Key area 3:control of gene expression

Question 1

Explain why only a fraction of genes in a cell are expressed

Answer 1

Only a fraction of the genes in a cell are expressed depending on the proteins required by that cell

Question 2

Name the molecules in a RNA nucleotide

Answer 2

Phosphate, ribose sugar and base

Question 3

Give the names of the 4 RNA bases

Answer 3

Adenine
Uracil
Guanine
Cytosine

Question 4

Describe the base pairing rules for RNA bases

Answer 4

Cytosine pairs with guanine

Adenine pairs with uracil

Question 5

Describe 3 differences between RNA and DNA molecules

Answer 5

DNA is double stranded while RNA is single stranded. DNA has deoxyribose sugar while RNA has ribose sugar.
DNA bases are A,T,C,G while RNA bases are A,U,C,G

Question 6

State what mRNA is and describe its role

Answer 6

messenger RNA is formed in the nucleus from free nucleotides and carries a copy of the DNA code from the nucleus to the ribosomes where protein synthesis occurs

Question 7

State what tRNA is and describe its role

Answer 7

Transfer RNA molecules collect amino acids and bring them to the ribosome to build proteins

Question 8

State the location of transcription

Answer 8

Transcription takes place in the nucleus

Question 9

State 4 things that must be present for transcription to occur

Answer 9

Template DNA
Supply of free RNA nucleotides
Enzymes
ATP

Question 10

Describe the process of transcription

Answer 10

The enzyme RNA polymerase unwinds and unzips the double helix of the gene to be expressed and aligns free RNA nucleotides against the exposed DNA nucleotides of the template strand. Complementary base pairing ensures correct positioning of RNA nucleotides which are joined to form a primary transcript. Each transcript has introns and exons. The exons are spliced together to form a mature mRNA transcript

Question 11

Describe the role of RNA polymerase in transcription

Answer 11

RNA polymerase binds at the promoter and unwinds the DNA . RNA polymerase adds nucleotides onto the 3’ end of the growing mRNA molecule

Question 12

Explain what introns are

Answer 12

Introns are non-coding region and are removed from the primary transcript

Question 13

Explain what exons are

Answer 13

Coding regions and are spliced together to form a mature mRNA transcript

Question 14

Describe RNA splicing

Answer 14

After the mRNA has been transcribed the introns are removed. The remaining exons are spliced together to form a continuous chain. This is called the mature transcript. The mature transcript then leaves to travel to the cytoplasm

Question 15

Describe the roll or rRNA

Answer 15

Ribosomal RNA molecules combine with proteins to create the ribosome-the organelle responsible for assembling proteins following the DNA code

Question 16

Where does transcription start from

Answer 16

Starts from a region of a DNA known as the promoter

Question 17

State the location in translation

Answer 17

The cytoplasm

Question 18

Define amino acid, polypeptide and protein

Answer 18

Proteins are made of long chains of smaller molecules called amino acids. Amino acids are joined together by peptide bonds. The proteins fold into different shapes which dictates the role it will have in the cell

Question 19

Describe the process of translation

Answer 19

Stage 1: the ribosome binds to the 5’ end of the mRNA at the start codon
Stage2: a tRNA carrying the amino acid methionine becomes attached to the ribosome
Stage3: the ribosome the moves to the next codon and looks for a tRNA with an anticodon that matches this second codon
Stage 4: the peptide Bon then forms between these two amino acids
Stage5: the ribosome then moves along to the next codon
Stage 6: steps 3-5 the rebate until it reaches a stop codon

Question 20

Describe the structure of tRNA

Answer 20

Transfer RNA is made of a single chain of nucleotides and it folds into a 3D structure held together by hydrogen bonds.

Question 21

Describe the function of tRNA

Answer 21

tRNA has an attachment site for a specific amino acids. The tRNA picks up its appropriate amino acid and takes it to the ribosomes to be matched with the mRNA

Question 22

Define codon

Answer 22

The mRNA is made up of sequences of three nucleotides (a triplet of bases) called codons. Each codon is code fo one amino acid

Question 23

Define anticodon

Answer 23

Each tRNA has an attachment site for a specific amino acid and a triplet of bases known as an anticodon

Question 24

Describe the structure of a ribosome

Answer 24

Small, roughly spherical structures found in all cells. They contain the enzymes essential for protein synthesis. The ribosomes function is to the the tRNA molecules beating amino acids in contact with the mRNA

Question 25

Describe the complementary pairing of bases between mRNA and tRNA

Answer 25

tRNA anticodons are recognised and align with mRNA codons and are attached together by peptide bonds

Question 26

Explain how codons on mRNA recognise incoming tRNA

Answer 26

tRNA has an attachment site which is specific to an mRNA amino acid

Question 27

Explain the function of start and stop codons

Answer 27

Start codons start the translation while stop codons causes translation to finish when the polypeptide is complete

Question 28

Name the bond formed between amino acids of a polypeptide

Answer 28

Peptide bond

Question 29

Explain the mechanism by which different proteins can be expressed from one gene

Answer 29

The same gene can be used to make different proteins by alternative RNA splicing or post translational modification

Question 30

Define alternative RNA splicing

Answer 30

Under certain conditions alternative segments of RNA may be treated as exons and introns do during splicing

Question 31

Define post translational modification

Answer 31

Once translation is completed proteins can be modified by cleavage or addition of other molecules

Question 32

Cleavage

Answer 32

A single polypeptide chain can be cleaved (cut) by enzymes to make it active.

Question 33

Addition of other molecules

Answer 33

Carbohydrates and phosphate groups can be added to proteins.

Question 34

Describe the overall shape of protein molecules

Answer 34

Proteins fall into two distinct groups which are fibrous or globular

Question 35

Describe what can happen to polypeptide chains as they are transformed into proteins

Answer 35

The polypeptide is folded to give a protein with a three dimensional shape held in place by a hydrogen bonds and other interactions between individual amino acids

Question 36

Explain how interactions of amino acids determine the final shape of a protein

Answer 36

The shoe of the protein is linked to its function