How DNA Codes for Polypeptides

Question 1

What is a gene?

Answer 1

Length of DNA coding for a polypeptide or length of RNA, involved in regulating gene expression

Question 2

What is a polypeptide?

Answer 2

Polymer made of many amino acid units joined together by peptide bonds

Question 3

How many amino acids are in the polypeptide insulin?

Answer 3

51

Question 4

What is a protein?

Answer 4

Large polypeptide of 100 or more amino acids, but terms are often used synonymously and insulin may be described as a small protein

Question 5

What is translation?

Answer 5

Formation of a protein, at ribosomes, by assembling amino acids into particular sequences according to coded instructions carried from DNA to ribosome by mRNA

Question 6

What is transcription?

Answer 6

Process of making messenger RNA from a DNA template

Question 7

What is RNA?

Answer 7

Structurally different from DNA:
• Sugar molecule in each nucleotide is ribose
• Nitrogenous base uracil (a pyrimidine) replaces pyrimidine base thymine
• Polynucleotide chain is usually single-stranded and shorter

Question 8

What are the 3 different forms of RNA?

Answer 8

• Messenger RNA (mRNA)
• Transfer RNA (tRNA)
• Ribosomal RNA (rRNA)

Question 9

How do proteins influence an organism’s dry mass?

Answer 9

• Accounts for 75% of an organism’s dry mass
• Some proteins are structural, (cytoskeleton threads inside cells or proteins in cell membrane)
• Others make up cell’s tool-kit, (enzymes), and may catalyse formation of non-protein molecules (lipids and carbohydrates)

Question 10

What is the role of the primary structure of polypeptides?

Answer 10

In each gene there’s a sequence of DNA base triplets determining amino acid sequence
• As long as this primary structure is correct, it will then fold correctly and be held in its tertiary structure or shape, enabling it to carry out its function

Question 11

What are four examples of polypeptides allowing the tertiary structure of the molecule to carry out its function?

Answer 11

• Shape of active site of enzyme molecule must be complementary to shape of substrate molecule
• Part of antibody molecule must have shape complementary to antigens on surface of invading pathogen
• Receptor on cell membrane must have shape complementary to shape of cell-signalling molecule
• Ion-channel protein must have hydrophilic amino acids lining inside of channel, and lipophilic amino acids on outside portion next to lipid bilayer

Question 12

What is the link between transcription and translation?

Answer 12

• As instructions inside genes can’t pass out nucleus, a copy of each gene muct be transcribed into length of mRNA
• In this form, sequence of base triplets (codons) can pass out nucleus to ribosome, ensuring coded instructions are translated and protein is assembled correctly from amino acids

Question 13

What is the nature of genetic code?

Answer 13

• Near universal, as in almost all living organisms, same triplet of DNA bases codes for same amino acid
• Described as degenerate as, for all amino acids (except methionine and tryptophan) there’s more than 1 base triplet and may reduce effect of point mutations, as change in 1 base of triplet could produce another base triplet still coding for same amino acid
• Non-overlapping, read starting from fixed point in groups of 3 bases. If base is added or deleted, it causes a frame shift, as every base triplet after that, and every amino acid coded for, is changed

Question 14

What are the exceptions to the universal code theory?

Answer 14

• In nearly all living organisms, AUG codes for methionine and starts off most proteins
• In our mitochondria, mRNA codon AUA codes for methionine instead of isoleucine, and codons AGA and AGG are stop codons instead of coding for arginine
• UGA codes for tryptophan instead of being a stop codon, helping support endosymbiont theory of origin of mitochondria, as in some other bacteria UGA also codes for tryptophan instead of being a stop codon

Question 15

What is the process of transcription?

Answer 15

• Gene unwinds and unzips
• Hydrogen bonds between complementary nucleotide bases break
• RNA polymerase catalyses formation of temporary hydrogen bonds between RNA nucleotides and complementary unpaired DNA bases on 1 strand of unwound DNA, called template strand
• Length of RNA complementary to template strand of gene is produced, therefore a copy of coding strand
• mRNA now passes out nucleus, through nuclear envelope and attaches to a ribosome

Question 16

How are ribosomes made?

Answer 16

• Made in nucleolus, in two smaller subunits, passing separately out nucleus, through pores in nuclear envelope, and come together to form ribosome
• Magnesium ions help bind 2 subunits together
• Made of ribosomal RNA and protein in roughly equal parts

Question 17

What is the process of translation?

Answer 17

• Transfer RNA molecules are made in the nucleolus and pass out nucleus into cytoplasm
• They are single-stranded polynucleotides, but can twist into a hairpin shape
• At one end is a trio of nucleotide bases recognising and attaching to specific amino acids
• At loop of hairpin is another triplet of bases, anticodon, complementary to specific codon of bases on mRNA

Question 18

How do ribosomes catalyse and synthesise polypeptides?

Answer 18

• Transfer RNA molecules bring amino acids and find place when anticodon binds by temporary hydrogen bonds to complementary codon on mRNA molecule
• Energy (ATP) needed for polypeptide synthesis
• After polypeptide has been assembled, mRNA breaks down • Component molecules can be recycled into new lengths of mRNA, with different codon sequences

Question 19

What do ribosomes do as they move along the length of mRNA?

Answer 19

Reads code and when 2 amino acids are adjacent to each other, peptide bond forms between them

Question 20

What is the sequence of amino acids for polypeptides ultimately determined by?

Answer 20

Sequence of triplets of nucleotide bases on length of DNA (gene)

Question 21

What do chaperone proteins do to help Newly synthesised polypeptides?

Answer 21

Fold correctly into 3D shape or tertiary structure to carry out its function

Question 22

What is a base triplet?

Answer 22

Triplet of bases on a DNA molecule

Question 23

What is a codon?

Answer 23

Triplet of bases on a length of mRNA

Question 24

What is an anticodon?

Answer 24

Triplet of bases on a tRNA molecule, complimentary to mRNA codon