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Definitions of the Gene. BIT 220 Chapter 15. A typical Gene. Figure 15.1 Prokaryotic Gene Initiator and terminator codon All done in cytoplasm Transcription and translation coupled Eukaryotic Gene Exons and introns; nucleus and cytoplasm Cap and poly A+ tail. Genes.

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Definitions of the gene l.jpg

Definitions of the Gene

BIT 220

Chapter 15


A typical gene l.jpg
A typical Gene

  • Figure 15.1

  • Prokaryotic Gene

    • Initiator and terminator codon

    • All done in cytoplasm

    • Transcription and translation coupled

  • Eukaryotic Gene

    • Exons and introns; nucleus and cytoplasm

    • Cap and poly A+ tail


Genes l.jpg
Genes

  • Focus in chapter on complementation test: controls the synthesis of one polypeptide or one mRNA molecule

  • Figure 15.4: Beadle and Tatum: led to one gene – one enzyme theory

    • Need only inorganic salts, simple sugar and biotin (vitamin)


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Recombination

  • Within a gene – Oliver in the 40s

  • Drosophila melanogaster – fruit fly- genetic organism of choice

  • Showed that genes could recombine between single nucleotide pairs (figures 15.5 and 15.6)


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Colinearity

  • Figures 15.7, 15.8 and 15.9

  • Nucleotide sequence co-linear with polypeptide sequence

  • First three bases in DNA is first amino acid in polypeptide, etc.

  • Introns do not invalidate this colinearity


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Complementation test for a Gene

  • Figures 15.10 and up

  • Terms to know:

    • wild-type – “normal” allele

    • Cis – same chromosome

    • Trans – opposite chromosome

    • Cis heterozygote – a+b+/ab

    • Trans heterozygote - ab+/a+b


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Gene Definition

  • Knew about 1 gene-1 enzyme, but how to know if a mutation was on same or different gene?

  • Complementation test for function allelism (Ed Lewis, 1942)

  • begin with Figure 15.11

  • Position effects - + designation for wild-type


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Gene Name Designation

  • With Mendel, used dominant gene - e.g., Tall and short would be T and t (learned in 120)

  • Now reclassified, would be S and s (named after mutant or recessive allele)

  • Fruit flies - white gene, normal eye red, so called w+; apricot mutation, wa

  • human gene designations later


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Complementation

  • Lewis’ experiment - Figure 15.11

  • fruit fly important genetic organism

    • white locus (where a gene is on a chromosome)

    • apricot mutation (apr in this textbook, now proper designation is wa)

      apr w/apr+ w - designation for 2 chromosomes (slash mark), give red eyes. ; apr w+/apr+ w - light apricot eyes (go over Fig. 15.11 here)


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Mutations on the same Gene

  • Following looking at Figure 15.11:

  • If apricot and white were on different genes, the eyes would be red

  • Why? Because the wild-type allele in each case would be dominant over the mutant allele, so they eyes would be wild-type color (red), but they are not.


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Lewis’ Discovery

  • The cis-trans position effect

  • This led to complementation test or trans test of functional allelism

  • can determine if mutations with same or similar phenotypes are in the same or different genes.


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Cis or Cis-trans test

  • Cis test important control (Figure 15.2)

  • phenotype wild-type whether mutation on same or different chromosome

  • Trans test (Figure 15.13) - phenotype mutant if both mutations on same chromosome (15.13a); phenotype wild-type if mutations on 2 different genes.


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Limitations

  • Complementation tests only work with simple genes and normal dominant/co-dominant relationships

  • Not useful in dominant mutations, or for multigenic traits


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Complex Gene-Protein Relationships

  • Figure 15.8 - family of proteins from a single gene

  • Figure 15.9 - antibody genes