DNA & Protein Synthesis
- Know the structures of DNA, including the structure of nucleotides (purines and pyrimidines), base pairing, the two sugar-phosphate backbones, phosphodiester bonds and hydrogen bonds
- Understand how DNA is replicated semi-conservatively including the role of DNA helicase, polymerase and ligase
- Know that a gene is a sequence of bases on a DNA molecule coding for a sequence of amino acids in a polypeptide chain
- Know the structure of mRNA including nucleotides, the sugar phosphate backbone and the role of the hydrogen bonds
- Know the structure of tRNA, including nucleotides, the role of hydrogen bonds and the anticodon
- Understand the processes oif transcription in the nucleus and translation at the ribosome, includiong the role of sense and anti-sense DNA, mRNA , tRNA and the ribosomes
- Understand the nature of the genetic code, including triples coding for amino acids,start and stop codons, degenerate and non overlapping nature and that not all the genome codes for proteins
- Understand the term gene mutation as illustrated by base deletions,insertions and substitutions.
- Understand the effect of point mutations on amino acid sequences, as illustrated by sickle cell anaemia in humans
DNA & RNA
DNA & RNA are nucleic acids (weak acids) and are found in the nuclei of cells. They are polymers composed of monomers called nucleotides.
- A phosphate group (PO42-)
- Is a negatively charged ion and gives nucleic acids their acidic properties
- A pentose sugar
- Carbon 2 has a hydroxyl group – the sugar is ribose, found in RNA
- Carbon 2 has a hydrogen atom – then the sugar is deoxyribose, found in DNA
- A nitrogenous base
- These are small organic basic groups that contain the elements C,H,O,N. There are four different bases in DNA nucleotides called Adenine, Cytosine, Guanine and Thymine. RNA also has four bases and Uracil
- Purines are a sub-group of nitrogenous bases, these are two rings which have nitrogen within them. Adenine and Guanine are examples
- Pyrimidines are a sub-group of nitrogenous bases, these are one ringed which also have nitrogen within them. Thymine and Cytosine are examples
Adenine (A) - Adenosine
Cytosine (C) - Cytidine
Guanine (G) - Guanosine
Thymine (T) - Thymidine
Nucleotides can have one, two or three phosphate groups. So for instance you can have adenosine monophosphate (AMP) and adenosine triphosphate (ATP). These are very important and play more of a role then just DNA. ATP is used as the energy transfer molecule while AMP is used as a messenger chemical
Nucleotides polymerise by forming a phosphodiester bond between carbon 3 of the sugar and an oxygen atom of the phosphate. This is a condensation reaction (water is produced). The bases are obsolete in this reaction.
Polymerisation ensures the sugar-phosphate backbone continues the structure. A polynucleotide has a free phosphate group at one end, called the 5 end as it is attached to the carbon 5 of the sugar and a free OH group on the other side coming out of the carbon 3 of the sugar
Two nucleotides - Dinucleotide
Three nucleotides - Trinucleotide
A few nucleotides - Oligonucleotide
Many nucleotides - Polynucleotide
Structure of DNA
- DNA is double- stranded
- This means there are two polynucleotide strands alongside each other. The strands are antiparallel (they run in opposite directions)
- The strands are wound round each other
- This causes the iconic double helix
- The two strands are joined together by hydrogen bonds
- This is situated between the bases, this makes the bases turn into base pairs and form the run like structure
- The base pairs are specific
- A only binds to T, C only binds to G. These are called complementary base pairs. This just means the other side of the pair is predictable
- DNA is very long
- It can store a great deal of information. The bases are protected on the inside and there are billions of hydrogen bonds making it a stable molecule
DNA contains genes and genes control characteristics. A gene is simply a section of DNA around 1000-2000 base pairs long. The base pairs can appear in any order along the DNA molecule so their sequence can encode information. DNA can control the protein synthesis which changes characteristics so it indirectly affects many thing.
A gene is defined as a sequence of bases on a DNA molecule coding for a sequence of amino acids in a polypeptide chain.#
Sequence of bases in DNA
Sequence of amino acids
Shape and function of protein
Characteristics of cell
DNA is transcribed into RNA which is then translated into a protein. The overall name for this is an expression
Separate post beneath
Meselson and Stahl – considered the semi – conservative model of DNA replication to be correct
- They grew several generations of bacteria (e.coli) in a medium with just 15N present. This is denser than normal nitrogen. This meant bacteria took up the radioactive isotope to make everything within the cell.
- They moved the bacteria to a medium containing 14N as there only nitrogen source. This meant the bacteria for a time bred in normal nitrogen.
- They found the DNA had roughly a middle density between 15N and 14N
- This would have shown that some of the DNA would be the original 15N and the new DNA would be 14N. This is because conservative meant that the parent was not changed whatsoever
- Fragmented / Dispersed
- This would have meant that parts of the DNA would have been 15N and parts of it would be 14N. This is because parts of the parent strand are divided between the daughters
- This would have meant that half the DNA would be 15N and half would have been 14N, this is because half the strand is completely new and half is from the parent.
- Double code would give us only 16 different products – 4 x 4.
- Triple code gives us 64 different options, which is more than enough without taking up too much space – 4 x 4 x 4
- Sequence of three bases of DNA or RNA is called a codon
- Start codon – a specific triplet code that tells the ribosome to start producing amino acids from the mRNA it is reading
- Stop codon – a specific triplet code that tells the ribosome to stop producing that particular amino acid. This means it is at the end of a protein.]
- Anticodon – a specific triplet code that is complimentary to the bases on the mRNA. This is on tRNA and allows it to “pick up “amino acids.
- Some amino acids are able to be obtained in more than one way (for example glycine which can be made using CG and either U , C , A or G
- This means that if there is an a mutation it could be possible that it would never affect the organism
- mRNA is formed in the nucleus and is the product of DNA being transcribed. The strand of mRNA formed from DNA being transcribed generally equates to one polypeptide. mRNA a sense strand that codes for a protein. The DNA being transcribed is caused by RNA polymerase. mRNA can pass out of the nucleus through the nuclear pores due to its small size.
- tRNA is found in the nucleus and is comprised of three bases that are the anti codon of the mRNA. This creates a cloved leaf like structure. This is imperative to the overall function as it allows the tRNA is correspond to the specific amino acid that the bases code for. The tRNA transports the amino acids, then the anti codon lines up with the codon on the mRNA then the correct sequence to form a polypeptide is made
- rRNA makes up 50% of the structure of a ribosome and is the most common form of RNA. It is made in the nucleus and then moves into the cytoplasm to bind with proteins to form ribosomes.
- Transcription - DNA is transcribed into mRNA which occurs in a replication fork
- mRNA carries information to the ribosome – The mRNA passes through the nuclear pores
- mRNA information is translated into a specific sequence of amino acids – These continue to stay in the specific line
- tRNA carries the amino acids to the ribosome – This creates the polypeptide which slowly grows until an anti-codon
- A full polypeptide chain is created