8.DNA genes and protein

Question 1

Define a gene

Answer 1

A section of DNA on a chromosome coding for one or more polypeptides / functional RNA

Question 2

Define a locus

Answer 2

The position of a gene on a chromosome

Question 3

Describe the features of the genetic code

Answer 3

• degenerate code
• non overlapping
• universal

Question 4

Why is the genetic code known as degenerate

Answer 4

Because most amino acids are coded for by more than one triplet

Question 5

Why is the genetic code known as non-overlapping

Answer 5

Each base is only read once, in one triplet

Question 6

Why is the genetic code known as universal

Answer 6

Each triplet codes for the same amino acids in all organisms

Question 7

Some triplets don’t code for an amino acid, what are these known as

Answer 7

Stop codons, they mark the end of a polypeptide chain

Question 8

Define an exon

Answer 8

A coding sequence within a gene

Question 9

Define an intron

Answer 9

A non-coding sequence within a gene

Question 10

What are the differences in DNA in prokaryotic cells and eukaryotic

Answer 10

PROKARYOTIC
-DNA molecules are shorter and circular
-they are not associated with protein molecules
EUKARYOTIC
-DNA molecules are longer and linear
-they are associated with proteins called histones

Question 11

When are chromosomes visible as distinct structures

Answer 11

When a cell is dividing (undergoing mitosis)

Question 12

What is a single thread of a chromosome called

Answer 12

Chromatid

Question 13

What is a homologous pair

Answer 13

A pair of chromosomes (one maternal one paternal) that have the same gene loci and therefore determine the same features

Question 14

Define diploid

Answer 14

A term applied to cells in which the nucleus contains two sets of chromosomes

Question 15

Define haploid

Answer 15

A term referring to cells that contain only a single copy of each chromosome

Question 16

What is the diploid number in humans

Answer 16

46

Question 17

Define allele

Answer 17

One of a number of alternative forms of a gene

Question 18

What is mRNA

Answer 18

A type of RNA that transfers the DNA code from the nucleus to the cytoplasm for protein synthesis where it acts as a template

Question 19

Define codon

Answer 19

The sequence of three bases on mRNA that codes for a single amino acid

Question 20

Define genome

Answer 20

The complete set of genes in a cell including those in mitochondria and/or chloroplasts

Question 21

Define proteome

Answer 21

The full range of proteins produced by the genome

Question 22

What does RNA stand for

Answer 22

ribonucleic acid

Question 23

Describe the structure of RNA

Answer 23

A polymer made of repeating mononucleotide sub-units forming a single strand. Each nucleotide is made of:

• ribose
• an organic base (AGCU)
• a phosphate group

Question 24

What is tRNA

Answer 24

• relatively small
• single stranded
• folded into clover leaf shape
• three base sequence (anticodon) on one end
• carries an amino acid
• each tRNA is specific to one amino acid and has an anticodon that is specific to that amino acid

Question 25

What is different about the bases in RNA

Answer 25

Thymine is replaced by uracil

Question 26

What is transcription

Answer 26

The process of making pre-mRNA using the DNA as template

Question 27

Describe the process of transcription

Answer 27

• an enzyme acts on a specific region of the DNA causing the two strands to separate and expose the nucleotide bases in that region
• nucleotides on template strand pair with complementary nucleotides from the pool present in the nucleus
• RNA polymerase moves along the strand and joins nucleotides together (via condensation reaction making phosphodiester bonds ) to form a pre-mRNA molecule
• DNA strand rejoins behind this
• RNA polymerase reaches the stop triplet it detaches

Question 28

What is the stop triplet

Answer 28

A particular sequence of bases on the DNA that causes RNA polymerase to stop bonding nucleotides on pre-mRNA molecules

Question 29

Describe how pre-mRNA is made into RNA

Answer 29

Via splicing, a process where introns are removed and the functional exons are joined together

Question 30

Why is splicing necessary

Answer 30

To get rid of introns (non coding parts of DNA) which would prevent the synthesis of a polypeptide

Question 31

Why is splicing not necessary in prokaryotic cells

Answer 31

Most prokaryotic cells do not have introns, transcription results directly in mRNA so splicing is unnecessary

Question 32

How does mRNA leave the nucleus

Answer 32

Via a nuclear pore as mRNA molecules are too large to diffuse out the nucleus where it attaches to a ribosome in the cytoplasm

Question 33

What is translation

Answer 33

The synthesis of a polypeptide

Question 34

Describe translation

Answer 34

• ribosome attaches to starting codon at one end of the mRNA
• tRNA molecule, carrying a specific amino acid, with the complementary anticodon moves to the ribosome and pairs with the codon on the mRNA
• another tRNA complementary anticodon pairs with the next codon on the mRNA, carrying a specific amino aid
• the ribosome moves along the mRNA binging the two tRNA molecules together
• two amino acids on the tRNA are joined by a peptide bond using an enzyme and ATP (which is hydrolysed to provide energy)
• as this happens the tRNA is released from its amino acid and is free to collect another amino acid from the amino acid pool in the cell
• this process continues until a ribosome reaches a stop codon causing the mRNA and the last tRNA molecule to separate