DNA and RNA

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Structure of DNA

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

The Genetic Code

• 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

Structure of RNA

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:

1. Adenine (A)
2. Uracil (U)
3. Guanine (G)
4. Cytosine (C)

Base pairing between DNA and RNA

Example: If DNA has sequence GCAATC along one strand, then the RNA will have the sequence CGUUAG

Differences between DNA and RNA

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)

Protein Synthesis (the making of protein)

• 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:
  1. Transcription = making of mRNA from DNA (occurs in nucleus)
  2. Translation = making of protein depending on the mRNA code (occurs in the ribosome)

Three types of RNA involved in production of protein:

1. Messenger RNA (mRNA)
2. Transfer RNA (tRNA)
3. 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:

1. A start codon
2. A series of codons represent different amino acids
3. 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

Review

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)

DNA Replication

• 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

DNA Profiling

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

Method of DNA profiling

1. DNA is released:
   • DNA is released by the breaking down of the cell
2. 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
3. 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
4. Patterns are compared:
   • If two different DNA samples are the same, then the two samples must have come from the same person

Applications of DNA profiles (DNA fingerprints)

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

• 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?

1. 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
2. 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

Experiment: To isolate DNA from tissue of a plant

1. Chop up a kiwi or an onion (this increases the surface area for the washing up liquid to act on)
2. 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
3. Add the kiwi to the washing up liquid and salt solution
4. 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
5. Place the solution in a water bath that is ice cold for 5 minutes (the breakdown of DNA is slowed down)
6. 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
7. 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
   • 
8. Using a syringe place some of the filtered solution into a boiling tube
9. Add protease enzyme which helps break down the proteins around the DNA
10. 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
    • 
11. Using a glass rod, DNA should attach to it as it is twisted

Important Definitions

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