2.3.3- How DNA Codes For Polypeptides

Question 1

What is a polypeptide?

Answer 1

A polymer that is made of many amino acids joined together by peptide bonds.

Question 2

explain what a protein is.

Answer 2

A large polypeptide of 100 or more amino acids.

Question 3

Explain what transcription is.

Answer 3

The process of making mRNA from a DNA (gene) template.

Question 4

Explain what translation is.

Answer 4

The formation of a protein by assembling a.acids into a particular sequence according to the coded instructions carried from DNA to the ribosome by mRNA. Happens at ribosomes.

Question 5

Define what a gene is.

Answer 5

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

Question 6

List the five ways in which RNA is structurally different to DNA.

Answer 6

The sugar is ribose (RNA), nitrogenous base thymine is replaced by uracil (pyrimidine) in RNA, polynucleotide shorter in RNA, chain is single stranded (RNA) usually, there are three forms of RNA; tRNA, mRNA and rRNA.

Question 7

What are that three forms of RNA?

Answer 7

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

Question 8

Explain how the gene contains a code that determines the sequence of amino acids in the polypeptide chain and how this will affect the molecules structure and function.

Answer 8

There is a sequence of base DNA triplets in the gene that determines that a.acid structure (primary) which will affect its folding into its tertiary structure to determine the shape and overall structure of the molecule. The shape allows it to carry out its function, eg; active site of an enzyme or receptor on a cell membrane must be specific to its function.

Question 9

Where are genes stored?

Answer 9

Inside the cell nucleus.

Question 10

Why is there a need for a copy of each gene to be transcribed into a mRNA length?

Answer 10

As the chromosomes cannot pass out of the genes. It’s the only way that the sequence of base triplets (codons), therefore the coded instructions for translation that’s held in them can be carried from the nucleus to the ribosomes.

Question 11

What three terms can be used to describe the nature of the genetic code?

Answer 11

Universal
Degenerate
Non overlapping

Question 12

Explain what is meant when calling the genetic code ‘universal’.

Answer 12

In almost all living things it is the same- the same triplet of DNA bases codes for the same amino acid.

Question 13

Explain what it means when calling the genetic code degenerate.

Answer 13

For all amino acids (excl methionine and tryptophan) there is more that one case triplet. This reduces the effect of point mutation as a change in the base triplets may produce another triplet that still codes for the same amino acid.

Question 14

explain what it means when calling the genetic code non overlapping.

Answer 14

It is read starting from a fixed point in groups of three bases. If a base is added/ deleted, it causes a frame shift, changing every base triplet after it and every amino acid coded for is changed.

Question 15

Explain the process of transcription.

Answer 15

A gene unwinds and unzips, hydrogen bonds between complementary base pairs break, RNA polymerase catalyses the formation of temp hydrogen bonds between RNA nucleotides and complementary free DNA bases (Uracil bonds with A), a RNA length complementary to the gene template is produced- it’s a copy of the other DNA strand, mRNA passes out of the nuclear envelope and attaches to a ribosome.

Question 16

What is the function of RNA polymerase?

Answer 16

It catalyses the formation of temporary hydrogen bonds between RNA nucleotides and complementary unpaired DNA bases during transcription.

Question 17

What two things are ribosomes made of?

Answer 17

Protein and rRNA.

Question 18

Explain briefly now ribosomes are made.

Answer 18

They’re made as two separate, smaller subunits that pass out of the nucleus through the nuclear envelopes pores and then come together to form a ribosome, using magnesium ions to help bind the two subunits together.

Question 19

What is the function of mRNA?

Answer 19

To carry the coded instructions that are carried in genes out of the nucleus to be translated into a protein at a ribosome.

Question 20

Explain the structure of a tRNA molecule and how this relates to its function.

Answer 20

Single stranded polynucleotides that can twist into a hairpin shape. At one end is a trio of nucleotide bases that recognises and attaches to a specific a.acid and at the loop of the hairpin is another triplet of bases called an anticodon that is complementary to a specific codon (triplet) of bases on the mRNA.

Question 21

What is a codon?

Answer 21

A triplet of bases on a length of mRNA.

Question 22

Explain the process of translation of a mRNA molecule into a protein.

Answer 22

tRNA brings the amino acids and find their place when the anticodon binds by temp hydrogen bonds to the mRNA’s complementary codon/ ribosome moves along the mRNA length and when two a.acids are adjacent, it places a peptide bond between them- needs energy in the form of ATP/ after the full polypeptide’s been assembled, mRNA breaks down to component molecules/ the polypeptide is folded into its 3D shape, helped by chaperone proteins in the cell.

Question 23

What is an anticodon?

Answer 23

A triplet of bases on the tRNA molecule that is complementary to the mRNA codon.