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Gene Expression. General Biology KEY. Central Dogma of Molecular Biology. DNA determines a cell or organism’s traits physical (the structures of a cell or organism) Physiological (how a cell or organism acts) Possibly behavioral traits (how an organism acts)

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Gene expression

Gene Expression

General Biology


Central dogma of molecular biology
Central Dogma of Molecular Biology

  • DNA determines a cell or organism’s traits

    • physical (the structures of a cell or organism)

    • Physiological (how a cell or organism acts)

    • Possibly behavioral traits (how an organism acts)

  • You know that DNA contains a code, but how does DNA’s code actually do anything?

    DNA RNAproteins  traits

Gene expression needs rna
Gene Expression needs RNA

  • Gene Expression = How traits result from genes.

  • DNA is stored and protected in the nucleus in eukaryotic cells.

  • Proteins are built at the ribosomes which are outside the nucleus.

  • RNA acts as a go-between to get DNA’s message out of the nucleus to where it can be used to build proteins.

What is a gene
What is a gene?

  • A gene can be defined in several ways:

    • A unit of heredity which can pass on traits by cell reproduction

    • A section (segment) of DNA that holds the code for the production of an RNA molecule

    • A section (segment) of DNA that holds the code for a polypeptide or protein

    • A section (segment) of DNA that holds the code for a trait (or part of a trait)

What s rna how is it different from dna
What’s RNA? How is it different from DNA?

  • RNA = Ribonucleic Acid

    • Made of nucleotides

      • Each nucleotide has a phosphate, sugar and Nitrogen base

    • Single Stranded

    • Three types: mRNA, rRNA, tRNA

    • Is able to leave the nucleus

    • Contains the Nitrogen Bases A, C, G, and U

      • Uracil instead of Thymine


There are 3 types of rna
There are 3 types of RNA




Protein synthesis
Protein Synthesis

  • Building Proteins (protein synthesis) takes place in two major steps:

    Transcription (in the nucleus)


    Translation (at the ribosomes)


  • =Building RNA using DNA as a template.

  • Takes place in the Nucleus.

  • Uses an Enzyme called RNA Polymerase.

  • Click

    DNA  RNA  protein  trait

Steps of transcription
Steps of Transcription

  • DNA unwinds & the DNA base pairs separate at the location of the gene to be expressed.

  • The enzyme RNA polymerase builds an RNA strand that is complementary to one side of the DNA at the gene location.

  • When it’s finished the RNA can leave the nucleus and the DNA returns to normal.

Transcription practice don t forget use u instead of t
Transcription Practice*Don’t forget – use U instead of T


  • DNA: T A C C C A A G G C CC A T T

  • mRNA: A U G GG U U C C G GG U A A

    EXAMPLE 2.

  • DNA: T A C G G C A T A G C G A C T

  • mRNA: A U G C C G U A U C G C U G A

How does the rna message get translated into a protein
How does the RNA message get translated into a protein?

  • Every 3 nucleotides on a mRNA is called a codon.

  • Each codon codes for a particular amino acid.

Mrna codon chart
mRNA codon Chart



  • =Building a protein (polypeptide) using RNA as a template.

  • The RNA message is “decoded” or “translated” by building a protein.

  • Takes place at the ribosomes.

  • Click

    DNA  RNA  protein  trait


  • Proteins have many functions and make up many structures in cells and organisms.

  • Amino acids are the monomer units of proteins.

  • The polymers are called polypeptides (chains of amino acids connected by covalent bonds called peptide bonds)

  • Once the polypeptide is properly folded and is functional, it can be called a PROTEIN.

Proteins are made of amino acids
Proteins are made of amino acids

Amino Acid


An immature polypeptide folds to become a functional PROTEIN

Proteins differ from

each other by which

amino acids they contain and what order they are in.




Practice translation
Practice: Translation


  • mRNA: A U G GG U U C C G GG U A A

  • Polypeptide: Met - Gly - Ser - Gly - stop

    EXAMPLE 2.

  • mRNA: A U G C C G U A U C G C U G A

  • Polypeptide: Met - Pro - Tyr - Arg - stop

Steps of translation
Steps of Translation

1 2 3

  • The ribosome attaches to the mRNA.

  • As the mRNA moves through the ribosomes, the tRNAs bring in the amino acids one by one and they bond to each other to form a long polypeptide chain.

  • The tRNA’s are recycled.

  • When the polypeptide is finished it is released and the mRNA is recycled.


  • Despite proofreading, sometimes mistakes are made when the DNA is copied.

  • Mutation = a spontaneous change in the DNA code.

  • Gene Mutations change the genetic code in a single gene (one or a few nucleotides). This may cause the gene to make a different protein or a nonfunctional one. This could change the resulting trait.

  • Mutations are not always bad. Some may be helpful or neutral (no effect).

What causes mutations
What causes mutations?

  • Sometimes mutations are just errors that occur during the DNA replication process

  • Sometimes cells can repair the damage, but when they cannot, the DNA base sequence changes permanently. The cell may die or malfunction.

  • Some mutations arise from MUTAGENS

    • Mutagens are chemical or physical agents in the environment

      • Some chemicals

      • Radiation (UV light, gamma rays, x-rays)

      • Some viruses (ex. HPV)

  • In sexually reproducing organisms, mutations only pass on to the next generation if they occur in the sex cells.

Point mutations
Point Mutations

  • Frameshift Mutations

    • Addition/Insertion: One or more nucleotides is added/inserted into the gene

    • Deletion: One or more nucleotides is removed/deleted from the gene

  • Substitution: One or more nucleotides is substituted or swapped with a different nucleotide. *usually the least harmful**

Chromosomal mutations
Chromosomal Mutations

  • Deletion: Part of the chromosome is deleted

  • Duplication: One or more gene(s) on the chromosome gets duplicated (repeated)

  • Inversion: Parts of a chromosome are reversed

  • Translocation: Part of one chromosome breaks off and attaches to another chromosome.

Do genes control everything
Do Genes Control everything?

  • No…a cell/organism’s DNA is only part of the story.

  • Gene expression can be heavily influenced by environmental factors.

  • Nature vs. nurture argument

Does every section of the genome dna code for something
Does every section of the genome (DNA) code for something?

  • Not necessarily…

  • Genes are known as “coding” regions, since they code for polypeptides and determine traits.

  • Scientists used to think that most of DNA’s code was made up of “non-coding” regions. These regions apparently did not code for any polypeptides.

  • Recent studies suggest that much of the ‘noncoding’ regions are important in regulating the expression of the ‘coding’ regions.

  • Simpler organisms (ex. Sponges) have less non-coding regions than more advanced organisms (ex. Humans)

Does every cell express every gene in the genome
Does every cell express every gene in the genome?

  • Not necessarily.

  • A cell will only express the genes that it needs for its own particular traits.

  • Each organism started life as just one cell. During the development process, an organism’s cells become more and more differentiated and specialized for particular functions. This is called cellular specialization.

Differentiation in c elegans
Differentiation in C. elegans

  • A fertilized egg develops into an adult worm after many cell divisions. Daughter cells from each cell division follow a specific path toward a role as a particular kind of cell. This involves each cell type expressing (making proteins from) only the genes it needs.

Stem cells
Stem Cells

  • Stem cells are undifferentiated cells.

  • They can specialize to become any type of cell in the body.

  • Early embryos are made entirely of embryonic stem cells.

  • Adults also have some stem cells for example in bone marrow.

Cloning combine an egg cell with a donor nucleus to make a cloned embryo
CloningCombine an egg cell with a donor nucleus to make a cloned embryo

Gene therapy use normal genes to add to or replace defective genes
Gene TherapyUse normal genes to add to or replace defective genes

Transgenic organisms
Transgenic Organisms

  • GMO = Genetically Modified Organism click

  • Ex) GM Glowing Animals click

  • Ex) GM crops - ex) insect resistance

  • EX) GM bacteria producing insulin and growth hormone

  • There are some concerns about the unintended consequences that a shift to GM farming and ranching may have on agriculture. click

  • Many (not all) companies label foods so consumers can decide whether or not to use GM products