bio 156 chapter 18 powerpoint

8/14/2019 BIO 156 Chapter 18 Powerpoint

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BIO 156

Chapter 18



How Genes Work and How Genes are Controlled



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.



DNA

Subunits



DNA



– 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.



• 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



• 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



• RNA synthesis is

called transcription

and takes place on aDNA template in the

nucleus of the cell.

Transcription



DNA and RNA Base Pairing



How Genes Work



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.



Messenger RNA



– 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.



Protein Synthesis



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



Posttranscriptional Control of Human Genes



Gene

Regulation



End of Chapter 18

bio 156 chapter 18 powerpoint

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