2.3-How DNA codes for polypeptides

Question 1

What does gene mean?

Answer 1

A length of DNA that codes for a polypeptide or fro a length of RNA that is involved in regulating gene expression.

Question 2

What does polypeptide mean?

Answer 2

A polymer made up of many amino acids units joined together by peptide bonds. Insulin is a polypeptide of 51 amino acids

Question 3

What does protein mean?

Answer 3

A large polypeptide of 100 or more amino acids.

Question 4

What does transcription mean?

Answer 4

The process of making mRNA from a DNA template

Question 5

What does translation mean?

Answer 5

Formulation of a protein at ribosomes, by assembling amino acids into a particular sequence according to the coded instructions carried from DNA to the ribosomes by mRNA

Question 6

What is RNA?

Answer 6

RNA is structurally different from DNA in a number of ways:

• the sugar molecules in each nucleotide is ribose
• the nitrogenous base uracil, which is a pyrimidine replaces the pyrimidine base thymine.
• the polynucleotide chain is usually single stranded
• the poly nucleotide chain is shorter
• there are 3 forms of RNA- messenger RNA, transfer RNA and ribosomal RNA.

Question 7

What are there on each chromosome?

GENES

Answer 7

On each chromosome there are specific lengths of DNA called genes. Each genes contains a code that determine the sequence of amino acids in a particular polypeptide or protein.

Question 8

What does protein account for?

GENES

Answer 8

Protein accounts for 75% of an organisms dry mass. Some proteins are structural, such as the cytoskeleton threads inside cells or proteins in the cell membrane, others make up the cells tool kit such as enzymes, and these may catalyse the formation of non protein molecules such as lipids and carbohydrates.

Question 9

What is within the cell?

GENES

Answer 9

Within each gene there is a sequence of DNA base triplets that determines the amino acids sequence or primary structure of a polypeptide. As long as this primary structure of polypeptide is correct it will fold correctly and beheld in its tertiary structure or shape, enabling it to carry out its function.

For example:

• The shape of the active site of an enzyme molecule must be complementary to the shape of the substrate molecule
• Part of an antibody molecule must have a shape complementary to that of its antigens on the surface of an invading pathogen.
• A receptor on a cell membrane must have a shape complementary to the shape of the cell signalling molecule, such as hormone or a drug that it must detect.
• An ion channel protein must have hydrophilic amino acids lining the inside of the channel and lipophilic amino acids on the outside portion that will be next to the lipid bilayer of the plasma membrane

Question 10

Where are genes?

GENES

Answer 10

Genes are inside the cell nucleus but proteins are made in the cytoplasm at ribosomes

As the instructions inside the genes on chromosomes cannot pass out of the nucleus, a copy of each gene has to be transcribed (copied) into a length of mRNA. In this format the sequence of base triplets, (CODONS) can pass out of the nucleus to the ribosome, ensuring that the coded instructions are translated and the protein is assembled correctly from amino acids.

Question 11

What is the nature of the genetic code?

Answer 11

• The genetic code is near universal because in almost all living organism the same triplet of DNA basses code for the same amino acids.
• The genetic code is described as degenerate because for all amino acids, except methionine and tryptophan, there is more than one base triplet. This may reduce the effect of the point mutations, as a change in one base of the triplet could produce another base triplet that still codes fro the sae amino acid.
• The genetic code is also non - overlapping and it is read staring from a fixed point in groups of three bases. If a base is added or deleted then it causes a frame shift, as every base triplet after that and hence every amino acid coded for it changes.

Question 12

How is transcription carried out?

Answer 12

• A gene unwinds and unzips
• Hydrogen bonds between complementary nucleotide bases break
• The enzyme RNA polymerase catalyses the formation of temporary hydrogen bonds between RNA nucleotides and their complementary unpaired DNA bases. A bonds with T, C bonds with G and U with A on one strand of the unwound DNA. This DNA strand is called the template strand.
• A length of RNA that is complementary to the template strand of the gene is produced. It is therefore a copy of the other DNA strand- coding strand
• The mRNA now passes out of the nucleus, through the nuclear envelope and attaches to a ribosome.

Ribosome are made in the nucleolus, in two smaller subunits. These passes separately out of the nucleus, through pores in the nuclear envelope, and d then come together to form the ribosome. Magnesium ions help to bind the two subunits together. Ribosomes are made of ribosomal RNA and protein in roughly equal parts.

Question 13

How is translation carried out?

Answer 13

Transfer RNA are made in the nucleus and pass out of the nucleus into the cytoplasm. They are single- stranded polynucleotides, but can twists into a hairpin shape. At one end is a trio of nucleotide bases that recognises and attaches to a specific amino acid. At the loop of the hair pin is another triplet of bases called anti codon, that is complementary to a specific codon (triplet) of bases on the mRNA.
Ribosomes catalyse the synthesis of polypeptides.
- Transfer RNA molecules bring the amino acids and find their place when the anticodon binds by temporary hydrogen bonds to the complementary codon on the mRNA molecule.
- A s the ribosome moves along the length of mRNA, it reads the code and when two amino acids are adjacent to each other \a peptide bond forms between them
-Energy in the form of ATP, is needed for polypeptide synthesis.
-The amino acids sequence for the polypeptide is therefore ultimately determined by the sequence of the triplets of nucleotide bases on the length of DNA- the gene
- After the polypeptide has been assembled, the mRNA breaks down. Its component molecules can be recycled into new lengths of mRNA with different codon sequences
- The newly synthesised polypeptide is helped by chaperone proteins in the cell, to fold correctly into its 3D shape or tertiary structure in order to carry out its function.