Section 4 - Genetic information, variation and relationships between organisms: 8. DNA, genes and protein synthesis

Question 1

What is a gene

Answer 1

A section of DNA that codes for a polypeptide (protein) or functional RNA (eg. mRNA, tRNA)

Question 2

What are the 4 bases in the DNA sequence

Answer 2

• Adenine
• Thymine
• Cytosine
• Guanine

Question 3

Why do 3 bases code for each amino acid

Answer 3

Triplet code gives 64 (4^3) possible combinations, as there are 4 possible bases.
(enough to code for the 20 naturally occurring amino acids)

Question 4

What are the features of the genetic code

Answer 4

• Degenerate
• Non-overlapping
• Universal

Question 5

Why is the genetic code described as degenerate

Answer 5

Most amino acids are coded for by more than one triplet, to limit the impact of mutations

Question 6

Why can the genetic code be described as non-overlapping

Answer 6

Each base sequence is read once, in one particular direction, where the triplets don’t overlap

Question 7

Why can the genetic code be describes as universal

Answer 7

Each triplet code codes for the same amino acid in all organisms

Question 8

What is always the first Amino acid coded for by a gene

Answer 8

Methionine, Start Codon AUG

Later removed if not part of the chain

Question 9

How is DNA stored in Prokaryotes (+ in Mitochondria and chloroplast)

Answer 9

Short, circular structures (plasmid), with no associated proteins

No chromosomes

Question 10

What is the structure of a chromosome

Answer 10

• DNA molecule wound up and wrapped around histones (proteins)
• DNA-Histone complex is coiled
• Coils form loops that pack together into chromatids
• Chromatids branch off of the centromere
  (contains only 1 DNA strand)

Question 11

What is a homologous pair

Answer 11

A pair of chromosomes, with one from the mother, and one from the father

Question 12

What is the Diploid number

Answer 12

Number of chromosomes in homologous pairs (Humans = 46)

Question 13

What is an allele

Answer 13

One of a number of forms of the same gene
- One from the mother and one from the father
- Each has different base sequence

Question 14

What is RNA

Answer 14

Single stranded nucleic acid transcribed from sections of DNA

Question 15

What is a Codon

Answer 15

Sequence of 3 bases on the mRNA that codes for one amino acid

Question 16

What is the genome

Answer 16

The complete set of genes in a cell (including those in the mitochondria/chloroplast)

Question 17

What is the proteome

Answer 17

The full set of proteins produced by the genome
(Complete proteome = Proteins produced by 1 type of cell under certain conditions)

Question 18

What is the structure of RNA

Answer 18

Polymer of nucleotides:
- Ribose sugar
- Organic base (AUCG)
- Phosphate group

Question 19

What is mRNA

Answer 19

Messenger RNA

Question 20

What is the function of mRNA

Answer 20

Acts as a template for protein synthesis

Question 21

What is the structure of mRNA

Answer 21

• Long, single stranded polymer of nucleotides
• No H bonds between nucleotides, so no folding
• Base sequence transcribed from DNA

Question 22

What is tRNA

Answer 22

Transfer RNA

Question 23

What is the function of tRNA

Answer 23

Binds to a specific amino acid, allowing for their arrangement in the polypeptide chain, for protein synthesis

Question 24

What is the structure of tRNA

Answer 24

• Single strand of nucleotides folded into a clover shape by H bonds
• One end binds to a specific amino acid
• The other end has a specific anticodon, complementary to the codon that codes for the carried amino acid

Question 25

What is the process of Protein synthesis

Answer 25

- DNA provides 'instructions' in the long base sequence - A complementary strand of pre-mRNA is made for a section of this sequence (transcription) - Pre-mRNA is spliced to form mRNA - mRNA is used as a template, to which tRNA molecules attach, aligning their amino acids to form the polypeptide chain (Translation)

Question 26

How is protein synthesis different in prokaryotes and eukaryotes

Answer 26

Prokaryotes directly transcribe mRNA form the DNA, whereas eukaryotes make pre-mRNA first, which needs to be spliced

Question 27

What is transcription

Answer 27

The formation of pre-mRNA from the DNA base sequence

Question 28

Where does transcription occur

Answer 28

In the Nucleus

Question 29

What is the process of transcription

Answer 29

- The enzyme DNA helicase breaks the hydrogen bonds between the two DNA stands, exposing the nucleotides - Free nucleotides align with the complementary bases of the template strands - The Enzyme RNA polymerase moves along the strand, forming phosphodiester bonds between the loose nucleotides - DNA strands re-join after the free nucleotides are joined together) - When the RNA polymerase reaches a terminal sequence on the DNA, it detaches, releasing the pre-mRNA

Question 30

What is the function of DNA helicase in transcription

Answer 30

Breaks the H bonds between the two strands of DNA, exposing the bases

Question 31

What is the function of RNA polymerase in transcription

Answer 31

Moves along the strand of free nucleotides, forming phosphodiester bonds between them

Question 32

How many DNA bases are exposed at the same time during transcription

Answer 32

~12, as the double helix re-attaches together after the RNA polymerase moves past

Question 33

What is RNA splicing

Answer 33

The process that forms mRNA from pre-mRNA, to be used in translation

Question 34

Where does RNA splicing occur

Answer 34

In the Nucleus

Question 35

What are the non-coding sections of DNA

Answer 35

Introns

Question 36

What are the sections of DNA that code for polypeptides

Answer 36

Exons

Question 37

What is the process of splicing

Answer 37

The non-coding introns are spliced out by the enzyme Spliceosome, and the exons join together to form the mRNA

Question 38

What is alternative splicing

Answer 38

When the exons re-join in a different order, so the same gene can code for multiple proteins, as this gives a different mRNA base sequence

Question 39

What is translation

Answer 39

The formation of a polypeptide from mRNA

Question 40

Where in the cell does translation occur

Answer 40

In the ribosomes

Question 41

What is the process of Translation

Answer 41

- Ribosome attaches to the start codon at one end of the mRNA - tRNA molecule with the complementary anticodon moves into the ribosome, carrying the specific amino acid - The tRNA binds to the mRNA, positioning the amino acid in the correct location in the polypeptide - A second tRNA moves into the ribosome, carrying the next amino acid, and attaches to the mRNA - A peptide bond forms between the two amino acids the bond between the first tRNA and the mRNA is broken - Ribosome moves along the the RNA to the next codon, repeating the process with two triplet codes within at any time - Synthesis continues until a stop codon is reached, when the polypeptide is released to begin folding into a protein

Question 42

What is the start codon

Answer 42

AUG - codes for Methionine