BIO 156 Chapter 18 Powerpoint
Transcript of BIO 156 Chapter 18 Powerpoint
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BIO 156
Chapter 18
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How Genes Work and How Genes are Controlled
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DNA and RNA
• DNA consists of a double helix heldtogether by hydrogen bonds. – Each strand of the double helix contains
nucleotides.
– Each nucleotide in the DNA moleculeconsists of a purine or pyrimidine base, thesugar deoxyribose, and a phosphategroup.
– The nucleotides are joined by covalentbonds between phosphate groups anddeoxyribose molecules.
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DNA
Subunits
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DNA
Subunits
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DNA
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– Complementary base pairing is an unalterable
coupling in which adenine on one strand of the DNA
molecule always binds to thymine on the other and
guanine always binds to cytosine. – Complementary base pairing ensures
• accurate replication of the DNA
•accurate transmission of genetic information from one cellto another and from one generation to another.
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• DNA unwinds then
serves as a template for
the production of new
DNA strands.
– DNA polymerase is
an enzyme that helps
align and pair
nucleotides to the
template strand.
DNA Replication
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• Three types of RNA exist,
each of which is involved in
protein synthesis:
– Transfer RNA – Ribosomal RNA
– Messenger RNA.
• All three RNA molecules aresingle-stranded
polynucleotide chains.
RNA
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• RNA synthesis is
called transcription
and takes place on aDNA template in the
nucleus of the cell.
Transcription
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DNA and RNA Base Pairing
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How Genes Work
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How Genes Work: Protein Synthesis
• Protein is synthesized on a mRNA template.
– This process is called translation. – The genetic information contained in the
DNA molecule is transferred to messenger
RNA. – Messenger RNA molecules carry this
information to the cytoplasm, where
proteins are synthesized.
– Messenger RNA serves as a templatefor protein synthesis.
– Ribosomes are required to produce
proteins
on the mRNA template.
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Messenger RNA
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– Transfer RNAmolecules deliver amino acid moleculesto the mRNA and
insert them in thegrowing chain.
– Each tRNA binds to aspecific amino acidanddelivers it to a specific
codon, a sequence of three bases on themRNA.
– The sequence of codons determinesthe sequence of
amino acids in theprotein.
– Messenger RNAserves as a templatefor protein synthesis.
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Protein Synthesis
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Controlling Gene Expression
• In humans, genetic expression is
controlled atfour levels: – At the chromosome—access to the genes is
controlled by coiling and uncoiling of the
chromosome during interphase. – At transcription—three control mechanisms
operate at the level of transcription:
• Induction
•Repression• Enhancement
– After transcription but before translation—byaltering the structure of mRNA.
– At translation—by masking mRNA
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Posttranscriptional Control of Human Genes
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Gene
Regulation
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End of Chapter 18