Bio Test 2.6, 2.7, 7 Flashcards
Similarities and differences of DNA and RNA
- Both the molecules of DNA and RNA are formed of monomers known as nucleotides.
- Both these molecules possess four nitrogenous bases.
- Both the molecules of DNA and RNA exhibit a phosphate backbone to which the attachment of bases takes place.
- DNA is a double-stranded molecule that has a long chain of nucleotides. RNA is a single-stranded molecule that has a shorter chain of nucleotides.
- DNA replicates on its own, it is self-replicating. RNA does not replicate on its own.
- Both DNA and RNA have four nitrogenous bases each—three of which they share (Cytosine, Adenine, and Guanine) and one that differs between the two (RNA has Uracil while DNA has Thymine).
What is the role/process of transcription?
Transcription: In the nucleus, the cell’s machinery copies the gene sequence into messenger RNA (mRNA), a molecule that is similar to DNA. Like DNA, mRNA has 4 nucleotide bases - but in mRNA, the base uracil (U) replaces thymine (T).
- RNA polymerase binds to the DNA double helix at the start of the gene
- RNA polymerase unwinds the double helix
- RNA polymerase breaks down hydrogen bonds, separating the two strands of the DNA molecule
- One strand acts as a template (the antisense strand)
- Free RNA nucleotides pair up with their complementary bases in the template strand
- Uracil replaces thymine in RNA and pairs up with adenine, cytosine with guanine
- RNA polymerase forms the mRNA molecule by joining RNA nucleotides with covalent bonds between the sugar and phosphate
- As RNA polymerase moves down the gene, the growing mRNA molecule separates from the template strand and the DNA strands re-join and then twist into a helix
- RNA polymerase reaches the end of the gene and releases the mRNA strand
What is the role/process of translation?
Translation: The protein-making machinery, called the ribosome, reads the mRNA sequence and translates it into the amino acid sequence of the protein. The ribosome starts at the sequence AUG, then reads three-nucleotides at a time. Each three-nucleotide codon specifies a particular amino acid. The “stop” codons (UAA, UAG, and UGA) tell the ribosomes that the protein is complete.
- There are many different tRNA molecules in the cytoplasm carrying different AA
- Ribosome attaches to mRNA (at the start codon)
- tRNA molecules can be positioned over the mRNA in one of three sites in the ribosome
- A tRNA with a complementary anticodon binds to the start codon on the mRNA
- A peptide bond forms between the two AA
- Ribosome moves along one codon
- The first tRNA (now without an attached AA) floats away
- A third tRNA (with an anticodon complementary to the third codon) now enters the ribosome and the AA chain grows
- Ribosome continues until stop codon
- Ribosome, mRNA, polypeptide chain are released
tRNA binding sites on ribosomes
The 3 binding sites for tRNA are called the aminoacyl site (abbreviated A), the peptidyl site (abbreviated P), and the exit site (abbreviated E), which are oriented 5’ to 3’ E-P-A with respect to the mRNA.
- The A site binds to the incoming aminoacyl tRNA, which carries the new amino acid to be added to the polypeptide chain.
- The P site holds the tRNA with the growing polypeptide chain.
- The E site serves as a threshold. It holds the tRNA without its amino acid, which is then released by the ribosome.
DNA Replication (including all enzymes involved)
- Helicase unwinds the double helix
- Helicase breaks hydrogen bonds, separating the two strands of DNA molecule
- Each strand acts as template
- Free DNA nucleotides pair up with their complementary base in each template
- Adenine with thymine and guanine with cytosine
- DNA polymerase forms each new strand by joining nucleotides together with covalent bonds between sugar and phosphate
- Two identical DNA molecules are formed
Explain the process and what is involved in gene expression
Gene expression consists of 2 major steps: transcription and translation.
Free vs bound ribosomes
Free ribosomes are located in the cytoplasm, and the bound ribosomes are attached to the endoplasmic reticulum. Free ribosomes produce proteins for the cell, while bound ribosomes produce proteins that are transported out of the cell.
What are nucleosomes/how do they work?
- Eukaryotic DNA supercoiling is organized by nucleosomes
- Nucleosomes both protect DNA and allow it to be packaged, this in turn allows DNA to be supercoiled
- Nucleosomes are formed by wrapping DNA around histone proteins
Sense vs antisense strands (direction of transcription of mRNA)
A sense strand, or coding strand, is the DNA strand within double-stranded DNA that carries the translatable code in the 5’ to 3’ direction. Its complementary strand is called antisense strand, which does not carry the translatable code and serves as a template during transcription.
Overview of Rosalind Franklin contribution
Her photo 51 revealed the double helix of DNA — a discovery that was essential in unlocking the mystery of how life is passed down from generation to generation.
Overview of Hershey and Chase’s findings
Scientists were unsure as to whether proteins or chromosomes were the genetic material of cells.
Alfred Hershey and Martha Chase wanted to solve this problem by finding out if protein or DNA was the genetic material of viruses.
Viruses infect cells and transform them into virus-producing factories:
- Versus inject their genetic material into cells
- The non-genetic part of the virus remains outside the cell
- Infected cells produce large numbers of the virus
- The cell bursts releasing the copies virus
Hershey and Chase chose to study T2 bacteriophage, which infects the E. coli bacterium, because of its very simple structure consisting of just:
- Protein coat (capsid)
- DNA inside the coat
Amino acids containing Radioactive isotopes were used to label the virus:
- Sulfur (35S) for the Protein coat (capsid)
- Phsophorus (32P) for the DNA
The experiment combined T2 bacteriophage with E. coli Bacteria. At the end of the experiment of a centrifuge was used to separate them:
- The smaller virus remained in the supernatant (liquid)
- The bacteria formed a pellet
Separate experiment with the two isotopes found that:
- Sulfur (35S) remained in supernatant
- Phsophorus (32P) was found in the pellet
Hershey and Chase deduced that DNA therefore was the genetic material used by viruses because DNA (labelled by 32P) was being transferred into the bacteria.
