© 2010 mcgraw-hill australia cell protein production chapter 3
TRANSCRIPT
© 2010 McGraw-Hill Australia
Cell Protein Production
Chapter 3
© 2010 McGraw-Hill Australia
Transcription
Translation
Protein Formation
Summary
To learn about the stages of protein synthesis click on the stage names in blue.
© 2010 McGraw-Hill Australia
• Transcription is the process by which the mRNA is formed using the code in a DNA molecule
• The genetic instructions are copied (or transcribed) from the DNA to the mRNA molecule
• The enzyme RNA polymerase begins the process of making mRNA
Transcription
© 2010 McGraw-Hill Australia
Transcription (cont.)• RNA polymerase makes
the double-stranded DNA molecule come apart, usually about 17 base pairs at a time
• RNA polymerase then transcribes (copies) the bases on one strand of the DNA to make a complementary molecule of mRNA
© 2010 McGraw-Hill Australia
Transcription (cont.)• RNA molecules have
the base uracil (U) instead of thymine
• At the end of the gene there is a sequence of bases that tells the RNA poly-merase to stop copying and as a consequence the mRNA molecule is released
© 2010 McGraw-Hill Australia
Transcription (cont.)• The strand that is copied
is called the template strand because it is the template from which the mRNA is made. The other strand is known as the coding strand
• Not all of the bases in the DNA molecule are code for an amino acid. The non- coding sequences are called introns
© 2010 McGraw-Hill Australia
Transcription (cont.)• The base sequences
that code for amino acids are called exons
• The functional mRNA will leave the nucleus and direct protein synthesis in the cytoplasm
Return
© 2010 McGraw-Hill Australia
Translation
• Translation is the production of a protein using the information that is coded in the mRNA molecule
• In the cytoplasm a ribosome attaches to one end of the mRNA molecule. using a sequence that is the binding site for the ribosome
© 2010 McGraw-Hill Australia
Translation (cont.)
• The ribosome then moves along the mRNA three bases at a time, reading the bases as it goes
• When the ribosome reaches the start codon (AUG) it starts making the protein Return
© 2010 McGraw-Hill Australia
Protein Formation
• Transfer RNA (tRNA) bring the individual amino acids to the ribosome to be joined together as proteins
• Each tRNA molecule has a section that binds to its corresponding amino acid
© 2010 McGraw-Hill Australia
• The loop of the tRNA has three nitrogen bases that form an anticodon. These three bases bind with the complementary bases of a codon on the mRNA molecule
• The anticodon determines the type of amino acid carried by the tRNA
Protein Formation (cont.)
© 2010 McGraw-Hill Australia
• As the ribosome reads the codons on the mRNA, tRNA molecules with the matching anticodon are brought in
• The amino acids carried by the tRNA are joined together so the protein is assembled with the amino acids in the correct sequence
Protein Formation (cont.)
© 2010 McGraw-Hill Australia
• Once the tRNA has delivered its amino acid it detaches from the ribosome and can pick up another amino acid from the cytoplasm
• One mRNA often has 10 or 20 ribosomes reading its code at the same time
• This means that a cell could produce over 150 000 protein molecules per second
Protein Formation (cont.)
Return
© 2010 McGraw-Hill Australia
• Each gene in the DNA in the cell nucleus carries the code for making a protein from amino acids
Transcription• The two strands of the DNA in a gene separate• mRNA forms with bases that are complementary to
those on the template strand of the DNA• The mRNA molecule that is formed undergoes
modification – the introns are removed leaving just the exons
• mRNA travels from the nucleus to the cytoplasmTranslation• mRNA attaches to a ribosome• The ribosome moves along the mRNA reading the
code. At the start codon it begins making the protein• For each codon on the mRNA, a tRNA with a matching
anticodon brings the correct amino acid• The amino acids are bonded together to form the
protein
Summary