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Protein Synthesis
Chapter 16, section 2
The sequence (order) of bases in a strand of
DNA makes the code for building proteins.
EX: The three bases “CCA” form the code for the
amino acid proline.
A long string of amino acids forms a protein.
Each gene is usually a set of instructions for
making a protein.
Proteins are responsible for most of the
differences in organisms that we see.
EX: height, curly or straight hair, etc.
RNA is needed to produce proteins. It is so
similar to DNA that it serves as a temporary
copy of a DNA sequence.
Three different types of RNA are involved in the
making of a protein.
1. mRNA (messenger RNA): mRNA creates a
complementary strand from DNA and carries it
out of the nucleus into the cytoplasm. It
replaces thymine with uracil.
For example:
AGCGTAAG
UCGCAUUC
DNA strand
RNA strand
2. rRNA (ribosomal RNA): makes up part of the
ribosome.
3. tRNA (transfer RNA): brings the correct amino
acids to the ribosome for protein construction.
Protein Synthesis Occurs in Two Steps
1. Transcription: Produces a single-stranded
molecule of RNA.
STEPS:
1.
2.
3.
The DNA molecule opens up along a gene.
RNA nucleotides (A,U,C,G) match up and join the open
DNA strand.
The complete RNA strand is released and moves to the
cytoplasm.
2. Translation: occurs at the ribosome. The
ribosome is the “protein factory” of the cell.
1.
2.
3.
4.
STEPS:
A ribosome attaches to the beginning of a
mRNA molecule.
The mRNA is fed through the ribosome three
bases at a time.
Molecules of tRNA deliver amino acids from
the cytoplasm to the ribosome.
The tRNA molecules are released after the
amino acids they carry are attached to the
growing chain of amino acids.
5. The ribosome completes the translation when
it reaches the end of the mRNA strand and the
newly made protein molecule is released.