DNA and RNA and protein synthesis Flashcards
(12 cards)
What is a nucleotide and what is it comprised of
-Nucleotides are the basic units of nucleic acids (DNA and RNA)
STRUCTURE
-Nucleotides are made up of 3 parts:
-Phosphate group
-Pentose sugar; which is deoxyribose in DNA and ribose in RNA
-and a nitrogenous base which can either be a purine; Adenine (A) and Guanine (G), or a pyrimidine; Cytosine (C), Thymine (T) (only in DNA) and Uracil (only in RNA)
Structure of DNA
-DNA is a double stranded polymer of nucleotides that forms a double helix structure
-The two strands are held together by hydrogen bonds between complementary bases:
-Adenine and Thymine have 2 hydrogen bonds between them
-Cytosine and Guanine have 3 hydrogen bonds between them
-DNA contains deoxyribose sugar
-The sugar-phosphate backbone is held together by phosphodiester bonds
-The two strands run anti parallell to one another (from 5 to 3 or from 3 to 5)
-DNA contains genetic material
Structure of RNA
-RNA is single stranded (does not form double helix)
-RNA contains ribose sugar
-RNA contains Uracil instead of thymine (Uracil pairs with adenine in RNA)
-RNA is usually shorter than DNA
Basic comparison points between DNA and RNA
-RNA is single-stranded, whereas DNA is double-stranded
-RNA contains ribose sugar, DNA contains deoxyribose sugar
-RNA contains Uracil instead of Thymine in DNA
-RNA is usually shorter than DNA
DNA as a double helix
-Complementary base pairing
-Forms hydrogen bonds
-Easily separated e.g for replication but keeps the DNA molecule stable
-DNA strands run antiparallel to each other
-Strands twist into a double helix, with the sugar-phosphate backbone on the outside and the nitrogenous bases on the inside
Protein synthesis overview
Transcription: DNA to mRNA (in the nucleus)
Translation: mRNA to protein (at the ribosome)
Process of transcription
-Transcription takes place in the nucleus
1.RNA polymerase binds to the start of the gene on the DNA
2. This causes DNA to unwind and unzip as it breaks the hydrogen bonds between the base pairs
3. The template (antisense) strand is used to form mRNA
4. Free nucleotides pair with the expose DNA bases on the template strand through complementary base pairing
5. Hydrogen bonds form between RNA bases and DNA bases
6. RNA polymerase joins RNA nucleotides using phosphodiester bonds
7. When the stop codon is reached, the mRNA strand detaches
8. DNA reforms hydrogen bonds and rewinds into a double helix shape
9. The mRNA strand leaves the nucleus via a nuclear pore and goes to the ribosome
Process of translation
-Translation takes place in the ribosome
1.The mRNA strand leaves the nucleus and attaches to a ribosome in the cytoplasm
2. The ribosome then reads the mRNA codons ( groups of 3 bases) one at a time
3. A tRNA molecule with complementary anti codons then binds to the first codon on the mRNA strand
4. Each tRNA molecule carries an amino acid that is specific to the codon
5. Once the first tRNA molecule leaves the ribosome, the second tRNA molecule attaches to the second codon on the mRNA strand, and the ribosome forms a peptide bond between the two amino acids
6. This process repeats, and a polypeptide chain forms
7. When the ribosome reaches a stop codon, tRNA can no longer bind, and translation stops
8. The polypeptide chain is released and the ribosome detaches from the mRNA
9. The polypeptide chain then folds into a specific 3D shape to from a functional protein
Nature of genetic code
TRIPLET CODE
-Each amino acid is coded for by a sequence of 3 bases on the mRNA strand (codons)
NON-OVERLAPPING
-The code is non overlapping
-Each triplet of bases is read once, and the next triplet starts when the previous one ends
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Degenerate
-Genetic code is degenerate, meaning that many amino acids can be coded for by more than one codon
-For example GCU and GCC both code for alanine
-This helps to protect against mutations as if one base is changed in the codon, then it may still produce the same amino acid
Semi conservative replication process
- DNA Helicase unwinds and unzips the DNA molecule by breaking hydrogen bonds between complementary base pairs
- This causes the polynucleotide strands to separate from each other
- Free DNA nucleotides in the nucleus then line up with their complementary base pair
- The nucleotides form complementary base pairs with both original strands, the nucleotides are held in place by hydrogen bonds
- DNA polymerase then catalyses the formation of phosphodiester bonds between complementary nucleotides, forming a sugar-phosphate backbone
- Two identical molecules of DNA are then formed; each one has one original strand and one newly synthesised strand (semi conservative replication)
Meselson and Stahl Experiment
Meselson and stahl used different isotopes of nitrogen; 15N (heavier nitrogen) and 14N (lighter nitrogen) to investigate semi conservative replication
PROCESS
1.The researchers grew E.Coli bacteria in a medium containing 15N for several generations
2. The 15N was incorporated into the DNA, making it more dense
3. The researchers then transferred the bacteria into a medium containing only 14N, and DNA replication continued
4. The DNA was then extracted after either one or two rounds of replication
5. The test tubes containing DNA were then spun in a centrifuge, which then separated the DNA into bands based on density
RESULTS
-The DNA from the bacteria grown in just the 15N medium formed a heavy band (lower down in the test tube), this suggests that the DNA was 100% 15N
-However, the DNA after one round of replication in the 14N medium, formed an intermediate band, this suggests that the DNA has one old strand (15N) and one new strand (14N), which provides support for semi conservative replication
-The seconds round of replication produced 1 light band and one intermediate band, supporting continued semi conservative replication
