Nov 18 Chapter 10 (continued)

 

2. Mechanism of gene expression

a.          Overview of gene expression

DNA (nucleus)

  ↓ transcription (nucleus)

RNA (nucleus ® cytoplasm)

  ↓ translation (ribosome)

protein

 

Genes

-          DNA contains genes aligned side by side.

-          One gene – one polypeptide - Each gene contains information necessary for making one polypeptide chain. 

Transcription

-          The process in which information in DNA is copied to mRNA (messenger RNA).

-          Occurs in the nucleus.

-          In eukaryotes, one RNA molecule contains information from one gene.

-          In prokaryotes, one RNA molecule may contain information from several genes.

-          DNA serves as template for RNA synthesis.  (RNA sequence is complementary to the template DNA sequence)

o       A pairs with U

o       T pairs with A

o       G pairs with C

o       C pairs with G

 

Example: If the first 10 nucleotide sequence of the template DNA is CGTATGCATC, the corresponding RNA sequence would be:

GCAUACGUAG

Translation

-          The process in which information in mRNA is used to produce polypeptides.

-          Occurs in the ribosome.

-          The language of mRNA is based on three-letter (triplet) words. Specific combinations of nucleotide triplets (codons) specify amino acids.

-          Codon table – dictionary of the genetic code.  Common to almost all living organisms.

-          Most codons specify amino acids.

Example: The codon UUU codes for the amino acid Phe (phenylalanine).

-          Many amino acids are coded by more than one codon because the number of possible combinations of triplets (64) is more than the number of amino acids (20).

Example 1: Codons UUU and UUC both specify Phe.

 

Can you find codons for the amino acid leucine (Leu)?

 

-          AUG is the start codon and also codes for Met (methionine).

-          Three codons UAA, UGA, and UAG are the stop codons and do not codes for any amino acids.

 

Example:

In the Figure 10.7B, p193 (See next page), the nucleotide triplet UUU codes for amino acid phenylalanine (Phe). 

 

Using the codon table, can you predict the three other amino acids in the same figure?

 

 

Length of DNA, RNA and polypeptides.

-          Length of DNA is expressed as the number of base pairs.

-          Length of RNA is expressed as the number of bases.

-          Length of a polypeptide is expressed as the number of amino acids.

 

 

b.          Transcription

RNA polymerase

-          Synthesizes RNA using DNA as templates. 

-          Normally, only one strand of DNA serves as a template.

-          The direction of synthesis is from the 5’ end to the 3’ end (Adds nucleotides at the 3’ end).  Similar to DNA synthesis.

Simple structure of a gene. 

 

 

 

-          Promoter: Specific binding site for RNA polymerase.  Signal for the initiation of transcription.

-          Terminator: Signal for the termination of transcription.  RNA polymerase detaches from the RNA molecule and the gene.

-          Three steps of transcription: initiation, elongation and termination (Figure 10.9B, p195)

 

c.          Processing of eukaryotic RNA (Fig 10.10)

-          In eukaryotes, newly transcribed RNA molecules are processed (modified) to make mature mRNAs.

o       CAP (an extra G) at the 5’ end and TAIL (multiple A’s) at the 3’ end.

o       Splicing – removal of internal noncoding sequences (introns).

 

introns vs. exons

-          Many eukaryotic genes contain noncoding sequences called introns.

-          The coding regions are called exons.

-          Both introns and exons are transcribed.  Introns are removed by splicing.  Mature mRNA do not contain introns.