Using models can help us understand more about some processes. The assembly line in a car factory can be used as a model to describe how proteins are put together.

When making a car, you start with a design of a complete car that is then broken down into its component parts. A plan is made of the order in which the parts must be put together to create a whole car. The plan is used to construct the assembly line, where the component parts are added one at a time until each car is complete.

To make the whole process simpler and faster, the cars move along the assembly line as they are being built and the component parts are fitted at the same point on the line each time. This means that robots specially designed to handle particular components don't have to move.

1Draw a flowchart to explain how a car is put together on an assembly line.

2Draw a flowchart to explain how a protein is put together in a cell.

3Match the main stages of building a car to the main stages in building a protein and explain your choices:

e.g. the DNA in building a protein is like the … in building a car because …

4Explain the advantages of using a model to explain a complex process such as protein manufacture.

5aDescribe any parts of the car assembly model that don't match protein manufacture well.

bIs this a problem for understanding protein manufacture? Explain your answer.

You are going to build an amino acid chain starting from the DNA code. Remember that in complementary base pairing, A (adenine) pairs with T (thymine), and C (cytosine) pairs with G (guanine). Also remember that in RNA the base T is replaced by U (uracil).

Here is a sequence of bases from the coding strand of some DNA. This sequence codes for a sequence of amino acids that begins with the amino acid Met (short for methionine).

T A C C T G C G C C T T C A T A G C

1Copy the sequence. Then write down the sequence of bases in the mRNA strand that will form during transcription.

The mRNA strand moves out of the nucleus into the cytoplasm and joins to a ribosome. The ribosome reads the base sequence in triplets (codons).

2Use brackets to mark the triplets on your mRNA strand.

In the cytoplasm there are many different kinds of tRNA. Each kind of tRNA has a different triplet of bases and the triplet controls which amino acid is attached to the tRNA, as shown in the diagram.

Some of the different tRNAs found in cytoplasm attached to their amino acids.

Only the tRNA with the complementary triplet of bases will attach to a triplet on the mRNA strand on the ribosome. Enzymes separate the amino acid from the tRNA and attach it to the growing amino acid chain.

3Match the correct tRNAs from the diagram to the triplets on your mRNA strand. Write down the sequence of amino acids in the chain that is formed.

4Check your amino acid sequence with another student. If the sequences aren't the same, compare each of your answers to find where the error occurred.

Jon and Felice were trying to make a model to explain how DNA controls the sequence of amino acids in which proteins are made. They had some coloured popper beads in red, green, white, yellow, blue and orange. They decided to use the first letter of each colour as the code for that colour of bead.

Jon wrote a code for a protein using these letters: G R R B W G Y B R W O Y.

1In this model of how proteins are made:

awhat do the coloured popper beads represent

bwhat do the letters represent

cwhat does the code that Jon wrote represent?

Jon gave Felice his letter code. Felice used the code to make a string of popper beads that matched the order of the code.

2Which colour was used most often in the string?

3What does the string of popper beads represent?

4What in DNA controls the order of amino acids when a protein is made?

When Jon and Felice wrote up their investigation they wrote:
Each of our letters is like the small groups of bases in DNA that code for different amino acids.

5Complete their write-up to explain how this model helps to show how DNA controls which proteins are made.

You do not need to remember the details on this sheet for your exam but you could be asked to apply your knowledge to unfamiliar situations.

The table shows how ribosomes decode the triplets of bases (codons) on an mRNA strand to make proteins. Each triplet is the code for one particular amino acid. The shortened names of the amino acids are shown in italics next to the triplet code.

Second position
U / C / A / G
First position / U / UUU / Phe / UCU / Ser / UAU / Tyr / UGU / Cys / U / Third position
UUC / Phe / UCC / Ser / UAC / Tyr / UGC / Cys / C
UUA / Leu / UCA / Ser / UAA / STOP / UGA / STOP / A
UUG / Leu / UCG / Ser / UAG / STOP / UGG / Trp / G
C / CUU / Leu / CCU / Pro / CAU / His / CGU / Arg / U
CUC / Leu / CCC / Pro / His / Arg / C
Leu / CCA / Pro / CAA / Gln / CGA / Arg / A
CUG / Leu / CCG / Pro / CAG / Gln / CGG / Arg / G
A / Ile / ACU / Thr / Asn / Ser / U
AUC / Ile / ACC / Thr / AAC / Asn / Ser / C
AUA / Ile / ACA / Thr / AAA / Lys / Arg / A
AUG / Met / ACG / Thr / AAG / Lys / Arg / G
G / GUU / Val / GCU / Ala / GAU / Asp / Gly / U
GUC / Val / GCC / Ala / GAC / Asp / Gly / C
GUA / Val / GCA / Ala / GAA / Glu / Gly / A
GUG / Val / GCG / Ala / GAG / Glu / Gly / G

1The table is not complete. Use the rules shown in the table to fill in the missing letters.

2Use the table to find:

ahow many different codons code for the amino acid serine (Ser)

bhow many different codons there are in total, which code for an amino acid

chow many different amino acids there are.

3The start of a DNA template strand of a gene looks like this:

TAC CGG TCG TGT AGG TGG TGG TAG

aThe complementary DNA strand to the template strand is called the coding strand. Write out the first 24 bases of the coding strand for this gene.

bWrite out the first 24 bases of the mRNA strand for this gene. (Remember that, instead of T in DNA, the base is U in RNA.)

cUse the table to write out the first eight amino acids that this gene codes for.

4What would be the coding strand of the gene that codes for the protein below? There is more than one answer. Give just one solution.

Met-Phe-Lys-Trp-Asp.

Extra challenge

5When scientists were trying to work out how many bases coded for one amino acid, they considered codes that used one base per amino acid and then two bases per amino acid. Explain why three bases was the smallest number that could possibly code for one amino acid.