B8 - DNA, Genes & Protein Synthesis

Question 1

What is the DNA in prokaryotes?

Answer 1

Have a nucleoid- a single, circular chromosomal DNA molecule, which is double stranded
They also have plasmids: small, circular DNA molecules
The DNA within prokaryotic cells is not associated with any proteins

Question 2

What is the DNA in eukaryotes?

Answer 2

In the nucleus of eukaryotic cells, DNA molecules are very long, linear and associated with proteins, called histones.
Tightly coiled DNA + histone proteins = Chromatin → coils & condenses even more → forms a Chromatid

Question 3

What is the function of histone proteins?

Answer 3

Organise & condense the DNA tightly → fits into the nucleus

Question 4

What joins together 2 chromatins to from a chromosome?

Answer 4

Centromere

Question 5

What is the DNA in chloroplasts and mitochondria like?

Answer 5

It is like the DNA of prokaryotes, is short, circular and not associated with protein

Question 6

What is a gene?

Answer 6

It is a base sequence of DNA that codes for:
The amino acid sequence of a polypeptide
A functional RNA (including ribosomal RNA and tRNAs)

Question 7

What are the functional RNA molecules?

Answer 7

Messenger RNA (mRNA): the base sequences on these molecules are used by ribosomes to form polypeptide chains
Transfer RNA (tRNA): amino acids are carried to the ribosome by these molecules
Ribosomal RNA (rRNA): these molecules form part of the structure of ribosomes

Question 8

What is loci/locus?

Answer 8

The position of a gene on a chromosome
Alleles may have slightly different nucleotide sequences, but still occupy the same position on the chromosome

Question 9

What is a triplet of bases called?

Answer 9

A codon

Question 10

What is the use of start and stop signal triplet codes?

Answer 10

This ensures that the cell reads DNA correctly, so the code is non-overlapping and produces the correct sequence of amino acids

Question 11

How is the effect of mutations limited?

Answer 11

Multiple codons can code for the same amino acids

Question 12

How is the genetic code universal?

Answer 12

As almost every organism on earth uses the same code, and the same set of amino acids

Question 13

What are non-coding sections of DNA within genes called?

Answer 13

Introns

Question 14

What are Non-coding DNA sections between genes called?

Answer 14

Introns

Question 15

What is the name for regions of the DNA that code for proteins?

Answer 15

Exons

Question 16

What happens to pre-mRNA before it leaves the nucleus?

Answer 16

The introns are removed, and exons are joined together in a process called splicing

Question 17

Are introns present in prokaryotic cells?

Answer 17

No

Question 18

What is a genome?

Answer 18

A genome is the complete set of genes present in a cell
The full genome of an organism is present within every one of it’s cells, but not every gene is expressed in every cell

Question 19

What is a proteome?

Answer 19

The proteome is the full range of proteins that a cell is able to produce
Usually larger than the genome of an organism

Question 20

What is the structure of mRNA?

Answer 20

Made up of one polynucleotide strand
Much shorter compared to DNA
It is made up of a sugar-phosphate backbone and exposed unpaired bases
Uracil bases are present instead of thymine bases

Question 21

What is the structure of tRNA?

Answer 21

A single-stranded molecule
Has a sugar-phosphate backbone
It has a folded shape
Hydrogen bonds between some of the complementary bases
Amino acids bind to specific regions of the molecule
The specific anticodon found on the tRNA molecule is complementary to a specific codon on an mRNA molecule

Question 22

What are the 2 stages of protein synethesis?

Answer 22

Transcription
Translation

Question 23

What occurs in the transcription stage of protein synthesis?

Answer 23

Helicase breaks hydrogen bonds between complementary base pairs, causes the DNA molecule to unwind, gene to be transcribed is now exposed
Free, activated RNA nucleotides pair up (via hydrogen bonding) with their complementary, exposed bases on the unzipped template strand
Sugar-phosphate backbone of mRNA molecule forms by RNA polymerase
Hydrogen bonds between template strand and newly-synthesised mRNA break → the original double helix re-forms
mRNA leaves the nucleus via the nuclear envelope, original DNA stays

Question 24

What’s the difference between the transcription of prokaryotic and eukaryotic cells?

Answer 24

Prokaryotic transcription of DNA results in the production of mRNA
Eukaryotic transcription of DNA results in the production of pre-mRNA

Question 25

What is alternative splicing?

Answer 25

When exons are spliced in many different ways → produces different mature mRNA molecules

Question 26

What happens during translation?

Answer 26

• Free molecules of tRNA bind with their specific amino acids in the cytoplasm
• mRNA molecules attaches to a ribosome
• The triplet of bases (anticodon) on each tRNA molecule pairs with a complementary codon on the mRNA molecule
• Second tRNA molecules binds to mRNA strand
• A peptide bond is formed between both amino acids (requires ATP)
• Process continued until a ‘stop’ codon on the mRNA molecule is reached
• Newly-synthesised amino acid chain forms - the polypeptide

Question 27

What is the difference between a triplet and a codon?

Answer 27

Triplet: sequence of three DNA bases
Codon: sequence of three mRNA bases

Question 28

What is an anticodon?

Answer 28

Anticodon: sequence of three tRNA bases that are complementary to a codon