Before a cell can divide, the DNA in the nucleus of the cell must be duplicated. Since the DNA molecule consists of two complimentary stands, if those two strands separate and the right conditions are present, two new stands that are the compliments of the originals will be produced.
Before a cell can divide, the DNA in the nucleus of the cell must be duplicated.
Since the DNA molecule consists of two complimentary stands, if those two strands separate and the right conditions are present, two new stands that are the compliments of the originals will be produced.
Each new DNA molecule will consist of one old stand, and a new complimentary strand.
The gray strands in the figure to the right are new strands in the process of being assembled.
Since the DNA molecule is very large, there must be a way to copy it faster than just unwinding from one end to the other!
This happens when the DNA molecule separates at many sites, forming thousands of replication bubbles. This allows parts of the DNA message to be replicated simultaneously in many locations.
DNA polymerase adds new nucleotides , while DNA ligase joints the DNA segments together.
There are 64 (4X4X4) possible triplet codes, but only 20 amino acids.
As seen in the table, more than 1 triplet may code for the same amino acid. This is no problem, as long as no triplet can code for more than one amino acid.
Note that several codons can also act as start (AUG) or stop (UAA) signals.
When the genetic message is copied to make mRNA, the message contains unwanted base sequences.
The ‘junk’ sequences (called introns) are removed from the message and the remaining sequences (exons) are linked together to produce a sequence of codons that will translate into a polypeptide.
This process occurs before the message leaves the nucleus.
A second type of RNA is transfer RNA, whose function is to attach to a specific amino acid and bring that amino acids to the site where polypeptides are being constructed.
This RNA strand is twisted and bonded into the shape seen on the right.
One end of the molecule attached to a specific amino acid.
The other end has an exposed sequence of 3-bases. These are called the anticodon.
How many kinds of tRNA must there be?
triplets. This means 64 anticodons.
The anticodons of the tRNAs each have a complimentary codon in the mRNA. For example the codon AUG would be the compliment of the anticodon UAC.
The third type of RNA is risosomal RNA (rRNA).
Ribosomes are the ‘decoding’ units of the cell.
Each ribosome consists of two subunits, and is an assemblage of rRNA and proteins.
Ribosomes have binding sites for both tRNA and mRNA molecules.
An mRNA molecule attaches to a ribosome.
As the ribosome moves along the mRNA, 3-base codons are exposed one at a time.
A tRNA with an anticodon that is complimentary to the codon of the mRNA temporarily bonds with the mRNA.
The ribosome positions the molecules so that this bonding occurs.
As the ribosome continues its journey along the mRNA additional tRNAs bring their a.a. to the site of peptide synthesis.
As new amino acids are brought to the ribosome, the growing peptide chain is attached to the new amino acid by a peptide bond.
Elongation of the chain continues until a stop codon is encountered. At that point the peptide chain is released from the tRNA.
A single mRNA can be read repeatedly to make many copies of a polypeptide.
Once a tRNA gives up its amino acid it can return to the cytoplasm and attach to another of its specified amino acid.
Information is stored in the triplet codes (codons) of DNA nucleotides.
This information is transcribed into 3 types of RNA.
mRNA carries the information to assemble a polypeptide.
In the nucleus, introns are removed and the remaining exons spliced together to make a functional mRNA strand.
tRNA molecules attach to specific amino acids.
rRNA and proteins form ribosomes.
mRNA attaches to a ribosome and the message is decoded when the anticodon of a tRNA is bonded to a mRNA codon.
Subsequent amino acids are attached to the growing peptide chain until a stop codon is reach and the chain is terminated.
A summary of these events can be seen in the next slide.