Gene Expression

Question 1

Gene Expression - Definition

Answer 1

• Gene expression is the process of of rewriting a gene into a protein
• it involves the transcription of the DNA into mRNA and translation of mRNA into protein

Question 2

Gene Expression

Answer 2

• Every cell in the body contains the same DNA and therefore contains the same genes
• Each gene is a section of DNA that codes for a particular type of protein. Proteins dictate the cell’s functions
• However, every cell does not produce every type of protein coded for by the genes e.g. stomach cells produce the protein pepsin but instestinal cells don’t
• This means that the genes can be ‘switched on’ to produce the protein and ‘switched off’ to stop producing its protein
• When a gene is switched on, we tend to say that the gene is being expressed

Question 3

Cellular control of Gene Expression

Answer 3

• Gene expression is tightly regulated. It begins when RNA polymerase attached to the gene for transcription
• To be transcribed, a gene must first be unpacked from its condensed state
• Many factors determine whether a gene is being expressed (turned on). Some factors include:
  1. Age of the cell
  2. Time of day
  3. Signals from other cells:
• hormones
• metabolites e.g. glucose or lactice acid
• ions
• antigens
• neurotransmitters
  4. The environment e.g. diet, exercise, temperature, UV exposure, education

Question 4

Chromosome

Answer 4

A thread-like structure of nucleic acids and protein (histones) found in the nucleus of most living cells, carrying genetic information in the form of genes

Question 5

Allele

Answer 5

Alternative forms of a gene that occur at a given point on a chromosome

Question 6

Homologous

Answer 6

Pair of chromosomes with the same structural features e.g. centromere, size, length and pattern of genes (defines the same characteristic)

Question 7

Dominant Allele

Answer 7

The allele that is expressed in the phenotype

Question 8

Phenotype

Answer 8

the set of observable characteristics of an individual resulting from the interaction of its genotype and the environment

Question 9

Homozygous

Answer 9

If an organism has two copies of the same allele for the same gene

Question 10

Gene

Answer 10

• the basic physical unit of heredity; a linear sequence of nucleotides along a segment of DNA that provides the coded instructions for synthesis of mRNA, which, when translated into protein, leads to the expression of hereditary character
• a part of a cell that controls or influences the appearance, growth, etc., of a living thing

Question 11

Locus

Answer 11

the chromosomal position of a gene as determined by its linear order relative to the other genes on that chromosome.

Question 12

Gamete

Answer 12

a mature sexual reproductive cell, as a sperm or egg, that unites with another cell to form a new organism

Question 13

Recessive

Answer 13

• relating to or denoting heritable characteristics controlled by genes which are expressed in offspring only when inherited from both parents
• not expressed in the phenotype if a dominant allele is present in the genotype

Question 14

Heterozygous

Answer 14

two different alleles for the same gene

Question 15

Genotype

Answer 15

the genetic makeup of an organism

Question 16

Co-Dominant

Answer 16

Having two dominant alleles in the genotype for the same gene of which both are expressed in the phenotype

Question 17

Multiple alleles

Answer 17

• Three or more alternative forms of a gene (alleles) that can occupy the same locus
• However, only two of the alleles can be present in a single organism. For example, the ABO system of blood groups is controlled by three alleles, only two of which are present in an individual.

Question 18

Monogenic Traits

Answer 18

• traits controlled by a single gene pair (2 alleles) e.g. ability to curl tongue or not (Aa, aa)
• these traits show ‘discontinous variation’ due to limited options

Question 19

Polygenic Traits

Answer 19

• traits controlled by more than one gene pair. The final phenotype is dependent on the interactions of all the gene pairs involved
• e.g. skin colour, hair colour, weight, height, intelligence
• polygenic traits are often influenced by the individuals environment (diet, exercise, education, disease-causing organisms, UV exposure) and as a result produce a wide range of variation in phenotypes
• this range of variation is said to be “continuous variation”

Question 20

Polygenic Inheritance of skin colour 1

Answer 20

• People’s skin colour varies from light to dark and many genes are thought to be involved. NOTE: for this discussion, we will assume 3 independent genes determine skin pigementation
• For each gene there are 2 alleles - each co-dominant (capital letter = dark, lowercase letter = light)
• Someone homozyous with the dark allele across all 3 genes will have the darkest skin possible with the genotype AABBCC
• Someone who is homozygous for the light allele across all 3 genes will have the lightest skin possible with the genotype aabbcc
• Depending on what the genotype is combined with the environmental influence, there are infinite skin colour phenotypes that could be seen

Question 21

Polygenic Inheritance of skin colour 2 - Melanin

Answer 21

• skin colour is due to the presence of the pigment melanin
• melanin is produced in specialised skin cells called melanocytes
• the melanin, synthesized from the amino acid tyrosine, catalyzed by enzymes, is contained in small sacs known as melanosomes
• the melanocytes send the melanosomes to the surruonding skin cells

NOTE: all human skin cells have similar number of melanocytes but the quantity and size of the melanosomes (the more melanin produced, the larger the melanosome) vary depending on what genes you have inherited

• individuals with dark skin contain larger melanosomes with more melanin
• individuals with light skin contain smaller melanosomes with less melanin

Question 22

Environmental influence on skin colour - Gene inheritance

Answer 22

• much of the variation in melanin appears due to variation in 2 major genes that affect the enzyme tyrosinase which affects melanin production
• if you have intereted the gene that allows for the production of the enzyme tyrozinase, required to produce melanin, then you will have larger melanosomes releasing more melanin which are more evently distributed to surrounding cells

Question 23

How can the environment influence this gene expression?

(skin colour)

Answer 23

if you have inherited the gene for enzyme tyrosinase:

• on exposure to UV light, a biochemical process occuring in the melanoctyes is stimulated and results in large amounts of melanin being produced

if you have NOT inherited the gene for the enzyme tyrosinase:

• the melanocytes won’t produce any melanin when stimulated by the UV light
• in this way, environment affects the expression of the genes for skin colour and contributes to a range of skin colours from light to dark*

Question 24

Gene Regulation

Answer 24

• gene expression is tightly regulated and begins when RNA polymerase attached to the gene on the DNA for transcription
• the section of DNA that contains the base sequence to code for a particular protein is called the structural gene
• Promoter gene: allows the RNA polymerase to bind and begin transcription

- Operator gene: the operator gene acts as the on/off switch for the transcription of the structural gene. If a protein structure (e.g. a repressor) binds to the operator gene it will prevent the RNA polymerase from moving past it to reach the structural gene and as a result prevent transcription from occuring

• Regulatory gene: located at one end of the operon or elsewhere. They code for sepcial proteins known as repressors. The repressor protein binds to the operator gene and blocks the movement of the RNA polymerase to the structural gene from the promoter

Question 25

Gene expression - the ‘Lac’ Operon

Answer 25

• E.coli bacteria are able to use both glucose and lactose as a food source
• in order to use lactose they must produce the correct enzyme to digest it. This will only be done when lactose is present in the bacteria’s environment
• when no lactose is present, the gene to code for this enzyme is switched off
• this is due to the regulatory gene producing the repressor which binds to the operator gene preventing the transcription of the structural gene
• But when lactose is present the gene becomes switched on
• The lactose binds to the repressor protein preventing it from binding to the operator gene
• This allows RNA polymerase to move from the promotor to the structural gene to being transcription

Question 26

Gene Regulation - Lac Operon Summary

Answer 26

In the ABSENCE OF lactose

• gene switched off
• regulatory gene produced repressor protein
• repressor binds to operator blocking RNA polymerase from transcribing structural gene

In the PRESENCE of lactose

- gene switched on

• lactose binds to repressor protein preventing it from binding to the operator gene
• RNA polymerase moves from the promoter to the structural gene to begin transcription

Question 27

Multi-Allelic

Answer 27

Characteristic controlled by ONE gene (monogenic) for which there are MORE THAN TWO possibel alleles e.g. ABO blood group. Only a discrete number of phenotypes are observed.

Question 28

Polygenic

Answer 28

Characteristics controlled by MORE THAN ONE GENE, with each gene having two possible alleles e.g. skin colour. A wide spectrum of phenotypes are observed.

Also possible for characteristics to be polygenic and multi-allelic