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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