Nucleic Acids and Protein Synthesis

Chromosomes are thread-like structures found inside the nucleus of a cell. Each body cell contains 46 chromosomes arranged as

23 pairs. Sex cells (eggs and sperm) contain only 23 chromosomes in total

Chromosomes are the stuctures that hold genes. Genes are made

from DNA

What are chromosomes?

• Chromosomes hold genes. Genes are made of DNA. A gene holds information that decides our colour of hair, colour of eyes etc. It also holds information for making proteins that may become enzymes or hormone for use within the body.

Two of these strands become joined together by weaker

hydrogen bonds forming between there bases. However this union

is temporary in that hydrogen bonds can be easily broken when

this becomes necessary e.g during the process of

transcription

Base pairing rules

• Each base can only pair with one other type of base: adenine(A) always bonds with thymine(T), and Guanine(G) always bonds with Cytosine(C). A-T and G-C are called Base Pairs. Each member of a pair are complimentary to its partner.

• This twisted strand a bit like a spiral ladder is called a double helix

How to carry out protein Synthesis

• In order to make a protein which may be a globular, fibrous or conjugated protein using the information from genes, we must start with DNA use it to make an RNA strand called a messenger RNA, this in turn is used to make another RNA strand called a Transfer RNA. The transfer RNA is used to link amino acids in a specific order. This in turns forms a polypeptide. Several polypeptides link together to form our completed protein

Structure of RNA

• Now that we understand the structure of DNA, we must now understand the structure of our second type of nucleic acid RNA. An RNA molecule is similar to DNA

Differences between DNA and RNA

Starting protein synthesis-

• A single stranded Messenger RNA (mRNA) must be Transcribed from a single strand of double stranded DNA. This process is called Transcription.

Stages of Transcription of a mRNA

• The 2 DNA strands becomes unwound at stage 1• The DNA strands separate as weak hydrogen bonds between the bases are

breaking at stage 2• Free RNA nucleotides join up with exposed bases on one of the DNA

Strands at stage 3. Uracil joins with Adenine, Cytosine with Guanine• Weak hydrogen bonds are forming between the new base pairs at stage 4• The nucleotides on the RNA strand are being linked in a chain by strong

chemical bonds between the sugar of one RNA nucleotide and the phosphate of the next one in the chain at stage 5. This linking into a chain is controlled by an enzyme called RNA Polymerase.

• At stage 6 the weak hydrogen bonds between the DNA and RNA bases are breaking allowing the molecule of transcribed mRNA to become separated from the DNA template.

• At stage 7 the transcribed mRNA is ready to begin its journey out of the nucleus and into the cytoplasm.

• At stage 8 the two exposed DNA strands reunite with weak hydrogen bonds forming between them. The DNA becomes wound into a double helix once more. This whole process is continuous.

mRNA

• The completed molecule of mRNA leaves the nucleus through the pore in the nuclear membrane and enters the cytoplasm. Each triplet of bases on mRNA is called a codon

tRNA

• A second type of RNA is found in the cell’s cytoplasm. This is called transfer RNA (tRNA). Each molecule of tRNA has a triplet of bases exposed. This triplet is known as an anticodon.

Picking up amino acids

• The anticodon corresponds to a particular amino acid. Each tRNA molecule picks up the appropriate amino acid from the cytoplasm at its site of attachment. There are 20 different amino acids.

Ribosomes

These are small almost spherical structures found in all cells. Some occur freely in the cytoplasm, others are found attached to

endoplasmic reticulum

Translation

• Ribosomes are the site of translation of mRNA into protein. The ribosome becomes attached to one end of the mRNA molecule about to be translated. Inside the ribosome there are sites for attachment of tRNA molecules, two at a time.

Stages of translation

• The anticodon of the first tRNA molecule forms weak hydrogen bonds with the complimentary codon on the mRNA

Stages of translation

• When the second tRNA molecule repeats this process, the first two amino acids molecules are brought into line with one another.The two amino acids become joined together by a strong peptide bond.

Stages of translation

• The first tRNA becomes disconnected from its amino acid and from the mRNA and leaves the ribosome

Stages of translation

• The amino acids continue to align form peptide bonds and disconnect. The growing chain of amino acids is known as a polypeptide chain.

Stages of translation

• The completed polypeptide chain consisting of very many amino acids is then released into the cytoplasm. The tRNA and mRNA are reused.

• The polypeptide then may be folded and rearranged to become the final protein. Sometimes several polypeptide chains combine to form the protein.

• Protein synthesised in free ribosomes is for use within the cell. Protein made in ribosomes attached to endoplasmic reticulum is for export.

Protein made in Ribosomes attached

To endoplasmicReticulum is for

export

Protein madeIn free

Ribosomes isFor use

Within the cell

Coiling and folding of the polypeptide

• When a protein is made on the ribosome that is attached to endoplasmic reticulum, the polypeptide is ‘injected’ into the ER and then coiled and folded. The protein is then passed to the golgi apparatus for packaging (adding for example a carbohydrate part to make it into a glycoprotein which is a conjugated protein) and secretion from the cell

Function of the nucleus and nucleolus

• Nucleus- contains DNA which holds the genetic instructions for the manufacture of proteins and the control of inherited characteristics

• Nucleolus- controls the synthesis of RNA and other components needed to build ribosomes

Nucleolus

Nucleus