Dna Replication Flashcards
Semi conserved replication requirements
Adenine ,guanine, cytosine or thymine, must be present.
• Both strands of the DNA molecule act as a template for the attachment of these nucleotides.
• The enzyme DNA polymerase.
• A source of chemical energy is required to drive the process.
Process of semi conservative replication
The enzyme DNA helicase breaks the hydrogen bonds linking the base pairs of DNA.
• As a result the double helix separates into its two strands and unwinds.
• Each exposed polynucleotide strand then acts as a template to which complementary free nucleotides bind by specific base pairing
• Nucleotides are joined together in a condensation reaction by the enzyme DNA polymerase to form the ‘missing’ polynucleotide strand on each of the two original polynucleotide strands of DNA.
• Each of the new DNA molecules contains one of the original DNA strands, that is, half the original DNA has been saved and built into each of the new DNA molecules (Figure 2). The process is termed
‘semi-conservative replication’.
Proper synthesis in a nutshell
DNA provides the instructions in the form of a long sequence of bases.
• A complementary section of part of this sequence is made in the form of a molecule called pre-mRNA - a process called transcription.
• The pre-mRNA is spliced to form mRNA.
• The mRNA is used as a template to which complementary tRNA molecules attach and the amino acids they carry are linked to form a polypeptide - a process called translation.
Transcription
An enzyme acts on a specific region of the DNA causing the two strands to separate and expose the nucleotide bases in that region.
• The nucleotide bases on one of the two DNA strands, known as the template strand, pair with their complementary nucleotides from the pool which is present in the nucleus. The enzyme RNA polymerase then moves along the strand and joins the nucleotides together to form a pre mRNA molecule. In this way an exposed bases are joined by free nucleotides . The exception is adenine, which links to uracil rather than thymine.
• As the RNA polymerase adds the nucleotides one at a time to build a strand of pre-mRNA, the DNA strands rejoin behind it.
• When the RNA polymerase reaches a particular sequence of bases on the DNA (called a terminator sequence), it detaches and the production of pre-mRNA is then complete.
Splicing of pre-mrna
In prokaryotic cells, transcription results directly in the production of mRNA from DNA. In eukaryotic cells transcription results in the production of pre-mRNA, which is then spliced to form mRNA… The DNA of a gene eukaryotic cells is made up of sections called exons that code for proteins and sections called introns that do not. In the pre-mRNA of eukaryotic cells. The base sequences corresponding to the introns are removed and the functional exons are joined together during a process called splicing. As most prokaryotic cells do not have introns, splicing of their DNA is unnecessary. The mRNA molecules are too large to diffuse out of the nucleus and so, once they have been spliced, they leave via a nuclear pore.
Synthesing a polypeptide
A ribosome becomes attached to the starting codon at one end of the mRNA molecule.
• The tRNA molecule with the complementary anticodon sequence moves to the ribosome and pairs up with the codon on the mRNA. This tRNA carries a specific amino acid
• A tRNA molecule with a complementary anticodon pairs with the next codon on the mRNA. This tRNA molecule carries another amino acid
• The ribosome moves along the mRNA, bringing together two tRNA molecules at any one time, each pairing up with the corresponding two codons on the mRNA.
• The two amino acids on the tRNA are joined by a peptide bond using an enzyme and ATP which is hydrolysed to provide the required energy.
• The ribosome moves on to the third codon in the sequence on the mRNA, thereby linking the amino acids on the second and third IRNA molecules
• As this happens, the first tRNA is released from its amino acid and is free to collect another amino acid from the amino acid pool in the cell.
• The process continues in this way, with up to 15 amino acids being added each second, until a polypeptide chain is built up
• Up to 50 ribosomes can pass immediately behind the first, so that many identical polypeptides can be assembled simultaneously
• The synthesis of a polypeptide continues until a ribosome reaches a stop codon. At this point, the ribosome, mRNA and the last tRNA molecule all separate and the polypeptide chain is complete.
The genetic code
scientists suggested that there must be a minimum of three bases that coded for each amino acid. Their reasoning was as follows:
• Only 20 different amino acids regularly occur in proteins.
• Each amino acid must have its own code of bases on the DNA.
• Only four different bases are present in DNA.
• If each base coded for a different amino acid, only four different amino acids could be coded for.
• Using a pair of bases, 16 different codes are possible, which is still inadequate.
• Three bases produce 64 different codes, more than enough to satisfy the requirements of 20 amino acids.
Features of the genetic code
few amino acids are coded for by only a single triplet.
• The remaining amino acids are coded for by between two and six triplets each.
• The code is known as a ‘degenerate code’ because most amino acids are coded for by more than one triplet.
• A triplet is always read in one particular direction along the
DNA strand.
• The start of a DNA sequence that codes for a polypeptide is always the same triplet. This codes for the amino acid methionine. If this first methionine molecule does not form part of the final polypeptide, it is later removed.
• Three triplets do not code for any amino acid. These are called ‘stop codes’ and mark the end of a polypeptide chain. They act in much the same way as a full stop at the end of a sentence.
• The code is non-overlapping, in other words each base in the sequence is read only once. Thus six bases numbered 123456 are read as triplets 123 and 456, rather than as triplets 123, 234, 345, 456.
• The code is universal, with a few minor exceptions each triplet codes for the same amino acid in all organisms. This is indirect evidence for evolution.
mRNA
Consisting of thousands of mononucleotides, mRNA is a long strand that is arranged in a single helix. The base sequence of mRNA is determined by the sequence of bases on a length of DNA in a process called transcription. There is a great variety of different types of mRNA. Once formed, mRNA leaves the nucleus via pores in the nuclear envelope and enters the cytoplasm, where it associates with the ribosomes. There it acts as a template for protein synthesis. Its structure is suited to this function because it possesses information in the form of codons (three bases that are complementary to a triplet in DNA). The sequence of codons determines the amino acid sequence of a specific polypeptide that will be made.
tRNA
Transfer RNA (IRNA) is a relatively small molecule that is made up of around 80 nucleotides. It is a single-stranded chain folded into a cloverleaf shape, with one end of the chain extending beyond the other. This is the part of the tRNA molecule to which an amino acid can easily attach. There are many types of tRNA, each of which binds to a specific amino acid. At the opposite end of the RNA molecule is a sequence of three other organic bases, known as the anticodon. Given that the genetic code is degenerate there must be as many tRNA molecules as there are coding triplets. However, each tRNA is specific to one amino acid and has an anticodon that is specific to that amino acid.
Genome and protemore
Genome - the complete set of genes in a cell, including those in mitochondria and/or chloroplasts.
• Proteome - the full range of proteins produced by the genome.
This is sometimes called the complete proteome, in which case the term proteome refers to the proteins produced by a given type of cell under a certain set of conditions.
