12.3 RNA and Protein Synthesis
12.3 RNA and Protein Synthesis
• Genes are coded instructions that control the
production of proteins within the cell
The Structure of RNA
• There are 3 main differences between RNA
and DNA: The sugar in RNA is ribose instead of
deoxyribose, RNA is generally single-stranded,
and RNA contains uracil in place of thymine.
Types of RNA
• There are three main types of RNA: messenger
RNA, ribosomal RNA, and transfer RNA.
• Messenger RNA (mRNA) carries copies of
messages encoded on DNA to the rest of the
cell.
• Ribosomal RNA (rRNA) makes up part of the
ribosome
• Transfer RNA (tRNA) carries amino acids to
ribosomes
Transcription
• During transcription, RNA polymerase binds to
DNA and separates the DNA strands. RNA
polymerase then uses one strand of DNA as a
template from which nucleotides are
assembled into a strand of RNA.
• RNA polymerase will only bind to regions of
DNA known as promoters.
RNA Editing
• The DNA of eukaryotic genes contains
sequences of nucleotides, called introns, that
are not involved in coding for proteins.
• The DNA sequences that code for proteins are
called exons because they are expressed in
protein synthesis.
The Genetic Code
• Proteins are made by joining amino acids into
long chains called polypeptides.
• A codon consists of three consecutive
nucleotides that specify a single amino acid to
be added to the polypeptide.
• There are 64 possible 3 base codons.
Translation
• The decoding of an mRNA message into a
polypeptide chain (protein) is known as
translation.
• Translation takes place on ribosomes.
• During translation, the cell uses information
from mRNA to produce proteins.
• A) mRNA is transcribed from DNA.
• B) Translation starts when an mRNA molecule
attaches to a ribosome. As each codon moves
through the ribosome, the proper amino acid
is brought to the ribosome by tRNA.
• Each tRNA has 3 unpaired bases, the
anticodon, which is complementary to one
mRNA codon.
• C) The ribosome forms a bond between the
first 2 amino acids, and breaks the bond that
held the first tRNA to its amino acid.
• D) The polypeptide chain continues to grow
until the ribosome reaches a stop codon on
the mRNA molecule.
The Roles of RNA and DNA
• The master plan is stored safely in an office,
and the blueprints are taken to the job site.
Similarly, the cell uses the DNA “master plan”
to prepare RNA “blueprints.”
Genes and Proteins
• A gene that codes for an enzyme to produce
pigment can control the color of a flower.
• Another gene controls the production of an
enzyme that determines blood type.