Biology 30 - Unit C: topic 3: the central dogma of biology

Chargaff

• a scientist who found that A and T are equal in DNA as well as C and D as well

Hershey and Chase

• used radioactive labelling of proteins and DNA

• They found DNA was the heritable information, transferred between bacteria

Franklin

• used X-ray photos to determine that DNA is a double helix

Watson and Crick

• proposed the first accurate model of DNA structure

nucleotides

• consists of a 5-carbon sugar and a phosphate group and a ring shaped nitrogen base (adenine, guanine, thymine, cytosine)

  • Pure as gold: Purine nucleotides are double ringed and adenine and guanine

  • Cut the Pye: pyrimidine nucleotides are single ringed and cytosine and thymine

Structures of DNA

• DNA is a double helix with two anti-parallel strands of nucleotides

• Each strand has nucleotides linked together with a sugar-phosphate backbone

• When the strands are connected, the backbone is the outer portion while the nitrogen bases are teh inner portio

• Nucleotides nitrogen bases extended inwards and link the chains via hydrogen bonding

Endosymbiosis theory

• suggested that mitochondria and the chloroplast were once self-sustaining prokaryotic and organisms

• They have their own ribosomes and DNA

Mitochondrial DNA

• duplicates when the whole organisms duplicates

• Through mitosis, every cell in an organism would end up wiht the same mitocondrial DNA

• Sperm DNA is only nuclear, there is no contribution of mitochondrial DNA, that is all passed down by the egg

DNA replication

1. Helicase: unzips the double helix, breaking the weak hydrogen bonds between nitrogen bases.The location of the unzipping is the replication fork

2. Topoisomerase: makes the unziping possible by preventing supercooling of the DNA

3. RNA primase: moves along the DNA (3’ to 5’) and adds a short primer that goes 5’ to 3’. This gives the next enzyme a place to attach

4. DNA polymerase lll: begins at the primar and moves along DNA 3’-5’ adding new DNA

5. DNA polymerase l: replaces the RNA primer with the DNA

6. Ligase: seals the backbone of the different sections of the DNA together

Leading strand

• goes 3’ → 5’

• The enzyme moves towards the replication fork making it then anti parallel

• Only one primer is needed, then DNA polymerase lll follows behind

• New strand is continuous

Lagging strand

• goes 5’ → 3’

• Enzymes move away from the replication form

• A primer is added then DNA is added and then it repeats as more DNA is exposed

• The new strand is produced discontinuously

• The segments are called Okazaki fragments

Semiconservative replication

• each double stranded DNA molecule consists of an “old” strand from the paren and a “new” strand produced during replication

Central dogma of Biology

• DNA is transcribed into mRNA which is then translated into proteins

RNA

• a different form of DNA

• Contains ribose sugars instead of deoxyribose

• Contains nitrogen base uracil which replaces thymus

• RNA is a single strand rather then double-stranded

• Much shorted then DNA and is just a copy then one gene not the whole chromosome

Transcription

• DNA must reman in the nucleus but protein synthesis occurs in the cytoplasm sI the RNA acts as a temporary copy to bring the genetic instructions to produce a certain protein

1. RNA polymerase: locates a promoter sequence at the beginning of a gene and unzips the DNA

2. RNA polymerase uses the template DNA strand (3’ → 5’) to make a complementary mRNA strand. The mRNA and the coding strand match

3. RNA polymerase reaches the terminator sequence at the end of the green and stops transcription, releasing the DNA

mRNA

• the RNA produces during transcription

• Contains the instructions to produce a certain protein

mRNA post transcription

• a 5’ cap and a poly-A tail are added to the mRNA molecule to prevent it from being degraded and to help wiht the Ribosomes binding in translations

• Spliceosomes splice the mRNA so that the proper protein will be produced

Amnio Acids

• DNA and RNA use nucleotides but proteins use amino acids

• The sequence of amino acids will determine the function of the protein

• Each set of 3 nucleotides will code for one amino acid

tRNA

• the conversion between mRNA codon and amino acid is done by transfer RNA, a clover shaped molecule of RNA

• Each tRNA has an anticodon which is complementary to an mRNA codon and is attached to the appropriate amino acid

Ribosomes

• the matching between mRNA and tRNA occurs at ribosomes

• Ribosomes consist of 2 subunit which will clamp onto the mRNA and then read it from start to stop codon (5’ → 3’) only one reading backwards

• Has 3 sites

  • A site (acceptor site), P site (protein site), E site (exit site)

Translation

1. Translation complex forms. The start codon AUG is recognized by the tRNA-methionine and the 2 ribosomal subunits clamp around the mRNA strand, positioning the tRNA-methionine in the P-site

2. A tRNA molecule enters the A site to match the codon there

3. A peptid bond forms between the amino acids in the A and P site, transferring the amino acid chain to the tRNA that just entered into the A site

4. The tRNA in the P site moves to the E site and exits, freeing the A

5. Repeat steps 2-4 until the codon in the A site is a stop codon at which the translation complex will be disassembled

Point mutation

• a mutation of just one nucleotide

Silent mutation

• a mutation that doesn’t change the amino acid

Missense mutation

• a mutation that changes the amino acid

• Not always serious

Nonsense mutation

• a mutation that introduces an early stop codon

Frameshift mutation

• the adding or subtracting of a nucleotide

• This shifts the whole reading frame of all codons downstream so a differnet sequence is coded

Translocation mutation

• moving a section of DNA to a different location in the genome

Inversion mutation

• when the section of DNA is reversed (3’ → 5’ to 5’ → 3’)

Restriction enzymes

• the enzyme scissors, cutting DNA at specific strands

• Cut at staggered/sticky ends

DNA ligase

• The molecular glue and enzyme that would joint DNA strands together to create a new, artificially recombined DNA molecule