DNA = deoxribonucleic acid
- DNA is located inside the nucleus of a cell.
- DNA can fit into a nucleus as it can coil and fold.
- DNA has two strands.
NOTE: DNA is also found in the mitochondria and chloroplast of cells
There are four bases used in DNA:
- A = adenine
- T = thymine
- G = guanine
- C = cytosine
What is meant by the term complimentary base pairs?
- These are bases that match (correspond) with each other.
- Adenine joins with thymine
- A -> T
- Guanine joins with Cytosine
- G -> C
- Apples (adenine) in the trees (thymine), Car (cytosine) in the garage (guanine)
What shapes does DNA have?
- DNA has a double helix shape
What is a gene?
- A gene is a section of DNA that has the code for the production of protein
- What is the genetic code? It is a sequences of three bases (codon or triplet) that represents a code for an amino acid
- How many common amino acids are used in the production of proteins? 20
- A gene carries different codes to control the formation of the different amino acids.
- Examples:
- A DNA triplet CAA is the code for the amino acid called valine
- A DNA triplet CGA is the triplet for an amino acid called alanine
What is non-coding DNA?
- They do not have a code for the formation of a protein
- Non-coding DNA is also known as junk DNA.
What is the structure of a nucleotide?
- DNA is made up of units called nucleotides
Sugar is Deoxyribose = D
Phosphate group = P
Nitrogen base = A or G or T or C
What is a polynucleotide?
- It is many nucleotides joined together
What holds base together?
- Bases are held together by a hydrogen bond
- Adenine and thymine form two weak hydrogen bonds.
- Guanine and cytosine form three hydrogen bonds.
What are purines and pyrimidines?
- There are four nitrogen bases, two are known as purines and two as pyrimidines.
- Two purine bases are (double ringed molecules): adenine (A) and guanine (G)
- Two pyrimidine bases are (single ringed molecules): thymine (T) and cytosine (C)
What scientists discovered the shape and structure of DNA?
- Frances Crick and James Watson
RNA = ribonucleic acid
- RNA also consist of four bases
- RNA contains the base uracil instead of thymine
- The bases in RNA pair up with those in a section of DNA
- RNA is a single stranded molecule
What are the four bases of RNA:
- Adenine (A)
- Uracil (U)
- Guanine (G)
- Cytosine (C)
Example: If DNA has sequence GCAATC along one strand, then the RNA will have the sequence CGUUAG
| DNA | RNA |
|---|---|
| Has the bases ATGC | Has the bases AUGC |
| Double stranded | Single stranded |
| Sugar = deoxyribose | Sugar = ribose |
| Found in the nucleus | Found in nucleus, cytoplasm, ribosome |
NOTE: DNA is also found in the mitochondria and the chloroplast (this is known as non-nuclear DNA)
- Genes are used to produce different proteins. A gene is a section of DNA that has the code for the production of protein
- The major steps involved in protein synthesis include:
- Transcription = making of mRNA from DNA (occurs in nucleus)
- Translation = making of protein depending on the mRNA code (occurs in the ribosome)
Three types of RNA involved in production of protein:
- Messenger RNA (mRNA)
- Transfer RNA (tRNA)
- Ribosomal RNA (rRNA)
- Remember: all produced in the nucleus!
Steps involved in transcription:
- 1: Enzymes start to unwind the DNA double helix in the nucleus
- 2: Complimentary RNA bases join to the exposed DNA strand to form mRNA (transcription)
- 3: The enzyme RNA polymerase joins the RNA bases together to form mRNA (messenger RNA)
NOTE: Each mRNA strand has:
- A start codon
- A series of codons represent different amino acids
- A stop codon
NOTE: start codon and stop codon are not involved in protein production
Steps involved in translation:
- 4: mRNA moves from the nucleus to the cytoplasm
- 5: rRNA (ribosomal RNA) are found in the ribosome
- 6: mRNA then moves into the ribosome and forms a weak bond with the rRNA (rRNA holds the mRNA in place in ribosome)
- NOTE: ribosome is the site for protein synthesis
- 7: tRNA (transfer RNA) is found in the cytoplasm
- tRNA contains an anticodon attached to an amino acid
- NOTE: an anticodon is a sequence of three bases on tRNA
- 8: tRNA is attracted to the mRNA in the ribosome. The anticodon on the tRNA compliments the codon on the mRNA
- 9: As the tRNA attach to the mRNA in the ribosome, it also brings with it an amino acid
_ 10: As the tRNA continue to enter the ribosome, the amino acids detach from the tRNA and bond together to form a new protein
- 11: RNA leave the ribosome without any amino acids and pull with it the mRNA strand out of the ribosome
- 12: The process stops once it has reached the stop codon. A new protein has been produced and becomes functional when it folds
Functions of RNA:
- mRNA (messenger RNA) – gets code from the DNA in nucleus/ then carries this code to the ribosome
- rRNA (ribosomal RNA) – binds (or holds) the mRNA in place in the ribosome
- tRNA (transfer RNA) – carries an amino acid to the ribosome/ it binds with the mRNA/ places amino acid in sequence
- NOTE: If a DNA sequence in a gene is not correct, the amino acids that join together will not form the correct protein (e.g cystic fibrosis)
- When mitosis occurs, one cell divides into two identical daughter cells
- DNA replication occurs so there is an exact copy of the DNA for mitosis to occur
- Takes place during the interphase of a cell
How does DNA replication occur?
- Enzyme breaks the hydrogen bond between the base pairs. The double helix unwinds
- Other DNA bases now enter the nucleus and join with the exposed DNA strand
- Two new strands of DNA are formed that are identical to the original strand
What is DNA Profiling?
- It is a method of making a unique pattern of bands from the DNA of a person which is then used to tell the difference of that DNA from other DNA
- DNA is released:
- DNA is released by the breaking down of the cell
- DNA is cut into fragments:
- DNA is cut into fragments using enzymes (restriction enzymes) depending on the sequence of bases
- The section of DNA bases cut will be of different lengths because of the different distances between the base sequences
- The fragments are separated:
- Fragments of DNA are separated based on their size
- They are separated by a process called gel electrophoresis. An electric current is passed through the gel which allows bands of small DNA fragments to be separated from bands of larger DNA fragments
- NOTE: The smaller DNA fragments move quicker through the gel
- Patterns are compared:
- If two different DNA samples are the same, then the two samples must have come from the same person
Solving Crimes:
- If a suspects DNA profile is compared to a victims DNA profile in a crime, and they are similar, then it can be said that this person was present at the crime scene
Medical (e.g maternity/ paternity tests):
- DNA profiles can be used to determine whether a person is the parent of a child (e.g financial inheritance cases)
- Genetic screening is testing DNA for the presence or absence of an altered gene
- Genes can be altered by mutations
- Altered genes do not have the correct code for the production of protein
- NOTE: Genetic disorders caused by altered genes include albinism, cystic fibrosis
Where is genetic screening used?
- Adult screening:
- Carried out on people who do not have a genetic disorder but may carry altered genes
- Tells them the chances if their children will have the genetic disease
- Examples: Carriers of sickle cell anaemia and cystic fibrosis
- Foetal screening:
- Cells can be removed from the placenta of a foetus
- Child can be tested for genetic disorders
What Ethical issues does genetic screening bring?
- Mothers may terminate a pregnancy if they find out their unborn child has a genetic disorder
- Chop up a kiwi or an onion (this increases the surface area for the washing up liquid to act on)
- Add sodium chloride (salt) to washing up liquid in distilled water
- NOTE: Washing up liquid causes the cell and nuclear membranes to break, which releases DNA from the cells
- NOTE: The salt causes the DNA to clump together
- Add the kiwi to the washing up liquid and salt solution
- Place this solution in a water bath at 60°C for 15 minutes (denatures the enzymes which stops the DNA being digested)
- NOTE: Should not be left more than 15 minutes as the DNA would break down
- Place the solution in a water bath that is ice cold for 5 minutes (the breakdown of DNA is slowed down)
- Place the solution into a blender for 3 seconds (the cell walls are broken down to release DNA)
- NOTE: If left any longer than 3 seconds will break down the DNA itself
- Filter the solution using coffee filter paper (using normal filter paper would slow down the process)
- NOTE: The DNA and protein pass through the filter paper
- Using a syringe place some of the filtered solution into a boiling tube
- Add protease enzyme which helps break down the proteins around the DNA
- Add ice cold ethanol slowly down the side of the boiling tube
- NOTE: Alcohol helps remove water from DNA which causes the DNA to float to top
- NOTE: DNA forms white threads
- Using a glass rod, DNA should attach to it as it is twisted
| Terms | Definitions |
|---|---|
| Triplet (codon) | A sequence of three bases |
| Genetic code | A sequence of three bases (codon or triplet) that represents a code for an amino acid |
| Gene | A section of DNA that has the code for the production of protein |
| Complementary bases | Complementary bases are when each base has a different corresponding base (or bases that match each other) |
| Nucleotide | A nucleotide consists of a phosphate group, deoxyribose (sugar) and a nitrogen base |
| Polynucleotide | A polynecleotide are many nucleotides joined together |
| Anti codon | A sequence of three bases on the tRNA that complement three bases on the mRNA |
| DNA profiling | DNA profiling makes a pattern of DNA bands of a person, which can be used to distinguish that DNA from other DNA |
| Restriction enzymes | Restriction enzymes are used to cut the DNA into different fragments |
| Genetic screening | The testing of DNA for an altered gene |
| Protein synthesis | The making of a protein |
| Translation | The making of a protein using the mRNA code |
| Transcription | The making of mRNA from DNA |
| Purines | Double ringed molecules e.g. adenine and guanine |
| Pyrimidines | Single ringed molecules e.g. cytosine and thymine |
| Non coding DNA | They don't have a code for the formation of a protein. Non-coding DNA is also known as junk DNA |