1. DNA replication
2. Transcription & Translation
Mission Objectives: You should be able to...
1. Explain what it means for DNA replication to be semi-conservative.
2. Describe and predict complementary base pairing.
3. Explain the purpose of helicase and DNA polymerase in the replication process.
4. Describe the process of transcription.
5. Describe the process of translation.
6. Explain the role of mRNA and tRNA in the production of amino acid sequences.
Links: DNA Replication
Textbook Readings: Chapter 2, section 2.7
Mission 1: Splitting the Atom, erm...Molecule. DNA's double helix structure is held together by weak hydrogen bonds (triple between C and G, double between A and T). The weakness of the bonds make it super-easy for the molecule to split and replicate, which of course, it HAS to do. Helicase is the enzyme responsible for splitting the molecule. It unzips the DNA molecule much like a zipper. DNA polymerase (which is a cluster of polymerases) is responsible for the construction of the two new strands. Recall that DNA forms from 5' to 3', and as a result, there is a leading strand and a lagging strand. The end result is two identical copies of the original strand. See the image below.
Mr. Rott breaks it down. This video addresses Objectives 1-3, so make sure you can follow the command terms and do what they say. For those of you who prefer Mr. Andersen, his video follows Rott's video.
SL may watch until the 11-minute mark, but HL...you must watch the entire video because the last four or so minutes talks about the history of DNA research, which you are required to know.
Mission 2: Turning Letters Into Meaning. Transcription and translation are processes by which proteins are formed. DNA transcribes a message to mRNA, which then exits the nucleus and goes to the ribosomes to form amino acids which eventually form proteins. It will help you to recognize (not memorize) the 20 natural amino acids.
Things to note about transcription: (a) One DNA strand is copied, (b) mRNA is always shorter than the DNA it is copied from because it is a complementary copy of one gene, (c) the presence of thymine identifies it as DNA, and (d) the presence of uracil identifies it as RNA.
The genetic code is written in triplets. The mRNA molecule is a complementary copy of one gene of DNA. The sequence of mRNA nucleotides is the transcribed version of the original DNA sequence. The sequence of nucleotides making up the mRNA is enough to make one polypeptide. The message written on mRNA is the message that determines the order of the amino acids. It was since discovered that the genetic code is written in a "language" of three bases that contain enough information to code for one of the 20 amino acids. These are called triplets. When a triplet is found in mRNA, it is called a codon or a codon triplet.
Once mRNA has its transcribed sequence, it leaves the nucleus and heads to the cytoplasm. Once there, it hooks up with a ribosome (or two, or three) so that translation may take place. The codons on the mRNA will sequence up with anticodons on tRNA molecules that are linked to specific amino acids. The anticodon is at the bottom of the molecule and the amino acid is at the top.
If you examine the image to the right (www.wiley.com), the mRNA codon is GCC. The corresponding anticodon on tRNA is CGG. CGG is the code for the amino acid arginine.
Let's practice! This is a really cool simulation that shows you how transcription and translation works. You will create a protein from a strand of mRNA. This is another simulation that shows the process in a slightly different way.
Practice problem: Imagine that an mRNA leaves the nucleus of a eukaryote with the following base sequence: AUGCCCCGCACGUUUCCAAGCCCCGGG. Find a mRNA codon chart and answer the following:
1. Determine in sequence the amino acids that are coded for by the mRNA molecule.
2. Determine the DNA code sequence that produced the mRNA codons.
3. What would the amino acid sequence be if the first cytosine of the mRNA molecule was replaced by uracil? **This would be the result of a change occurring in the DNA molecule that transcribed the mRNA**
Links: Amino acid codons mRNA/DNA codons
Homework: This is an online quiz. You cannot take it until you have completed Missions 1 & 2. There are some questions being asked that only HL will need to know, but for the other 20 or so questions, the material is covered by Missions 1 & 2. Email your results to me on the date I give you.
Mission 3: This Don't Make No Sense! A question you may be wondering about is WHICH STRAND OF DNA IS COPIED? Recall that the two strands are complementary. This means that there is a difference in the code of the strands. One may have the base sequence ATATCGGA. Consequently, the other strand must have the complementary base sequence TATAGCCT. These sequences lead to different codons on mRNA. Because codons are specific to amino acids, complementary strands mean different codons, different aminos, and therefore different proteins. Therefore, it is important that the mRNA hooks up with the "correct" strand. See below.
The promoter determines which strand is the antisense strand, but it depends on the gene or sequence of genes. Examine the below image and determine which strand is the sense strand and which is the antisense strand. The image is from your textbook.
Homework: Corresponding lessons in IB workbook.