IB Biology - Curriculum Notes
2.7 DNA Replication, Transcription and Translation
Genetic information in DNA can be accurately copied and can be translated to make the proteins needed by the cell.
∑ - The replication of DNA is semi-conservative and depends on complementary base pairing.
Complementary base-pairing ensures two identical DNA strands are formed after replication is complete.
In replication, the original strands are used as templates, allowing complementary bases to be added according to base-pairing rules.
DNA replication is semi-conservative, meaning the new DNA that is created consists of one old strand (template) and one new strand (synthesized strand).
The significance of complementary base pairing means that the two daughter cells have the exact same DNA genome as the parent cell.
Gene sequences (if no mutations occur) are therefore successfully passed on from generation to generation.
Adenine is always matched with thymine with two hydrogen bonds and guanine is always matched with cytosine with three hydrogen bonds.
B - Skill: Analysis of Meselson and Stahl’s results to obtain support for the theory of semi-conservative replication of DNA.
Read through the article for Obtaining evidence for the theory of semi-conservative replication and complete the data-based question on the Analysis of Meselson and Stahl’s results on page 113- and 114
∑ - Helicase unwinds the double helix and separates the two strands by breaking hydrogen bonds.
The DNA strand is unwound and separated by an enzyme called helicase.
The separation is completed by breaking the hydrogen bonds between the base pairs
Energy from ATP is required for Helicase to move along the DNA and break the bonds
∑ - DNA polymerase links nucleotides together to form a new strand, using the pre-existing strand as a template.
Free nucleotides found in the nucleus are added to the strands of DNA by an enzyme called DNA polymerase.
DNA polymerase brings the nucleotide into position so a hydrogen bond can form between the base pairs
A covalent bond is formed between the phosphate on the free nucleotide and the sugar on the existing chain
Nucleotides are added to complementary bases on the DNA template strands according to base-pairing rules (adenine pairs with thymine and guanine pairs with cytosine).
Bases are added in one direction on one strand and are added in the opposite direction on the other strand.
Very few mistakes occur
The newly formed DNA strands rewind to form a double-helix spiral staircase shape once again.
β - Applications and skills:
β -Application: Use of Taq DNA polymerase to produce multiple copies of DNA rapidly by the polymerase chain reaction (PCR).
Go to http://www.dnalc.org/resources/animations/pcr.html
Click on Amplification to start the animation
Follow the animation and write down the steps involved in PCR
PCR (polymerase chain reaction) is a laboratory technique that takes a single or few copies of DNA and amplifies them to generate millions or more copies of a particular DNA sequence.
When you collect DNA from different sources such as sperm samples or small drops of blood, there are usually very little usable cells to collect DNA.
Therefore, PCR is used to create enough DNA to be analyzed for investigations such as forensics or custody cases.
Once large quantities of the DNA have been created, other methods such as gel electrophoresis are used to analyze the DNA.
∑ - Transcription is the synthesis of mRNA copied from the DNA base sequences by RNA polymerase.
Genes contained within a specific region of DNA code for a specific protein.
Protein synthesis occurs outside the nucleus on the ribosomes. Therefore a molecule needs to be synthesized that will relay the DNA code to allow the specific protein to be made correctly.
Transcription is the formation of an mRNA strand that is complementary to the DNA strand contained within the gene.
Transcription begins when the area of DNA that contains the gene is unwound by RNA polymerase
RNA nucleotides found in the nucleus are added to the template strand of the DNA by the enzyme RNA polymerase according to base-pairing rules.
RNA polymerase also creates covalent bonds between the nucleotides of the mRNA strand.
The mRNA strand contains the nitrogenous base uracil instead of thymine.
Once the gene has been transcribed, the mRNA strand falls off and exits the nucleus through the nuclear pore. It is then transported to the ribosomes for protein synthesis.
The DNA strand with the same base sequence as the mRNA is the called the sense (coding) strand and the other is called the antisense (template) strand
Good Video on transcription and translation
∑- Translation is the synthesis of polypeptides on ribosomes.
The translation is the synthesis of polypeptides with a specific amino acid sequence that is determined by the base sequence on the mRNA molecule
That base sequence is determined by the specific gene
Translation takes place at the ribosomes in the cytoplasm or on the rough ER
The ribosomes consist of a large and a small subunit
Ribosomes are made of rRNA and protein
∑ - The amino acid sequence of polypeptides is determined by mRNA according to the genetic code.
Messenger RNA (mRNA) carries the information from the specific gene to the ribosomes in order to create the correct polypeptide
The mRNA that is created is specific for that polypeptide only
∑ - Codons of three bases on mRNA correspond to one amino acid in a polypeptide.
The mRNA strand created in transcription consists of triplet bases called codons.
Each triplet codon on the mRNA codes for a specific amino acid.
The protein synthesized is made up of a series of amino acids coded for by the mRNA strand which is a complimentary copy of the gene contained in the DNA.
All the different combinations of bases that make up the 64 triplet codons can code for one of the 20 amino acids.
However, 3 triplets, UAA, UAG, and UGA do not code for amino acids and are called stop codons.
The mRNA codon AUG, codes for the amino acid Methionine and is called the START codon because it signals the start of translation.
The genetic code is considered “degenerate” because more than one triplet codon can code for a specific amino acid.
B Skill: Use a table of the genetic code to deduce which codon(s) corresponds to which amino acid.
B 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.
B Skill: Deducing the DNA base sequence for the mRNA strand.
Use the DNA strand below to transcribe a strand of mRNA and then identify the correct amino acids in the polypeptide strand.
Template Strand GCC TAC TCG CCT TTT AAA GCT AGT ACT GGG CGC
Coding Strand CGG ATG AGC GGA AAA TTT CGA TCA TGA CCC GCG
mRNA Strand _____________________________________________________
Amino Acids ________________________________________________________
Also, complete the Decoding base sequences in your text.
∑ - Translation depends on complementary base pairing between codons on mRNA and anticodons on tRNA.
The process where amino acids are combined to form proteins (polypeptides).
mRNA has a sequence of codons (3 base pairs) that specifies the AA sequence of the polypeptide
tRNA has an anticodon that matches and binds to their complementary codon carrying the AA corresponding to that codon
rRNA binding site for the mRNA and tRNA and catalyzes the reaction to put together the polypeptide
After transcription occurs the transcribed mRNA moves out from the nucleus through the nuclear pore into the cytoplasm and binds to the ribosome unit either in the cytoplasm or attached to the rough ER
mRNA binds to the small subunit of the ribosome with its first two codons contained within the binding sites of the ribosome.
The first codon is called the start codon (AUG) which codes for methionine.
The corresponding tRNA attaches to the mRNA bringing the amino acid methionine to the ribosome to start the polypeptide chain.
While still attached, a second tRNA attaches to the mRNA at the second binding site on the ribosome, carrying the amino acid that corresponds to the mRNA codon.
The two amino acids are combined by a condensation reaction, forming a covalent dipeptide bond.
The bond between the first amino acid and the tRNA that carried it to the ribosome is broken by an enzyme.
The ribosome slides along the mRNA, moving down one codon releasing the tRNA back into the cytoplasm so it can go pick up another amino acid (in this case methionine).
Another tRNA moves into the empty site bringing the next amino acid that corresponds to the mRNA codon.
Again, the amino acid is attached to the polypeptide and the previous tRNA is released back into the cytoplasm as the ribosome moves along the mRNA.
This process continues until 1 of the 3 stop codons (UAA, UGA, and UAG) is reached. These tRNA's have no attached amino acid.
Finally, when the ribosome moves along the mRNA, the polypeptide will fall off and be released into the cytoplasm.
Watch the following animation on translation. http://highered.mheducation.com/sites/0072943696/student_view0/chapter3/animation__protein_synthesis__quiz_3_.html
β - Application: Production of human insulin in bacteria as an example of the universality of the genetic code allowing gene transfer between species.
A gene produces a certain polypeptide in an organism.
Since the genetic code is universal, when a gene is removed from one species and transferred to another, the sequence of amino acids in the polypeptide produced remains unchanged.
Animal insulin has been used for the treatment of diabetics for many years; however, some people develop an allergic reaction to the animal insulin
Since 1982, human insulin created by the pancreas has been produced using gene transfer techniques with E. coli bacteria
Gene transfer is taking one gene from an organism and inserting it into another organism.
First, mRNA that codes for insulin produced in the pancreatic cells is extracted.
The enzyme reverse transcriptase is mixed with the mRNA. This enzyme produces a strand of coding DNA called cDNA.
Plasmids are small circles of DNA found in bacteria cells. These plasmids are cut with a restriction enzyme, leaving sticky ends to which the cDNA can attach.
DNA ligase is used to seal the nicks between the cDNA and the plasmid.
Linking sequences are added to the cDNA allowing them to be inserted into the plasmid.
The bacterial plasmid carrying the insulin gene is now inserted into the plasmid-free bacterial cell such as e.coli bacteria (with plasmid removed). This is known as the host cell.
These insulin-producing bacterial cells will now reproduce rapidly during fermentation, creating millions of insulin-producing bacteria cells.
Finally, the insulin produced is extracted from the cell and purified to be used by diabetics.
Watch the video on the production of insulin and genetic engineering