II. Connections
1. 5' & Electronegativity: The 5' carbon on a nucleotide is the highest carbon on the sugar component. This carbon attaches to a phosphate group on the end. A free nucleotide has a tail of three phosphate groups dangling from the 5' carbon. A DNA polymerase detaches two of the phosphates. The last phosphate is driven towards the -OH group dangling off from the 3' carbon, the carbon on the bottom of the sugar component, of another nucleotide. Since the electronegativities of the phosphate group and the -OH are different, their energies connect and form a bond, linking two nucleotides together.
2. Start Codon & Incomplete Dominance: A codon is a base triplet of three nucleotides in mRNA. There are 64 different codons, making up the genetic code for an organism. One of these codons is called a start codon, that specifically codes as a start signal for translating three bases at a time. The typical start codon is AUG, but one change in the nucleotide can alter the purpose of the codon. This variation of codons cause genetic mutations in the codes for proteins. Some of the mutations cause genetic variations of traits, including incomplete dominance, when a heterozygous gene codes for a new trait than a homozygous gene.
3. Semiconservative & Barr Body: Semiconservative replication is a mechanism by which a DNA molecule is duplicated. For this to occur, the double helix unzips along its length, exposing the bases. The bases are used as a template, and a new strand is assembled. The two stands are wound back up into a double helix. Semiconservative replication can only occur if the chromosome decondenses into its chromatin form, so that the strands of DMA can be read and copied. This form of replication cannot occur then in a Barr Body, a chromosome that does not decondense into its chromatin form, but stays condensed.
4. RNA Polymerase & Nucleolus: RNA polymerase is an enzyme that adds ribonucleotides one at a time to the end of a growing strand of RNA. The enzyme is primarily used in transcription of RNA, resulting in mRNA. To ensure the mRNA has no mistakes on it, the transcripts are modified of their mistakes before leaving the nucleolus, where the transcription is occuring.
5. DNA Polymerase & Glycosidic Linkage: DNA polymerase is an enzyme that catalyze the formation of two brand-new strands of DNA from free nucleotides. These enzymes also catalyze the hydrogen bonding of each new strand to the unwound region of one of the two parent DNA strands. The nitrogen bases connect to each other based on pairing, A with T and C with G, by a glycosidic linkage.
6. Helicase & G2 Karyotype: DNA replication occurs during interphase in a cell's life cycle. Interphase starts with the G1 stage, when the DNA is unwinding and preparing to be replicated. S phase is the phase of replication. G2 stage is the when the replicated DNA is checked for mistakes in the replication. To start this replication process, the DNA needs to be unwound into its chromatin form. An enzyme called helicase unzips the DNA and breaks the hydrogen bonds between the nucleotides. Helicases are primarily found in the S phase of a cell's life because that is when the replication occurs. In G2, the DNA is already replicated, which is why in the karyotype, there will be twice as much DNA as before.
III. Few Essentials
1. For DNA replication, the DNA polymerases can only assemble new strands in a 5'-3' direction. DNA in a double helix are formed by two strands of nucleotides that are anti-parallel, meaning they line up in different directions. One strand in a 5'-3' direction is opposite of a strand in a 3'-5' direction. Since DNA polymerases can only assemble in one direction, the nucleotides are assembled in short stretches. DNA ligases are in charge of sealing the short stretches together into a continuous strand, so that it is complementary with the 3'-5' direction strand.
2. 3 structural differences between RNA and DNA:
- RNA has 3 classes of molecules for protein synthesis, while DNA only has one. mRNA is the outcome of of transcription, where the protein-building codes are carried. rRNA becomes a component of ribosomes. tRNA delivers amino acids one by one to ribosomes in the orders specified by mRNA.
- RNA molecules have four kinda of ribonucleotides, like DNA. Adenine, guanine, and cytosine are the same. But, instead of thymine, RNA has uracil, a new nitrogen base. Uracil can bind with adenine.
- Transcription results in one free RNA strand, not a hydrogen-bonded double helix like DNA.
3. RNA has 3 classes of molecules for protein synthesis, while DNA only has one. mRNA is the outcome of of transcription, where the protein-building codes are carried. rRNA becomes a component of ribosomes. tRNA delivers amino acids one by one to ribosomes in the orders specified by mRNA.
4. Protein Synthesis
a. Transcription
- 1st step in protein synthesis
- sequence of nucleotide bases is exposed in an unwound region of DNA strand
- sequence: template strand RNA is assembled from
- RNA uses adenine, cytosine, guanine, and uracil
- RNA polymerase adds nucleotides one at a time to end of growing strand of RNA
- promoter: start signal that RNA polymerase binds to so it start adding nucleotides
- when RNA polymerase reaches sequence that marks "the end": it stops
b. RNA Splicing
- occurs in the nucleus before being released
- pre-mRNA is modified with a cap at the beginning and a poly-A tail at the end
- all introns, noncoding sequences, are snipped from pre-mRNA strand
- exons, protein-coding base sequences, are all kept
- can be combined together to form a mature mRNA
c. Translation
- initiation stage: small ribosomal subunit and an initiator tRNA arrive at an mRNA transcript's start codon
- large ribosomal subunit binds to them
- anticodon and codon meet up here
- elongation stage: tRNAs deliver amino acids to a ribosome in the order said by the sequence of mRNA codons
- polypeptide chain lengthens as peptide bonds form between the amino acids
- termination stage: stop codon triggers events that cause polypeptide chain and the mRNA to detach from ribosome
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