RNA
¨ RNA uses the information from DNA to synthesize proteins
¨ RNA is a polymer of nucleotide monomers and subunits
Differs
from DNA:
1. consists of only one strand of nucleotides
2. ribose
is the five-carbon sugar
3. the nitrogen base uracil rather than thymine
¨ You can think of an RNA molecule as a disposable copy of a segment
of DNA.
Types of RNA
1. messenger RNA (mRNA) – transmits information from DNA for use during protein
synthesis and serves as a template for the assembly of amino acids.
2. ribosomal RNA (rRNA) – major constituent of the ribosomes but its function is
unknown
3. transfer RNA (tRNA) – allows complimentary bases to pair
and exists in 20 or more varieties each bonding to only one specific amino acid
Transcription
¨ transcription is the process of RNA being produced from DNA
¨ RNA molecules are transcribed according to the information encoded
in the base sequence of DNA
1. RNA polymerase (enzyme) binds to a DNA molecule and separates the DNA strands
2. Bonds form between the bases of a DNA strand and complementary bases of floating RNA nucleotides
3. Enzyme establishes sugar-to-phosphate bonds between RNA nucleotides
4. When the termination signal on DNA is reached, the new RNA is released.
¨ The enzyme will bind only to regions of DNA known as promoters,
which have specific base sequences.
¨ Promoters are signals in DNA that indicate to the enzyme where to
bind to make RNA.
¨ All three types of RNA are transcribed in this manner and move from
the nucleus into the cytoplasm, where they are involved in the synthesis of protein.
¨ RNA has a base sequence complimentary to DNA
Protein Synthesis
¨ Protein synthesis is the formation of proteins using information coded on DNA and carried out by
RNA.
¨
An organism may have from several hundred
to several thousand different proteins.
¨ Each protein is made of polypeptides which are made of amino acids
(20 different ones)
Codons
¨ Protein synthesis is the assembly of amino acids into a specific protein.
¨ The genetic code is the system that contains information needed
by cells for proper functioning.
¨
A codon is
a specific group of three sequential bases of mRNA.
¨ Each codon codes for, or recognizes, a specific amino acid using
tRNA as an intermediary.
¨ There are 64 possible codons and amino acids may have several different
codons, which mainly differ, in the third base position.
Example:
UUU and
UUC = Phenylalanine
UUA,
UUG, CUU, CUC, CUA and CUG = Leucine
¨ A few amino acids do not encode amino acids at all. They are a “start” and “stop” signal for reading a mRNA molecule.
¨ The coding for amino acids is universal with AUG being the universal “start”
signal.
¨ The “stop” codons act like a period at the end of a sentence;
they signify the end of polypeptide.
Translation
¨ Translation is the process of assembling protein molecules for information
encoded in mRNA.
¨ Translation takes place on ribosomes.
1. mRNA moves out of the nucleus through the pores towards a group of ribosomes.
2. Amino acids floating freely in the cytoplasm are transported to the ribosomes
by tRNA
3. The opposite end of tRNA bears a sequence of three bases called an anticodon,
which pairs with the codon of mRNA.
4. Assembly begins at an AUG codon on the mRNA and pair with the anticodon UAC on
the tRNA.
5. The first amino acid is always methionine but is sometimes removed from the chain.
v Several ribosomes are translating at the same time.
6. Enzymes catalyze a reaction that binds the amino acids together and a peptide
bond is formed with the “stop” codon ending the process.
¨ A gene is the region of DNA that directs the formation of a polypeptide.
¨ Since most proteins consist of more than one polypeptide, information
from several genes may be required to produce a particular protein.
