Mission 1: Replication
Mission Objectives. You should be able to...
1. Explain how DNA replicates itself.
2. Describe the function of helicase.
3. Describe the function of DNA polymerase.
Mission Objectives. You should be able to...
1. Explain how DNA replicates itself.
2. Describe the function of helicase.
3. Describe the function of DNA polymerase.
Replication
There are four nitrogen bases in DNA (adenine, thymine, guanine and cytosine). Adenine forms a double hydrogen bond with thymine and guanine forms a triple hydrogen bond with cytosine. These hydrogen bonds are very weak, which allows for the molecule to be split in replication.
Adenine and guanine are known as purines (double ring structures) and guanine and thymine are known as pyrimidines (single ring structures). Single-rings always pair with double-rings which form complementary base pairs based on the distance between the sugar-phosphate backbones. As a result, this dual-stranded molecule forms a double helix when twisted. |
Source: study.com
Recall that DNA replication occurs during the S sub-phase of interphase. Hanging out in the nucleus during this time are two kinds of molecules to facilitate this process: the enzymes helicase and DNA polymerase, and free nucleotides. Helicase is responsible for separating the double helix into two single strands. Remember that the helix is held together by extremely weak hydrogen bonds. Helicase begins at some random point within the molecule or at the end of it and splits one complementary base pair (A-T, C-G) at a time. The unpaired nucleotides on each single strand is now a template to create two new strands. The free nucleotides hanging out in the nucleus can now bond to the single strands, but they only bond in complementary base pairs: A to a T, G to a C. This process is catalyzed by DNA polymerase. See the image to the right. New DNA strands are rebuilt only from the 5' end, so the two new strands are not constructed at the same rate. There is a leading strand and a lagging strand; the lagging strand forms in the opposite direction of the leading strand, from 5' to 3'. Obviously, it takes a little longer for the lagging strand to complete. This process ensures that an exact copy of DNA is produced. |
Source: nitro.biosci.arizona.edu
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Mission 2: Transcription & Translation
Mission Objectives. You should be able to...
1. Explain the process of transcription and its purpose.
2. Identify which strand of DNA is transcribed.
3. Explain the process of translation and its purpose.
4. Explain the role of mRNA and tRNA in the processes of transcription and translation.
Mission Objectives. You should be able to...
1. Explain the process of transcription and its purpose.
2. Identify which strand of DNA is transcribed.
3. Explain the process of translation and its purpose.
4. Explain the role of mRNA and tRNA in the processes of transcription and translation.
Transcription & Translation
Protein Synthesis. This is the process that determines the control DNA has over a cell. DNA controls proteins produced in a cell; some being enzymes. The production of a particular enzyme can have an effect on the biochemistry of a cell. Therefore it can be inferred that DNA controls the biochemistry of macromolecules with enzyme production.
The sections of DNA that code for polypeptides are called genes. Any gene is a specific sequence of nitrogenous bases found in a specific location on a DNA molecule. DNA is found in the nucleus, but proteins are synthesized in the cytoplasm. So a molecule called mRNA is used to carry the DNA message from the nucleus to the cytoplasm where the enzymes, ribosomes, and amino acids are found. "m" stands for messenger.
The sections of DNA that code for polypeptides are called genes. Any gene is a specific sequence of nitrogenous bases found in a specific location on a DNA molecule. DNA is found in the nucleus, but proteins are synthesized in the cytoplasm. So a molecule called mRNA is used to carry the DNA message from the nucleus to the cytoplasm where the enzymes, ribosomes, and amino acids are found. "m" stands for messenger.
The process of transcription goes as follows: A section of DNA (a gene) unzips into two single strands. RNA, a single-stranded molecule, will link with one of the strands to form the mRNA molecule with RNA polymerase used to catalyze the reaction. Free RNA nucleotides float into place by complementary base pairing. Uracil, a nitrogenous base specific only to RNA, pairs with adenine.
Only one of the DNA strands is copied. mRNA is a short strand that is a complementary copy of a single gene. The image above shows the transcription process. The center oval is the gene that's been split by RNA polymerase. mRNA (in red) begins transcribing the separated strand using free nucleotides in the cell. Only one strand of DNA is copied.
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.
Image courtesy of ck12.org
Only one of the DNA strands is copied. mRNA is a short strand that is a complementary copy of a single gene. The image above shows the transcription process. The center oval is the gene that's been split by RNA polymerase. mRNA (in red) begins transcribing the separated strand using free nucleotides in the cell. Only one strand of DNA is copied.
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.
Image courtesy of ck12.org
There are three kinds of RNA, and they are all single-stranded molecules that are transcribed from genes.
i. mRNA (messenger RNA) is a complementary copy of a DNA gene and has enough genetic information to code for a single polypeptide. ii. rRNA (ribosomal RNA) each ribosome is composed of rRNA and ribosomal protein. iii. tRNA (transfer RNA) each type of tRNA transfers one of the 20 amino acids to the ribosome for polypeptide formation. Image below courtesy of hyperphysics.phys-astr.gsu Image to the right courtesy of www.wiley.com |
The process of translation is like a train that picks up codon triplets and drops off amino acids to form polypeptides. If you examine the above image, the messenger RNA has come from the nucleus with the complementary copy of the DNA molecule. It passes through the ribosome where it meets up with a tRNA molecule that has a complementary anticodon to the first codon triplet of mRNA. Then a second tRNA "train car" enters the ribosome with its own specific anticodon to bond with the codon on mRNA, and so on.
In the image, start from the 5' end of the mRNA molecule and move right. The tRNA chunks move right to left. The GAG codon of mRNA bonds with the CUC codon on the tRNA to form the amino acid glutamine (GLU). The CAC anticodon on tRNA bonds with the GUG codon on mRNA to form the amino acid valine and then the AAA codon on mRNA bonds with the UUU anticodon on tRNA to form the amino acid lysine. When the tRNA chunks are "used up," they leave the ribosome to return to the cytoplasm and collect more nitrogen bases. The ribosome moves the finished codon triplet down the mRNA molecule to make room for the next one. This process repeats.
Once there are at least two amino acids formed, an enzyme then catalyzes a condensation reaction between the aminos to create a peptide bond. The final codon is not an amino acid, but a signal to stop the translation process.
There are 20 naturally occurring amino acids. Each one has a specific genetic code.
PCR, or polymerase chain reaction, is a process that is used frequently in forensic science. It is a method by which short segments of DNA can be replicated in the lab to produce enough of the molecule for testing. An enzyme used in PCR called Taq polymerase, found in hot springs bacteria, is very stable at high temperatures and as a result, does not denature in the PCR process.
In the image, start from the 5' end of the mRNA molecule and move right. The tRNA chunks move right to left. The GAG codon of mRNA bonds with the CUC codon on the tRNA to form the amino acid glutamine (GLU). The CAC anticodon on tRNA bonds with the GUG codon on mRNA to form the amino acid valine and then the AAA codon on mRNA bonds with the UUU anticodon on tRNA to form the amino acid lysine. When the tRNA chunks are "used up," they leave the ribosome to return to the cytoplasm and collect more nitrogen bases. The ribosome moves the finished codon triplet down the mRNA molecule to make room for the next one. This process repeats.
Once there are at least two amino acids formed, an enzyme then catalyzes a condensation reaction between the aminos to create a peptide bond. The final codon is not an amino acid, but a signal to stop the translation process.
There are 20 naturally occurring amino acids. Each one has a specific genetic code.
PCR, or polymerase chain reaction, is a process that is used frequently in forensic science. It is a method by which short segments of DNA can be replicated in the lab to produce enough of the molecule for testing. An enzyme used in PCR called Taq polymerase, found in hot springs bacteria, is very stable at high temperatures and as a result, does not denature in the PCR process.
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.