Discuss how cells use the genetic code on chromosomes to make proteins
Your task is to cut out and read each statement carefully, then, using the cues provided in the
language of each statement as well as your knowledge of the processes involved, put them in
order to construct an answer to the question above that flows logically. All statements MUST
be used. Once you think you have the statements in the correct order, glue them in place.
Note: Each statement is numbered ONLY to make checking against the model answer easier.
18
In the nucleus of a eukaryotic cell, there are structures called chromosomes which bear
the instructions for making all the proteins needed by the cell.
2
Chromosomes are made up of deoxyribose nucleic acid or DNA, which is a molecule
made of two strands of nucleotides twisted into a double helix. The two strands are
joined together by the complementary pairing of bases, of which there are four: adenine,
cytosine, guanine and thymine.
21
Bases always pair as follows: adenine pairs with thymine and cytosine pairs with guanine.
It is the order of bases along a length of DNA which comprises the genetic code.
7
A length of DNA which contains the genetic code or instructions to make a particular
protein is a gene. The genetic code is “written” in 3-base sequences called triplet codes
along the length of a gene.
25
While genes remain on chromosomes in the nucleus, proteins are assembled in the
cytoplasm by ribosomes. Therefore, in order for a cell to make a protein, it must first
make a copy of a gene, then get that copy of the instructions for a protein out of the
nucleus to the ribosomes.
23
The process by which a cell makes a copy of the instructions carried by a gene is called
transcription. Firstly, the DNA unwinds at a gene, and the two strands unzip between the
base pairs, exposing the two strands.
19
The strand of the gene that actually contains the code for the protein, the sense strand, is
then used as the template for the construction of a molecule of messenger ribose
nucleic acid or mRNA, a single-stranded nucleic acid, by the enzyme RNA polymerase.
10
This enzyme joins free nucleotides together in the order determined by the exposed bases
of the sense strand of the unzipped DNA.
13
The free nucleotides form a complementary RNA strand to the sense strand of DNA using
the base pairing rules given above, except that uracil replaces thymine.
26
For example, if the sense strand contained the triplet codes ACT GGC TAT, then the
strand of mRNA that would form would be UGA CCG AUA.
15
These 3-base sequences of mRNA are known as codons.
8
When enough molecules of mRNA have been formed from the entire length of the gene,
the DNA then rewinds back into a double helix until it is needed again. At the same time,
the completed molecules of mRNA move through the nuclear pores in the nuclear
membrane into the cytoplasm, carrying the ‘message’ of the gene.
17
Once in the cytoplasm, the mRNA molecules each join up with a ribosome to begin the
process of translation.
1
This is the process by which the genetic code of the mRNA is used in ribosomes to join
amino acids together in the particular order needed to make the protein coded for by the
original gene.
20
Ribosomes, made of another type of RNA called ribosomal RNA or rRNA, are found
either free in the cytoplasm or attached to the membranes of the endoplasmic reticulum.
3
Within a ribosome, there is enough space to fit two codons of a mRNA molecule.
14
Also involved in translation and present in the cytoplasm are molecules of a third type of
RNA called transfer RNA or tRNA.
5
Each tRNA molecule has a particular 3-base sequence at one end called an anticodon
which is complementary to a particular codon in an mRNA molecule. At their other end of
a tRNA molecule is one of the 20 possible amino acids.
22
Translation begins when a tRNA molecule that has the anticodon UAC, complementary to
the first codon of the mRNA, the start codon AUG, temporarily joins with the mRNA inside
a ribosome.
12
This first tRNA molecule always carries the amino acid methionine.
6
Then a second tRNA molecule with the right anticodon to match the second codon on the
mRNA will also temporarily join the mRNA, right next to the first tRNA.
27
This brings the two amino acids carried by the tRNA molecules close together enough for
a peptide bond to form between them. The first tRNA molecule now has no amino acid
and is released from the ribosome to go find another methionine. Meanwhile, the ribosome
moves along the mRNA, putting the second and third codons into position.
9
A third tRNA molecule bringing yet another amino acid will enter the ribosome, and the
two-amino-acid sequence will peptide bond to this third tRNA, releasing the second tRNA
into the cytoplasm where it will recharge with its particular amino acid.
4
As the ribosome continues along the length of the mRNA, tRNA molecules enter and
leave the ribosome-mRNA complex while their amino acids are added to the growing
polypeptide chain.
16
Eventually the ribosome will reach a codon on the mRNA for which there is no matching
tRNA molecule.
24
This codon is known as the stop codon (either UAA, UAG or UGA) because at this point,
no more amino acids can be joined and the polypeptide chain is finished and is released
from the ribosome.
11
The completed polypeptide usually folds in a specific way, depending on its amino acid
sequence, and may join with other polypeptides before finally forming a complete
functional protein.
28
In summary, the genetic code of genes, found on chromosomes, is first transcribed into
mRNA inside the nucleus, which is then translated by ribosomes in the cytoplasm to form
the polypeptide chains which make up proteins.
Discuss how cells use the genetic code on chromosomes to make proteins
Your task is to cut out and read each statement carefully, then, using the cues provided in the
language of each statement as well as your knowledge of the processes involved, put them in
order to construct an answer to the question above that flows logically. All statements MUST
be used. Once you think you have the statements in the correct order, glue them in place.
Note: Each statement is numbered ONLY to make checking against the model answer easier.
1
This is the process by which the genetic code of the mRNA is used in ribosomes to join
amino acids together in the particular order needed to make the protein coded for by the
original gene.
2
Chromosomes are made up of deoxyribose nucleic acid or DNA, which is a molecule
made of two strands of nucleotides twisted into a double helix. The two strands are
joined together by the complementary pairing of bases, of which there are four: adenine,
cytosine, guanine and thymine.
3
Within a ribosome, there is enough space to fit two codons of a mRNA molecule.
4
As the ribosome continues along the length of the mRNA, tRNA molecules enter and
leave the ribosome-mRNA complex while their amino acids are added to the growing
polypeptide chain.
5
Each tRNA molecule has a particular 3-base sequence at one end called an anticodon
which is complementary to a particular codon in an mRNA molecule. At their other end of
a tRNA molecule is one of the 20 possible amino acids.
6
Then a second tRNA molecule with the right anticodon to match the second codon on the
mRNA will also temporarily join the mRNA, right next to the first tRNA.
7
A length of DNA which contains the genetic code or instructions to make a particular
protein is a gene. The genetic code is “written” in 3-base sequences called triplet codes
along the length of a gene.
8
When enough molecules of mRNA have been formed from the entire length of the gene,
the DNA then rewinds back into a double helix until it is needed again. At the same time,
the completed molecules of mRNA move through the nuclear pores in the nuclear
membrane into the cytoplasm, carrying the ‘message’ of the gene.
9
A third tRNA molecule bringing yet another amino acid will enter the ribosome, and the
two-amino-acid sequence will peptide bond to this third tRNA, releasing the second tRNA
into the cytoplasm where it will recharge with its particular amino acid.
10
This enzyme joins free nucleotides together in the order determined by the exposed bases
of the sense strand of the unzipped DNA.
11
The completed polypeptide usually folds in a specific way, depending on its amino acid
sequence, and may join with other polypeptides before finally forming a complete
functional protein.
12
This first tRNA molecule always carries the amino acid methionine.
13
The free nucleotides form a complementary RNA strand to the sense strand of DNA using
the base pairing rules given above, except that uracil replaces thymine.
14
Also involved in translation and present in the cytoplasm are molecules of a third type of
RNA called transfer RNA or tRNA.
15
These 3-base sequences of mRNA are known as codons.
16
Eventually the ribosome will reach a codon on the mRNA for which there is no matching
tRNA molecule.
17
Once in the cytoplasm, the mRNA molecules each join up with a ribosome to begin the
process of translation.
18
In the nucleus of a eukaryotic cell, there are structures called chromosomes which bear
the instructions for making all the proteins needed by the cell.
19
The strand of the gene that actually contains the code for the protein, the sense strand, is
then used as the template for the construction of a molecule of messenger ribose
nucleic acid or mRNA, a single-stranded nucleic acid, by the enzyme RNA polymerase.
20
Ribosomes, made of another type of RNA called ribosomal RNA or rRNA, are found
either free in the cytoplasm or attached to the membranes of the endoplasmic reticulum.
21
Bases always pair as follows: adenine pairs with thymine and cytosine pairs with guanine.
It is the order of bases along a length of DNA which comprises the genetic code.
22
Translation begins when a tRNA molecule that has the anticodon UAC, complementary to
the first codon of the mRNA, the start codon AUG, temporarily joins with the mRNA inside
a ribosome.
23
The process by which a cell makes a copy of the instructions carried by a gene is called
transcription. Firstly, the DNA unwinds at a gene, and the two strands unzip between the
base pairs, exposing the two strands.
24
This codon is known as the stop codon (either UAA, UAG or UGA) because at this point,
no more amino acids can be joined and the polypeptide chain is finished and is released
from the ribosome.
25
While genes remain on chromosomes in the nucleus, proteins are assembled in the
cytoplasm by ribosomes. Therefore, in order for a cell to make a protein, it must first
make a copy of a gene, then get that copy of the instructions for a protein out of the
nucleus to the ribosomes.
26
For example, if the sense strand contained the triplet codes ACT GGC TAT, then the
strand of mRNA that would form would be UGA CCG AUA.
27
This brings the two amino acids carried by the tRNA molecules close together enough for
a peptide bond to form between them. The first tRNA molecule now has no amino acid
and is released from the ribosome to go find another methionine. Meanwhile, the ribosome
moves along the mRNA, putting the second and third codons into position.
28
In summary, the genetic code of genes, found on chromosomes, is first transcribed into
mRNA inside the nucleus, which is then translated by ribosomes in the cytoplasm to form
the polypeptide chains which make up proteins.
Maddie Ware, Kerikeri High School