Heredity

Genes

• Genes are carried on DNA. A gene is a section of DNA that causes the production of protein which is the production of the animal or plant part. (Living material is built up by proteins.)
• Each one codes for a specific protein by specifying the order in which amino acids must be joined together.

GENE EXPRESSION

• Gene expression is the process by which inheritable information from a gene is made into protein or RNA.
• Gene expression functions in the environment of the living organism. 
• If the environmental factors are not correct then the characteristic produced by the gene may not be expressed. 
• The way an organism physically looks is called its phenotype. 

DNA

• DNA (deoxyribose nucleic acid) molecules are large and complex. 
• They carry the genetic code that determines the characteristics of a living thing.
• Except for identical twins, each person’s DNA is unique. 
• This is why people can be identified using DNA fingerprinting. 
• DNA can be cut up and separated, forming a sort of “bar code” that is different from one person to the next.

Structure of DNA

• Each base can only join with one other base.
  
• A can only join with T G can only join with C. Each base pair has a purine and a pyrimidine
• The A and G bases are PURINES. The T and C bases are PYRIMIDINES.
• The forces holding the bases together are hydrogen bonds.

Structure of DNA

• Each pair of bases are held on the side strands (some times called the ‘backbones’) of the DNA. 
• These side strands are made up of a pentose sugar (a 5-carbon sugar) and a phosphate group. 
• The entire unit is then made of three molecules: a pentose sugar (a 5-carbon sugar) and a phosphate group a nitrogenous base. 
• This smallest unit of DNA is called a nucleotide.
  
• The pentose sugar is called deoxyribose.
  
• The phosphate is PO4 but is represented a P.
• DNA IS FOUND IN CELLS IN A TWISTED FORM CALLED A DOUBLE HELIX

THE GENETIC CODE

• Proteins are made up of amino acids.
• Amino acids are made up of a set of 3 nucleotides called triplets or codons. 
• There are 20 amino acids that are used to form a variety of proteins. 
• There are many combinations of amino acids that make up the proteins that are an organism’s body.

DNA Replication

• It takes place during interphase of mitosis. 

• 1. Nucleotides are made in huge quantity in the cytoplasm.
  
• 2. An enzyme unzips the two complementary strands of DNA.
  
• 3. New complementary nucleotides link to the exposed bases on the separated strands.
• 4. A new complementary strand is built along each ‘old’ strand.
  
• 5. Two DNAs, identical to the original and each other, are now present. 

DNA Profiling

• DNA profiling is the process of making patterns of bands from a persons DNA to compare with other DNA patterns.
• The process identifies the DNA contained in a tissue sample, which is unique to each organism.
• The DNA is cut using enzymes and the segments are separated according to the size, using an electric current, along a piece of gel.
• The fragments are made visible by staining. The fragments form a pattern that can be analysed. A permanent record can be made of the pattern.

Applications

• DNA profiles can be used to identify people from crime sites, for paternity case, identifying pathogens or tissue matching

Genetic Screening 

• Genetic Screening is used to find out if the parents of a child or the child itself (through removal of cells from the foetus) carry defective genes that could develop into health problems. 
• Although the parent may not have the disorder he/she could be a carrier for the condition. 
• With genetic screening, prospective parents can determine if there is a possibility that their future children could have the health problem.

RNA

• Uracil takes the place of Thymine in RNA. So, the bases in RNA are:
  
• A = Adenine
• U = Uracil
  
• G = Guanine
  
• C = Cytosine
  
• A = Adenine is complementary (combines with) U = Uracil
  
• G = Guanine is complementary with C = Cytosine
  
• Unlike DNA, RNA is single stranded. 
• The RNA produced is complementary to the DNA which produced it. 
• Also, RNA can move out into the cytoplasm while DNA remains in the nucleus.
• GC, AU 

Types Of RNA

mRNA (messenger RNA):

• Produced in the nucleus. Used at ribosome to make protein.
• Copy of genetic message. Used as template to make protein.

rRNA (ribosomal RNA):

• A component of the ribosome.
• Reads the message on mRNA. Used to attach tRNA to the mRNA.

tRNA (transfer RNA):

• It attaches mRNA at the ribosome.
• Places amino acids in the correct sequence to make a protein.

mRNA and tRNA

Each mRNA strand carries:

• A start codon (AUG below)
  
• A series of codons each of which will form a particular amino acid
  
• A stop codon (UAA below)

Each tRNA is composed of:

• A codon of DNA which formed an amino acid
  
•  The tRNA itself
  
• A special anti-codon- each anti-codon is complementary to the amino acid codon. As a result, the tRNA and the mRNA are attracted to each other.

Protein Synthesis

• Each ribosome is composed of a large sub-unit and a small sub-unit. The 2 are depicted below. The large subunit is the upper, thinner; part of the ribosome and the small sub-unit is the lower, wider, part of the ribosome.
  
• Ribosomes are made up of ribosomal RNA (rRNA) and protein.
• The mRNA strand forms weak bonds with the rRNA in the small sub-unit. That is where the protein will be synthesised (made).
  
• The cytoplasm contains transfer RNA (tRNA) which is depicted below.
• The first tRNA molecule will attach to the mRNA just after the start codon. Remember: This is happening in the cytoplasm.
• The tRNA molecules attach to the mRNA two at a time. This combination is called the binding site. The amino acids are, by this method, brought to the ribosomes.

Protein Synthesis

• The mRNA and the tRNA detach in the ribosomes and the amino acids are bonded together to form proteins.
  
• The tRNA molecules move out of the ribosome. As they move out they pull the strand of mRNA through the ribosome.
  
• tRNA molecules continue to bind with mRNA until a stop codon is reached.

Transcription and Translation

Transcription:

Is the copying of a section of DNA, a gene, into mRNA.

Translation:

Is the manufacture of protein based on the sequence of bases on the mRNA.