4 Flashcards
James Watson and Francis Crick
Credited with discovering the structure of DNA in1953
Describe the structure of DNA
1) DNA is located in the nuclues of eukaryotic cells ( or nucleoid region of prokaryotic cells
2) DNA is a double -stranded helix that looks like a twisted ladder. It contains
- The nitrogenous bases
A ( adenine)
G ( guanine)
T ( thymine), and C ( cytosine) that make up the rungs ( steps) of the “ladder.”
( A always pairs with T; G always pairs with C) these pairs are held together by hydrogen bonds.
- A sugar phosphate backbone
Semiconservative Replication
how DNA replicates itself. “ Old” strands are used as the template for making “ new” strand of DNA.
Accuracy of DNA replication
On average there are about 140 million bas pairs per chromsome
DNA polymerase
” proofreads” the DNA strand checking for mismatched pairs. Most, but not all, mismatches are repaired. After “ proofreading”, the error rate is only about on per 10 billion base pairs.
DNA ligase
seals the nick or crack left in the sugar-phosphate backbone after the mismatched base (s) are excised and replaced with the correct bases.
One gene-one polypeptide hypothesis
One gene codes for the production of one specific polypeptide ( protein,enzyme, horomone).
Structure of RNA
- single-stranded
- found in nucleus and cytoplasm
-contains A,G,C, and U (uracil)
uracil substitutes for thymine
-contains ribose sugar
-relatively small molecule
Structure of DNA
-Double stranded
-found in nucleus
- contains A,G,C, and T
-contains deoxyribose sugar
-very large molequle
messegner RNA
copies genetic “ recipe” from DNA in the nuclues and carries the “ recipe” out into the cytoplasm
What is Transcription and where does it occur
synthesis of mRNA using DNA template ( occues in the nuclues)
Describe the process of Transcription
1) chemical message sent to the chromosome with the gene that produces a certain polypeptide
2) DNA unzips to expose the gene
3) mRNA is built as complementary nucleotides pair up with bases on the DNA strand
4) once mRNA is built, it leaves the gene and moves out into the cytoplasm; DNA zips up again
What is translation and where does it occur?
production of a polypeptide from the genetic meddagr carried by mRNA ( occurs in cytoplams)
Descripe the process of translation
1) mRNA associates with ribosomes in the cytoplasm
2) the genetic message is read 3 bases at a time ( 3 bases = one codon)
3) transfer RNA brings an amino acid to the mRNA-rRNA complex; the anticodon on tRNA pairs up with the codon on mRNA
4) a second tRNA brings an amino acid and holds it in place until a peptide bond forms between the two amino acids
5) the first tRNA leaves the complex, the rRNA (ribosome) moves down the mRNA by one codon, and the process continues until the polypeptide is completed.
Codon
A sequence of 3 nucleotide bases on mRNA that codes for a signle amino acid ( or stop signal) Of the 64 codons in the gentic code, 61 specify amino acids and 3 specify “ STOP”
Anticodon
A sequence of 3 nucleotide bases on tRNA that have the complimentary sequence of codons on mRNA
Triplet Code
This refers to the genetic code of mRNA codons that each consist of three nucleotide bases.
The genetic code is degenerate or redundant what does this mean?
Some amino acids are specified by more than one codon, BUT each codon specifies one amino acid ( or a “stop” signal). Redundancy is good because it allows some mutations to “ slip through the cracks” and the correct amino acid is specified anyway.
What is meant by the universe salinity of the genetic code
The genetic code is a universal code in that most organisms that we know of use this code. That is why we can splice genes from one organism into another, and the recipient organism can express those forgein genes.
Gene mutation
A change in the DNA sequence of a gene. Mutations can be passed to the next generation only if they occur in the eggs or sperm.
Frameshift Mutation
Involves an addition or deletion of one or more nucleotide bases. The result is a shift in the “ reading frame” of mRNA. The consequences are not as bad if the addition or deletion is a multiple of 3. If the mutation exist in DNA, mRNA will copy the error each time, resulting in a faulty gene product. Sometimes, mRNA will make an error in copying even though the DNA is correct. This results in a a bad batch of mRNA. Hopefully, next time the mRNA will copy correctly.
Point mutation
The subsistution of one nucleotide for another. The result is that the incorrect amino acid may be inserted into the protein chain. This depends upon where the point mutation occurs. If it is present in the third position of the codon, then the correct amino acid may be inserted anyway.
Mechanisms protecting against gene mutation
1)Semiconservative replication
2) base pairing rules
3) DNA polymerase
4) having new mRNAs produced each time a protein product is needed
Mutagen
any substance or agent that would cause a mutation or change int eh DNA sequence Ex. Radiation, chemicals in tobacco, pollutants in air and water
