Understandings:
NATURE OF SCIENCE
Obtaining evidence for scientific theories: Meselson and Stahl obtained evidence for the semi-conservative replication of DNA.
Application and skills:
- DNA replication involves ‘unzipping’
Cells must prepare for a cell division by doubling the DNA content of the cell in a process called DNA replication. This process doubles the quantity of DNA and also ensures that there is an exact copy of each DNA molecule. In the nucleus of cells are two types of molecules that are particularly important for the process of DNA replication; they are:
- Formation of two complementary strands
Once DNA has become unzipped, the nitrogenous bases on each of the single strands are unpaired. In the environment of the nucleoplasm, there are free-floating nucleotides. These nucleotides are available to form complementary pairs with the single-stranded nucleotides of the unzipped molecule. This does not happen in a random fashion. A free nucleotide locates on one opened strand at one end, and then a second nucleotide can join the first. This requires these two nucleotides to become covalently bonded together, because they are the beginning of a new strand. The formation of a covalent bond between two adjoining nucleotides is catalysed by one of the DNA polymerase enzymes that are important in this process.
A third nucleotide then joins the first two, and the process continues in a repetitive way for many nucleotides. The other unzipped strand also acts as a template for the formation of another new strand. This strand forms in a similar fashion, but in the opposite direction to the first strand.
The two strands are absolutely identical to each other. This is because the original double-stranded molecule had complementary pairs of nucleotides and it was the complementary nucleotides that used the unzipped single-stranded areas as templates.
This also means that no DNA molecule is ever completely new. After replication, every DNA molecule consists of a strand that is ‘old’ paired with a strand that is ‘new’. DNA replication is described as a semi-conservative process because half of a pre-existing DNA molecule is always conserved (saved).
- The replication of DNA is semi-conservative and depends on complementary base pairing.
- Helicase unwinds the double helix and separates the two strands by breaking hydrogen bonds.
- DNA polymerase links nucleotides together to form a new strand, using the pre-existing strand as
a template.
- Transcription is the synthesis of mRNA copied from the DNA base sequences by RNA polymerase.
- Translation is the synthesis of polypeptides on ribosomes.
- The amino acid sequence of polypeptides is determined by mRNA according to the genetic code.
- Codons of three bases on mRNA correspond to one amino acid in a polypeptide.
- Translation depends on complementary base pairing between codons on mRNA and anticodons on tRNA.
NATURE OF SCIENCE
Obtaining evidence for scientific theories: Meselson and Stahl obtained evidence for the semi-conservative replication of DNA.
Application and skills:
- Application: Use of Taq DNA polymerase to produce multiple copies of DNA rapidly by the polymerase chain reaction (PCR).
- Application: Production of human insulin in bacteria as an example of the universality of the genetic code allowing gene transfer between species.
- Skill: Use a table of the genetic code to deduce which codon(s) corresponds to which amino acid.
- Skill: Analysis of Meselson and Stahl’s results to obtain support for the theory of semi-conservative replication of DNA.
- Skill: Use a table of mRNA codons and their corresponding amino acids to deduce the sequence of amino acids coded by a short mRNA strand of known base sequence.
- Skill: Deducing the DNA base sequence for the mRNA strand.
Guidance
● The different types of DNA polymerase do not need to be distinguished.
- DNA replication involves ‘unzipping’
Cells must prepare for a cell division by doubling the DNA content of the cell in a process called DNA replication. This process doubles the quantity of DNA and also ensures that there is an exact copy of each DNA molecule. In the nucleus of cells are two types of molecules that are particularly important for the process of DNA replication; they are:
- enzymes needed for replication, which include helicase and a group of enzymes collectively called DNA polymerase
- free nucleotides, which are nucleotides that are not yet bonded and are found floating freely in the nucleoplasm, some contain adenine, some thymine, some cytosine, and some guanine.
- Formation of two complementary strands
Once DNA has become unzipped, the nitrogenous bases on each of the single strands are unpaired. In the environment of the nucleoplasm, there are free-floating nucleotides. These nucleotides are available to form complementary pairs with the single-stranded nucleotides of the unzipped molecule. This does not happen in a random fashion. A free nucleotide locates on one opened strand at one end, and then a second nucleotide can join the first. This requires these two nucleotides to become covalently bonded together, because they are the beginning of a new strand. The formation of a covalent bond between two adjoining nucleotides is catalysed by one of the DNA polymerase enzymes that are important in this process.
A third nucleotide then joins the first two, and the process continues in a repetitive way for many nucleotides. The other unzipped strand also acts as a template for the formation of another new strand. This strand forms in a similar fashion, but in the opposite direction to the first strand.
The two strands are absolutely identical to each other. This is because the original double-stranded molecule had complementary pairs of nucleotides and it was the complementary nucleotides that used the unzipped single-stranded areas as templates.
This also means that no DNA molecule is ever completely new. After replication, every DNA molecule consists of a strand that is ‘old’ paired with a strand that is ‘new’. DNA replication is described as a semi-conservative process because half of a pre-existing DNA molecule is always conserved (saved).
- Protein synthesis
The control that DNA has over a cell is determined by a process called protein synthesis. In simple terms, DNA controls the proteins produced in a cell. Some of the proteins produced are enzymes. The production (or lack of production) of a particular enzyme can have a dramatic effect on the overall biochemistry of the cell. Thus DNA indirectly controls the biochemistry of carbohydrates, lipids, and nucleic acids with the production of enzymes.
Protein synthesis involves two major sets of reactions, transcription and translation. Both either produce or require a type of nucleic acid called RNA
- Transcription produces RNA molecules
The sections of DNA that code for polypeptides are called genes. Any one gene is a specific sequence of nitrogenous bases found in a specific location in a DNA molecule. Molecules of DNA are found within the confines of the nucleus, yet proteins are synthesized outside the nucleus in the cytoplasm. This means that there has to be an intermediary molecule that carries the message of the DNA (the code) to the cytoplasm where the enzymes, ribosome, and amino acids are found. This intermediary molecule is called messenger RNA (mRNA).
The nucleoplasm (fluid in the nucleus) contains free nucleotides, as mentioned earlier. In addition to the free nucleotides used for DNA replication, the nucleoplasm also contains free RNA nucleotides. Each of these is different from the DNA counterpart, because RNA nucleotides contain the sugar ribose not deoxyribose. Another major difference is that no RNA nucleotides contain thymine; instead there is a nitrogenous base unique to RNA, called uracil.
- The transcription process
The process of transcription begins when an area of DNA of one gene becomes unzipped (see Figure 2.40). This is very similar to the unzipping process involved in DNA replication, but in this case only the area of the DNA where the particular gene is found is unzipped. The two complementary strands of DNA are now single-stranded in the area of the gene. Recall that RNA (which includes mRNA) is a single-stranded molecule. This means that only one of the two strands of DNA will be used as a template to create the mRNA molecule. An enzyme called RNA polymerase is used as the catalyst for this process.
As RNA polymerase moves along the strand of DNA acting as the template, RNA nucleotides float into place by complementary base pairing. The complementary base pairs are the same as in double-stranded DNA, with the exception that adenine on the DNA is now paired with uracil on the newly forming mRNA molecule. Consider the following facts concerning transcription:
The control that DNA has over a cell is determined by a process called protein synthesis. In simple terms, DNA controls the proteins produced in a cell. Some of the proteins produced are enzymes. The production (or lack of production) of a particular enzyme can have a dramatic effect on the overall biochemistry of the cell. Thus DNA indirectly controls the biochemistry of carbohydrates, lipids, and nucleic acids with the production of enzymes.
Protein synthesis involves two major sets of reactions, transcription and translation. Both either produce or require a type of nucleic acid called RNA
- Transcription produces RNA molecules
The sections of DNA that code for polypeptides are called genes. Any one gene is a specific sequence of nitrogenous bases found in a specific location in a DNA molecule. Molecules of DNA are found within the confines of the nucleus, yet proteins are synthesized outside the nucleus in the cytoplasm. This means that there has to be an intermediary molecule that carries the message of the DNA (the code) to the cytoplasm where the enzymes, ribosome, and amino acids are found. This intermediary molecule is called messenger RNA (mRNA).
The nucleoplasm (fluid in the nucleus) contains free nucleotides, as mentioned earlier. In addition to the free nucleotides used for DNA replication, the nucleoplasm also contains free RNA nucleotides. Each of these is different from the DNA counterpart, because RNA nucleotides contain the sugar ribose not deoxyribose. Another major difference is that no RNA nucleotides contain thymine; instead there is a nitrogenous base unique to RNA, called uracil.
- The transcription process
The process of transcription begins when an area of DNA of one gene becomes unzipped (see Figure 2.40). This is very similar to the unzipping process involved in DNA replication, but in this case only the area of the DNA where the particular gene is found is unzipped. The two complementary strands of DNA are now single-stranded in the area of the gene. Recall that RNA (which includes mRNA) is a single-stranded molecule. This means that only one of the two strands of DNA will be used as a template to create the mRNA molecule. An enzyme called RNA polymerase is used as the catalyst for this process.
As RNA polymerase moves along the strand of DNA acting as the template, RNA nucleotides float into place by complementary base pairing. The complementary base pairs are the same as in double-stranded DNA, with the exception that adenine on the DNA is now paired with uracil on the newly forming mRNA molecule. Consider the following facts concerning transcription:
- only one of the two strands of DNA is ‘copied,’ the other strand is not used
- mRNA is always single-stranded and shorter than the DNA that it is copied from, as it is a complementary copy of only one gene
- the presence of thymine in a molecule identifies it as DNA (the presence of deoxyribose is another clue)
- the presence of uracil in a molecule identifies it as RNA (the presence of ribose is another clue).
- The genetic code is written in triplets
DNA → (transcription) → mRNA triplet (codon)
The mRNA molecule produced by transcription represents a complementary copy of one gene of DNA. The sequence of mRNA nucleotides is the transcribed version of the original DNA sequence. This sequence of nucleotides making up the length of the mRNA is typically enough information to make one polypeptide. As you will recall, polypeptides are composed of amino acids covalently bonded together in a specific sequence. The message written into the mRNA molecule is the message that determines the order of the amino acids. Researchers found experimentally that the genetic code is written in a language of three bases. In other words, a set of three bases contains enough information to code for one of the 20 amino acids. Any set of three bases that determines the identity of one amino acid is called a triplet. When a triplet is found in an mRNA molecule, it is called a codon or codon triplet.
- Translation results in the production of a polypeptide
There are three different kinds of RNA molecule. They are all single-stranded and each is transcribed from a gene (a section of DNA).
- The translation process
The mRNA will locate a ribosome and align with it, so that the first two codon triplets are within the boundaries of the ribosome.
A specific tRNA molecule now floats in: its tRNA anticodon must be complementary to the first codon triplet of the mRNA molecule. Thus the first amino acid is brought into the translation process. It is not just any amino acid: its identity was originally determined by the strand of DNA that transcribed the mRNA being translated. While the first tRNA ‘sits’ in the ribosome holding the first amino acid, a second tRNA floats in and brings a second (again specific) amino acid. The second tRNA matches its three anticodon bases with the second codon triplet of the mRNA. Two specific amino acids are now being held side by side. An enzyme then catalyses a condensation reaction between the two amino acids, and the resulting covalent bond between them is called a peptide bond.
The next step in the translation process involves breaking the bond between the first tRNA molecule and the amino acid that it transferred in. This bond is no longer needed, as the second tRNA is currently bonded to its own amino acid, and that amino acid is covalently bonded to the first amino acid. The first tRNA floats away into the cytoplasm and invariably reloads with another amino acid of the same type. The ribosome that has only one tRNA in it now moves one codon triplet down the mRNA molecule. This, in effect, puts the second tRNA in the ribosome position that the first originally occupied, and creates room for a third tRNA to float in, bringing with it a third specific amino acid. The process now becomes repetitive: as another peptide bond forms, the ribosome moves on by another triplet, and so on. The process continues until the ribosome gets to the last codon triplet. The final codon triplet will be a triplet that does not act as a code for an amino acid, instead it signals ‘stop’ to the process of translation. The entire polypeptide breaks away from the final tRNA molecule, and becomes a free-floating polypeptide in the cytoplasm of the cell.
DNA → (transcription) → mRNA triplet (codon)
The mRNA molecule produced by transcription represents a complementary copy of one gene of DNA. The sequence of mRNA nucleotides is the transcribed version of the original DNA sequence. This sequence of nucleotides making up the length of the mRNA is typically enough information to make one polypeptide. As you will recall, polypeptides are composed of amino acids covalently bonded together in a specific sequence. The message written into the mRNA molecule is the message that determines the order of the amino acids. Researchers found experimentally that the genetic code is written in a language of three bases. In other words, a set of three bases contains enough information to code for one of the 20 amino acids. Any set of three bases that determines the identity of one amino acid is called a triplet. When a triplet is found in an mRNA molecule, it is called a codon or codon triplet.
- Translation results in the production of a polypeptide
There are three different kinds of RNA molecule. They are all single-stranded and each is transcribed from a gene (a section of DNA).
- mRNA, messenger RNA, as described above, each mRNA is a complementary copy of a DNA gene and has enough genetic information to code for a single polypeptide
- rRNA, ribosomal RNA, each ribosome is composed of rRNA and ribosomal protein
- tRNA, transfer RNA, each type of tRNA transfers one of the 20 amino acids to the ribosome for polypeptide formation.
- The translation process
The mRNA will locate a ribosome and align with it, so that the first two codon triplets are within the boundaries of the ribosome.
A specific tRNA molecule now floats in: its tRNA anticodon must be complementary to the first codon triplet of the mRNA molecule. Thus the first amino acid is brought into the translation process. It is not just any amino acid: its identity was originally determined by the strand of DNA that transcribed the mRNA being translated. While the first tRNA ‘sits’ in the ribosome holding the first amino acid, a second tRNA floats in and brings a second (again specific) amino acid. The second tRNA matches its three anticodon bases with the second codon triplet of the mRNA. Two specific amino acids are now being held side by side. An enzyme then catalyses a condensation reaction between the two amino acids, and the resulting covalent bond between them is called a peptide bond.
The next step in the translation process involves breaking the bond between the first tRNA molecule and the amino acid that it transferred in. This bond is no longer needed, as the second tRNA is currently bonded to its own amino acid, and that amino acid is covalently bonded to the first amino acid. The first tRNA floats away into the cytoplasm and invariably reloads with another amino acid of the same type. The ribosome that has only one tRNA in it now moves one codon triplet down the mRNA molecule. This, in effect, puts the second tRNA in the ribosome position that the first originally occupied, and creates room for a third tRNA to float in, bringing with it a third specific amino acid. The process now becomes repetitive: as another peptide bond forms, the ribosome moves on by another triplet, and so on. The process continues until the ribosome gets to the last codon triplet. The final codon triplet will be a triplet that does not act as a code for an amino acid, instead it signals ‘stop’ to the process of translation. The entire polypeptide breaks away from the final tRNA molecule, and becomes a free-floating polypeptide in the cytoplasm of the cell.