gene expression

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Controls related to transcription include (Brown
Box p. 242)

• Gene AmplicationDNA replicates hundreds or
  thousands of copies prior to transcription
• DNA Rearrangements Some genes have base
  sequences in the DNA, and they are put together
  in different ways to make different product
  molecules

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• Chemical Modification histones and other
  proteins interact with eukaryotic DNA in
  organized ways. The DNA-protein packaging, plus
  chemical modifications to particular sequences

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Post-transcriptional controls

• Transcript Processing exons and introns
• Transport Controls dictates which mature
  transcripts will be shipped to the cytoplasm for
  translation
• Post translational controls govern the
  modifications to polypeptides
• All found on p. 243 Figure 15-4 (b-d)

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Types of Control MechanismsHomeotic Genes and
Body Plans

• Homeotic gene interact with one another and with
  control elements to brin about the formation of
  tissues and organs in accordance with the basic
  body plan.
• These genes code for regulatory proteins that can
  bind to promoters and enhancers to control
  transcription

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X Chromosome Inactivation

• In mammalian females, the gene products of only
  one X chromosome are needed the other is
  condensed and inactive ---called the Barr Body.
• Because in some cells the parental X chromosome
  is inactivated while in other cells the maternal
  X chromosome is inactivated, each adult female is
  a mosaic of X-linked called mosaic tissue effect

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• This mosaic effect is seen in human females
  affected by anhidrotic ectodermal dysplasia in
  which a mutant gene on one X chromosome results
  in patches of skin with no sweat glands
• Figure 15.6b and 15.7 -- page 245

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Hormones Signals

• Hormones are major signaling molecules that can
  stimulate or inhibit gene activity in target
  cells
• Some hormones bind to membrane receptors on cells
  surfaces
• Other enter cells to bind with regulatory
  proteins to initiate transcription, often with
  the aid of enhancer sequences

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• In the salivary glands of insect larvae, the
  polytene chromosomes responds to the hormones
  ecdysone by puffing out during transcription
• In vertebrates, some hormones such as
  somatotropin have widespread effects because most
  of the bodys cells have receptors for it
  whereas prolactin affects only the mammary glands

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Sunlight as a Signal

• Plant seedlings will respond to a single burst of
  light by making chlorophyll
• Phytochrome is a blue-green pigment that helps
  plants adapt to the changing light conditions of
  day/night and seasons by signaling genes
  responsible for germination, stem elongation,
  branching, leaf expansion, and formation of
  flowers, fruits, and seeds