Unit 1: DNA and the Genome Sub-Topic (1.3) Gene Expression€¦ · Higher Biology Pupil Course Notes Duncanrig Secondary LS 2016 page 1 of 17 Unit 1: DNA and the Genome Sub-Topic

Unit 1: DNA and the Genome

Sub-Topic (1.3) Gene Expression

Higher Biology Pupil Course Notes

Duncanrig Secondary LS 2016 page 1 of 17

Unit 1: DNA and the Genome

Sub-Topic (1.3) Gene Expression On completion of this subtopic I will be able to

State the meanings of the terms genotype, phenotype and allele. State that the phenotype of an organism is determined by the proteins produced as

a result of gene expression. This is affected by environmental factors acting inside and outside the cell.

Know that only a fraction of the genes inside a cell are expressed. State that gene expression is controlled by the regulation of transcription and

translation.

State the differences between DNA and RNA. Describe the process of transcription. State that RNA polymerase is the enzyme responsible for transcription. It unwinds

and opens up (unzips) the DNA strand to bring about the synthesis of an mRNA molecule.

State that introns are non-coding regions within a gene and play no part in coding for a polypeptide.

State exons are coding regions within a gene that have a part to play in coding for a polypeptide.

Know that during RNA splicing introns (non-coding regions) within the primary mRNA transcript are removed and exons (coding regions) are joined together to form the mature transcript.

Know that translation of mRNA into a protein takes place at the ribosome.

Describe the process of translation. Describe the structure and function of mRNA, tRNA and rRNA. Different mRNA molecules are produced from the same primary transcript depending

on which exons are included in the mature mRNA pranscript.

State that many proteins can come from one gene as a result of alternative mRNA splicing.

Know that proteins can be modified after the translation process by cutting and combining polypeptide chains or by adding phosphate or carbohydrate groups.

Know that proteins have a three dimensional shape; peptide bonds link amino acids together in the polypeptide chain which then folds as a result of hydrogen bonds forming between amino acids. Cross connections may also form between amino acids in one or more chains to form the protein’s final structure.

Know that the final three dimensional structure of the proteins allows it to carry out its function.



1. Gene Expression

Genes are DNA sequences that code for particular proteins. The instructions

contained within the DNA molecule must be interpreted and translated into proteins

that carry out actions in the cell. Although every individual will have genes for the

same proteins, there may be different forms of the gene, these are called alleles. In

humans, since you have two copies of each chromosome you will have two copies of

each gene. The copies may not be in the same form.

The phenotype of an organism is determined by the proteins produced as a result of

the genes expressed. This is influenced by environmental factors acting inside and

outside the cell. Only a fraction of the genes in a cell are expressed.

State the meaning of the word phenotype:

___________________________________________________________________

State the meaning of the word genotype:

____________________________________________________________________

Prior Learning

Unit 1.4 DNA and the production of proteins

Each section of DNA which codes for a protein is known as a gene.

Proteins are built from subunits called amino acids.

A polypeptide is a chain of amino acids linked together.

The sequence of bases on the DNA molecule encodes information for the sequence of

amino acids in proteins.

The sequence of the bases on the DNA molecule therefore determines the function of

the proteins they code for.

Messenger RNA (mRNA) is a single stranded molecule which carries a

complementary copy of the code from the DNA, in the nucleus, to a ribosome.

The ribosome is the site of protein synthesis.

Proteins are assembled from amino acids at the ribosome.



Gene Expression through protein synthesis

A protein’s exact molecular structure, shape and ability to carry out its function

depends on the sequence of its amino acids. The order of the amino acids is

determined by the sequence of bases on the DNA molecule. Therefore, the

information for the manufacture of a protein has to be carried from the nucleus to

the ribosome where proteins are synthesised.

This is carried out by a nucleic acid called ribonucleic acid (RNA).

RNA is a single strand of RNA nucleotides. They are similar to DNA nucleotides but

the sugar is a ribose sugar rather than a deoxyribose sugar and one of the bases is

different- Uracil replaces thymine.

Complete the diagram of the RNA nucleotide below:

-

Complete the table below to summarise the differences between a DNA and an

RNA molecule:

DNA

RNA

Found in

Number of strands

Sugar in nucleotide

Adenine paired to

Bases:



2. The Genetic Code

As we have learned, the order of the amino acids in a protein is determined by

the sequence of bases on the DNA molecule. The sequence of the bases along

the strand represents a sequence of ‘codewords’. This is called the genetic code.

Each group of three bases on the DNA molecule, or triplet, is called a codon.

These three letter codes are designated by the code letters of the bases e.g. CGC,

AAT depending on the order of the nucleotides. Each amino acid is coded for by one

or more triplets of bases. The triplet code allows for 64 combinations which is more

than enough to code for 20 amino acids.

Gene expression is controlled by the regulation of transcription and

translation.

3. Transcription

Transcription is the first step in gene expression. In this process a molecule of mRNA

is formed using the DNA as a template. This occurs in the nucleus.

An enzyme called RNA polymerase attaches to a sequence of DNA called the

promoter. It moves along the DNA unwinding the double helix and opens up

(unzips) the DNA molecule breaking the hydrogen bonds holding the base pairs

together.

Free RNA nucleotides align with the complementary base pairs on the DNA

moving from 3’ to 5’. RNA nucleotides are held in place by hydrogen bonds while

strong covalent bonds form between the phosphate of one nucleotide and the 3’

carbon of the adjacent nucleotide. The molecule lengthens until a terminator

sequence of nucleotides is reached on the DNA strand and the RNA polymerase

enzyme is released.

As a result of the base pairing rule, the mRNA strand gets a nucleotide sequence

complementary to one of the two DNA strands. The mRNA strand becomes

separated from the DNA template. This is known as the primary transcript of

mRNA.



Modification of the primary transcript

Eukaryotic organisms have long stretches of DNA that exist within a gene that do

not play a part in coding for a polypeptide chain. These non-coding regions are

called introns. These are located between coding regions called exons. This

means that the region in the primary transcript responsible for coding for the

polypeptide is fragmented.

RNA splicing

The primary transcript needs to be modified in a process called RNA splicing. The

introns are cut out of the primary transcript and the exons are spliced

together to form a mature mRNA strand with a continuous sequence of

nucleotides.

The mature mRNA passes out of the nucleus and enters the cytoplasm. Once in the

cytoplasm ribosomes and another RNA molecule called transfer RNA (tRNA) work

together to translate mRNA into a polypeptide chain.



RNA splicing

Complete the diagram below by:

1. labelling the template strand of DNA. 2. adding the missing complementary base-pairs on the DNA strand. 3. labelling the mRNA strand and adding the complementary base pairs. 4. labelling the sugar phosphate bonds between adjacent nucleotides on the

mRNA molecule.

A T C

C

A G T A T C C A T C C

G G T T

C A A

Mature mRNA transcript



Answer the questions that follow:

1. How many bases in the genetic code are needed to code for one amino acid?

2. Complete the diagram to show the mRNA strand that would be transcribed

from the section of DNA in the diagram below.

ATA TCG CGA CCT TGA DNA strand

mRNA stand

3. Name the enzyme that would direct this process.

4. State the difference between an exon and an intron.

5. Which of these is removed during modification of the primary transcript of

mRNA?

6. State the name of the process in which this occurs.

Ribosomes Translation of mRNA into a protein takes place at the ribosome



Ribosomes are small spherical structures found in all cells. They can be found

free in the cytoplasm or attached to endoplasmic reticulum. They are formed from a

type of RNA called ribosomal RNA (rRNA) and protein. They contain enzymes

required for protein synthesis.

Ribosomes allow mRNA and transfer RNA (tRNA) to come together during a process

called translation.

4. Translation

Translation is the process that translates the mRNA into a polypeptide.

The ribosome reads the information carried by the mRNA molecule as three letter ‘codewords’. Each group of three bases on the mRNA (or triplet) is called a codon. Remember! The information contained in the triplet of bases on the mRNA molecule is complementary to a triplet of bases on the original DNA molecule. Transfer RNA (tRNA) Transfer RNA (tRNA) is a type of RNA found in the cell’s cytoplasm. It is made up of a single strand of nucleotides that folds due to base pairing to form a site for attachment for a specific amino acid at one end and a triplet anticodon at the other. amino acid attachment site

anticodon

The triplet anticodons on the tRNA are complementary to codons on the mRNA

strand. Each anticodon corresponds to a specific amino acid. The specific amino

acid is carried at the attachment site of the tRNA molecule to the ribosome.



Start and Stop codons

Before translation can begin the ribosome must bind with the mRNA molecule at the

mRNA binding site. A start codon at the beginning of an mRNA molecule signals

where translation is to begin.

A tRNA molecule carrying the specific amino acid for the start codon becomes

attached to the binding site within the ribosome by hydrogen bonds between the

anticodon and the start codon.

After the start codon there are further codons on the mRNA that are particular for

the polypeptide to be synthesised. tRNA molecules carry specific amino acids for

each of these codons to the ribosome one at a time.

As they enter the ribosome the codons on the mRNA strand form hydrogen

bonds with the anticodon of a tRNA molecule carrying the specific amino acid for

that codon.

When two amino acids are aligned next to each other they become joined by a

peptide bond to synthesise a growing polypeptide chain.

As the ribosome moves along to the next codon the tRNA molecule is released

from the ribosome to be reused and the next tRNA molecule carrying its specific

amino acid moves into the binding site.

This process is repeated many times allowing the mRNA to be translated into a

complete polypeptide chain.

Eventually a triplet sequence called a stop codon is reached within the mRNA that

signals termination of translation. A release factor frees the polypeptide from the

ribosome.



One gene, many proteins

One gene can produce many proteins as a result of the way in which RNA splicing

occurs.

Different mRNA molecules are produced from the same primary transcript

depending on which exons are included in the mature mRNA transcript.

This results in several different mRNA molecules each with a different sequence of

base triplets coding for a different polypeptide.



Overview

There are 3 forms of RNA involved in protein synthesis:

Messenger RNA (mRNA)

Transfer RNA (tRNA)

Ribosomal RNA (rRNA)

Each type of RNA molecule has its own form and function: Complete the table

below using the class resources:

Complete the diagram showing translation by labelling the parts A-G

Abbreviation Full name Where it is found in

the cell Function

mRNA

tRNA

rRNA



Complete the diagram by naming the stages shown in protein synthesis:

DNA code

section = gene

Primary mRNA transcript

Excised Introns

+

Mature mRNA transcript

Polypeptide chain

amino acid

amino acid

amino acid

amino acid

amino acid



5. Proteins

Proteins are chains of amino acids held together by peptide bonds. The chain is

called a polypeptide. During protein synthesis amino acids are linked together in

a specific order which is determined by the order of the bases on the DNA

molecule.

amino acids

amino acids become joined together by peptide bonds in a particular genetically determined sequence

hydrogen bonds form between certain amino acids

peptide chain coiled peptide chain folded

peptide chains become

arranged in long parallel

strands

peptide chains become folded

fold e together into a spherical shape

which incorporate another

chemical

peptide chains become folded

together into a spherical shape



An enormous number of different proteins are found in living cells. A human has

over 10,000 different proteins. Proteins can be fibrous, globular or conjugated

(contain a non-protein chemical) and each has structural properties that suit their

role in the organisms body. Use the class resources to complete the table

below.

Post-Translational Modifications

After translation is complete proteins may be modified by cutting and

combining polypeptide chains or by adding phosphate groups and

carbohydrate groups to the polypeptide chain.

The hormone insulin consists of two polypeptide chains that originated as one

chain.

Answer the questions that follow:

1. State the function of a ribosome.

_______________________________________________________________

2. Describe the structure of a tRNA molecule.

_______________________________________________________________

_______________________________________________________________

_______________________________________________________________

3 Name the type of bond that forms between the mRNA molecule and tRNA

molecule within the ribosome.

_______________________________________________________________

Protein Function



4 Name the type of bond that forms between amino acids during protein

synthesis.

_______________________________________________________________

5. Explain how different functional mRNA molecules can be produced from the

same primary transcript sequence.

_______________________________________________________________

_______________________________________________________________

_______________________________________________________________

6. State 3 ways in which a polypeptide chain may be modified following

translation.

_______________________________________________________________

_______________________________________________________________

_______________________________________________________________



1.3 Gene Expression

How well do you rate your knowledge and understanding?

1 2 3

State the meanings of the terms genotype, phenotype and allele.

State that the phenotype of an organism is determined by the proteins produced as a result of gene expression. This is affected by environmental factors acting inside and outside the cell.

Know that only a fraction of the genes inside a cell are expressed.

State that gene expression is controlled by the regulation of transcription and translation.

State the differences between DNA and RNA.

Describe the process of transcription.

State that RNA polymerase is the enzyme responsible for transcription. It unwinds and opens (unzips) up the DNA strand to bring about the synthesis of an mRNA molecule.

State that introns are non-coding regions within a gene and play no part in coding for a polypeptide.

State exons are coding regions within a gene that have a part to play in coding for a polypeptide.

Know that during RNA splicing introns (non-coding regions) within the primary mRNA transcript are removed and exons (coding regions) are joined together to form the mature transcript.

Know that translation of mRNA into a protein takes place at the

ribosome.

Describe the process of translation.

Describe the structure and function of mRNA, tRNA and rRNA.

Different mRNA molecules are produced from the same primary

transcript depending on which exons are included in the mature RNA

transcript.

State that many proteins can come from one gene as a result of alternative mRNA splicing.

Know that proteins can be modified after the translation process by cutting and combining polypeptide chains or by adding phosphate or carbohydrate groups.

Know that proteins have a three dimensional shape- peptide bonds link amino acids together in the polypeptide chain which then folds as a result of hydrogen bonds forming between amino acids. Cross connections may also form between amino acids in one or more chains to form the protein’s final structure.

Know that the final three dimensional structure of the proteins allows it to carry out its function.

Complete:

Column 1 before your Unit assessment

Column 2 before your Prelim

Column 3 before your May exam

Documents

Unit 1: DNA and the Genome Sub-Topic (1.3) Gene Expression€¦ · Higher Biology Pupil Course Notes Duncanrig Secondary LS 2016 page 1 of 17 Unit 1: DNA and the Genome Sub-Topic