Genes and gene expression - University College...
Transcript of Genes and gene expression - University College...
Genes and gene
expression
EH1008 : Biology for Public Health : Biomolecules
Lecture 6
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What is a Gene?
• Segment of DNA
• Functional unit of heredity
• Heredity – transmission of genetic traits from
parent to offspring
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What is DNA?
• 4 different chemical building blocks or bases/nucleotides – A, T, G, C
• Attached to a chemical backbone forming a strand of DNA
• 2 strands wind around each other to make a double helix structure (twisted zip)
• Each set of interlocking ‘teeth is’ a base-pair of DNA – A paired with T; C with G
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Gene Expression
• A gene is a segment of DNA
• The segment of DNA produces a protein or
RNA – ie. gene expression
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Gene Expression
• Transcription
– A copy of the gene is made
• Translation
– A protein is constructed from the copy
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Transcription
• Section of DNA unwinds
• Strands separate
• One of the strands serves as a template
• Nucleotides align with the nucleotides on the template strand by complementary base pairing
• An enzyme (RNA polymerase) joins the new nucleotides together
• New strand forms RNA
• RNA detaches – mRNA
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Genetic Code• A codon is a set of 3
nucleotides
• Each codon instructs for
an amino acid
• 4 x 4 x 4 = 64
combinations
• 20 ‘standard’ amino acids
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Translation
• Synthesis of a protein in response to the codons of mRNA
• Process that occurs on ribosomes
• Turns mRNA into a polypeptide
• Involves rRNA, tRNA, and mRNA
• tRNA anticodons match with mRNA codons, and the rRNA catalyzes the formation of a peptide bond between the amino acids at the opposite end of the tRNA
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Regulation of Gene Expression
• All nucleated cells except germ cells have the full complement of DNA
• During development, differentiation occurs and some segments of DNA are turned off in some cells while those segments remain “on” in other cells
• During the lifetime of a cell, the rate of protein synthesis varies depending upon chemical signals that reach the cell
– Example: thyroxine from the thyroid causes cells to increase their metabolic rate. More thyroxine, higher metabolic rate; less thyroxine, lower metabolic rate
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Cell Life Cycle
• Cell Life Cycle = changes a cell undergoes from
the time it is formed until it divides to
produce 2 new cells
• 2 phases
– Interphase
– Cell division
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Interphase
• Phase between cell divisions
• 90% or more of the life cycle of a typical cell is
spent in interphase
• Cell carries out the metabolic activities
necessary for life and performs its specialised
functions
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Interphase
• Cell also prepares for division:
– Increase in cell size (many cell components double
in quantity)
– Replication of the cell’s DNA
• Consequently, when the cell divides, each
new cell receives the organelles and DNA
necessary for continued functioning
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Interphase
• Sub-phases: G1, S, G2
• G1 (first Gap phase)
– Routine metabolic
activities
• S (Synthesis phase)
– DNA is replicated
• G2 (second Gap phase)
– Prepares for cell division
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DNA Replication
• The 2 strands of a DNA molecule separate
from each other
• New strands of complementary DNA pair with
the original strands
• This results in 2 identical DNA molecules (from
1 original DNA molecule)
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DNA
Replication
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Cell Division
• Produces the new cells necessary for growth and repair
• Parent cell divides to form 2 daughter cells
• Each daughter cell has same:
– Amount and type of DNA as parent cell
– Structure and performs same function as parent cell (exceptions, eg. stem cells; sperm & eggs)
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Cell Division
• Two major events
• Mitosis
– Division of the chromosomes into 2 new nuclei
• Cytokinesis
– Division of the cytoplasm to form 2 new cells
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Mitosis
• The 2 new (replicated) strands of DNA condense to form chromatids
• The 2 chromatids are joined at the centromere
• As 2 daughter cells form, the chromatids separate and move into one of the 2 newly forming nuclei
• Each daughter cell receives a copy of the chromosomes of the parent cell – genetically identical
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Mitosis
• 4 phases:
– Prophase
– Metaphase
– Anaphase
– Telophase
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Cytokinesis
• Division of the cell’s cytoplasm to produce 2 new cells
• Begins in anaphase and continues through telophase
• Plasma membrane ‘indents’ roughly in the centre (cleavage furrow)
• Plasma membrane gets pulled inward, dividing the cell into 2 halves
• Membrane of the halves separate at the cleavage furrow to form 2 separate cells
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Cell
Division
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Centrioles and the Cell Cycle
• Before cell division,
centrioles divide, move
• to ends of cell and
organize spindle fibers
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