[IB Biology: Chemistry of Life]


Ch. 15

Transcription & Translation

RNA types

1. Ribosomal RNA (rRNA): make up ribosomes

2. Transfer RNA (tRNA): transport amino acids to ribosomes

3. Messenger RNA (mRNA): copied from DNA, conveys information from chromosomes to ribosomes


Transcription: production of mRNA copy of the DNA gene.

Gene: Section of DNA within a chromosome that codes for the synthesis of one protein. There are roughly 20-30,000 genes on the 23 pairs of human chromosomes.

Only 1 side of DNA in a gene gets transcribed:

A. Sense Strand: Strand of DNA in a gene which has the same nucleotide sequence as the m-RNA (except with T instead of U).

B. Antisense Strand: Name of strand of DNA that is transcribed to create m-RNA. It is the template used for making m-RNA.

RNA polymerase: enzyme that initiates transcription by binding to promoter (TAC code on DNA) at the 3' end of DNA

Promoter site: sequences in the DNA strand which accept RNA polymerase and initiate transcription

Elongation: mRNA bonds with DNA in small units, transcription bubble, adding bases Adenine to uracil & Guanine to cytosine, etc.. in the 5' to 3' direction (on the new m-RNA).

Termination: stop signal disengages RNA polymerase

[Transcription Diagram #1]

[Transcription Diagram #2]

Codon: sequence of 3 nucleotides on m-RNA that codes for one amino acid. Each amino acid has one to several different codons.

A Site with a good beginning Animation of Transcription (Needs Shockwave)


Much of DNA is non-coding base sequences, not genes

Intron: part of gene (DNA or mRNA) that does not code for polypeptide. It must be removed before translation.

Exon: coding part of DNA (or mRNA). The exon is translated during protein synthesis.

[How Introns are removed from m-RNA]


Translation: synthesis of protein by ribosomes

Anticodon: the 3 nucleotide sequence on t-RNA which the ribosome must fit against m-RNA to ensure that the correct amino acid is placed in the growing protein during translation.

Initiation: Initiates translation. rRNA polymerase of ribosome binds to mRNA strand. 1st tRNA is bonded to mRNA.

Elongation: Ribosome reads mRNA chain in three nucleotide groups (codon) & inserts 2nd tRNA.

tRNA anti-codon (with amino acid) binds to mRNA codon

Translocation: 5' to 3' sequence continues building amino acid polymers (protein), one codon at a time.

Termination: tRNA recognizes release factors of nonsense codon. Newly completed polypeptide is released from ribosome

A Site with a good beginning Animation of Translation (Needs Shockwave)

[Translation Diagram & Description]


General information:

1. Universality of Nucleic Acids: All organisms contain nucleic acids. It is a common thread of all life! It's possible to transplant nucleic acids from one species to another as a result!

1. Degeneracy of genetic code: 64 codons possible for 20 amino acids (Ensures several possible codon combinations for each amino acid which helps prevent mutations.) (See Table 15.1)

2. Anticodon & codon both three nucleotides long

3. 45 types of tRNA

4. Much of DNA is base sequences, not genes

5. Intron: part of gene (DNA or mRNA) that doesn't code for polypeptide

6. Exon: coding part of DNA (or mRNA)

7. Splicing: as mRNA matures, non-coding (introns) parts are removed (See Figs. 15.17 & 15.18)

8. nonsense (stop) codons are UAA, UAG, UGA

9. Start (promotor codon) signal is AUG

10. A gene is a DNA base sequence, a polypeptide is the protein coded by the gene.

 

[Gene Technology: Reverse Transcriptase]

Long & Slichter