Cellular Control

Cellular Control

state that genes code for polypeptides including enzymes

Gene – a length of DNA that codes for one or more polypeptides, including enzymes.

Polypeptide – a polymer consisting of a chain of amino acids residues joined by peptide bonds.

Protein – a large polypeptide – usually 100 or more amino acids. Some proteins consist of one polypeptide chain and some consist of more than one polypeptide chain.

Genome – the entire DNA sequence of that organism. The human genome consists of about 3 billion nucleotide base pairs.

explain the meaning of the term genetic code

Genetic Code – the sequence of nucleotide bases on a gene that provides the codes for the construction of a polypeptide of a protein. The characteristics of the genetic code includes:

Triplet code – a sequence of three nucleotide bases codes an amino acid.
Degenerate code – all amino acids (except methionine) have more than one code.
Stop codes – indicates the end of a polypeptide chain (doesn’t correspond to an amino acid).
Widespread but not universal – where the same base sequence codes for similar polypeptides in different organisms, but are not always identical

describe, with the aid of diagrams, the way in which a nucleotide sequence codes for the amino acid sequence in a polypeptide

Transcription – the creation of a single-stranded mRNA copy of the DNA coding strand.

Messenger RNA (mRNA) – activated nucleotides that uses complementary base pairing to the template strand to be arranged identically to the coding strand, other than uracil being present and thymine being absent. They carry the codons that are used to be make the polypeptide.

The gene unwinds and unzips, by the length of the DNA that makes up the gene dips into the nucleolus. The hydrogen bonds between the complementary bases break forming two

Template strand – the strand of DNA that is used to help make mRNA. RNA nucleotides use the template strand to make mRNA through complementary base pairing between the template strand and mRNA.
Coding strand – the strand of DNA that is identical to mRNA, other than the base uracil being present in preference to thymine.

Activated RNA nucleotides bind, using hydrogen bonds, to the exposed bases on the template strand, through complementary base pairing (U binds with A, C binds with G), catalysed by the enzyme RNA polymerase.
Two extra phosphoryl groups are released, which releases energy for bonding adjacent nucleotides.
mRNA is formed and passes out of the nucleus through a pore in the nuclear envelope, to a ribosome.

describe, with the aid of diagrams, how the sequence of nucleotides within a gene is used to construct a polypeptide, including the roles of messenger RNA, transfer RNA and ribosomes

Translation – the assembly of polypeptides (proteins) at ribosomes.

Transfer RNA (tRNA) – lengths of RNA that fold into hairpin shapes and have three exposed bases at one end where a particular amino acid can bind. At the other end of the molecule are three unpaired nucleotide bases, known as an anticodon.

mRNA binds to a ribosome. Two codons are attached to the small subunit of the ribosome. The first codon is always AUG, which codes for methionine. A tRNA with methionine and the anticodon UAC forms hydrogen bonds with this codon.
The next tRNA, with another amino acid, binds to the second exposed codon on the mRNA with its complementary anticodon.
A peptide bond forms between the two adjacent amino acids. This repeats forming a polypeptide.
The last codon on the mRNA is called the stop codon (UAA, UAC or UGA), which stops translation.
The rough endoplasmic reticulum packages the polypeptide into a vesicle, which moves to the Golgi apparatus to give the protein its final secondary/tertiary structures.

state that mutations cause changes to the sequence of nucleotides in DNA molecules

Mutation – a change in the amount of or arrangement of the genetic material in a cell, by base deletion, addition, substitution or by inversion or repeat of a triplet. Mutations cause changes to the sequence of nucleotides in DNA molecules.

Mutations may occur during DNA replication. Certain substances (mutagens) may cause mutations, including tar found in tobacco, UV light, X-rays and gamma rays. Mitotic mutations are somatic mutations and are not passed on to offspring. Meiotic mutations and gamete formation can be inherited (passed on to offspring).

What types of mutation are there?

Insertion/deletion mutations – in which one of more nucleotide pairs are inserted or deleted from a length of DNA, causing a frameshift – the amino acid sequence is altered after the insertion/deletion point.
Point mutations/Substitution – in which one base pair replaces

Nonsense – introduces a premature stop codon, stopping translation early, giving a truncated
Missense – changes the codon, changing the amino acid produced, so there is a change in the tertiary structure.
Silent – changes the codon, but the amino acid produced is not changed, so the amino acid sequence remains the same.

explain how mutations can have beneficial neutral or harmful effects on the way a protein functions

Allele – an alternative version of a gene. It is still at the same locus on the chromosome and codes for the same polypeptide but the alteration to the DNA base sequence may alter the protein’s structure.

Mutations with Neutral Effects:

If a gene is altered by a change to its base sequence, it becomes another version of the same gene – an allele. It may produce no change to the organism if:

The mutation is in a non-coding region of the DNA.
It is a silent mutation – the base triplet changes but still codes for the same amino acid, so the protein is unchanged.

If a mutation does cause a change to the structure of the protein, and therefore different characteristics, but the changed characteristic gives no particular advantage or disadvantage to the organism, then the effect is also neutral.

Mutations with Harmful or Beneficial Effects:

Early humans in Africa almost certainly had dark skin. The pigment melanin protected from the harmful effects of ultraviolet light. However, they could still synthesise vitamin D from the action of the intense sunlight on their skin. This is an important source of vitamin D, because much of the food that humans eat contains very little vitamin D.

The Inuit people have not lost all their skin pigments, although they do not live in an environment that has intense sunlight. However, they eat a lot of fish and seal meat, including the blubber, both rich sources of dietary vitamin D. Depending on the environment, the same mutation for paler skin can be beneficial or harmful. Individuals within a population who have a certain characteristic may be better adapted to the new environment. The well-adapted organisms can out-compete those in the population that do not have the advantageous characteristics. This is natural selection, the mechanism for evolution. Without genetic mutations there would be not evolution.

state that cyclic AMP activates proteins by altering their three-dimensional structure

Some proteins have to be activated by a chemical, cyclic AMP that, like ATP, is a nucleotide derivative. Cyclic AMP activates proteins by altering their three-dimension structure, so that their shape is a better fit to their complementary molecules.

explain genetic control of protein production in a prokaryote using the lac operon

coli normally respires glucose but it can also use lactose as a respiratory substrate. E. coli grown in a culture medium with no lactose can be placed in a medium with lactose. At first they cannot metabolise the lactose because they only have tiny amounts of the two enzymes needed to metabolise it. A few minutes after lactose is added to the culture medium, E. coli bacteria increases the rate of synthesis if the enzymes by about 1000 times. Lactose must trigger the production of the two enzymes, and is known as the inducer.

The lac operon is a section of DNA within the bacterium’s DNA, consisting of:

Structural genes – the enzymes:
β-galactosidase – breaks down lactose to glucose and galactose.
Lactose permease – helps the cell to absorb lactose/increase uptake of lactose.
Control sites:
Operator region – binds to the repressor and can switch on and off the structural genes.
Promoter region – binds to RNA polymerase and controls transcription.

The regulator gene controls the production of repressor protein. This repressor molecule binds to the operator region, preventing RNA polymerase binding to the promoter region and preventing transcription. Therefore the structural genes are switched off and lactose is not broken down.

explain that the genes that control the development of the body plans are similar in plants, animals and fungi, with reference to homeobox sequence

Homeobox genes – regulatory genes that codes for proteins that controls the development of body plans.

Homeobox genes each contain a sequence of 180 base pairs (homeobox) coding for the homeodomain. The homeodomain on the protein is able to bind to DNA, switching it on or off controlling transcription (the protein is the transcription factor). Homeobox genes are arranged into clusters known as Hox clusters. Some organisms have more Hox clusters than others.

Homeobox genes genetically mediate development of organisms:

Maternal-effect genes – determine the embryo’s polarity (which end is the head (anterior) and which end is tail (posterior)).
Segmentation genes – specify the polarity of each segment.
Homeotic selector genes – specifies the identity of each segment and direct the development of individual body segments. These are the master genes in the control networks of regulatory genes. There are two gene families:
The complex that regulates development of thorax and abdomen
The complex that regulates development of head and thorax

Mutations of these genes can change one body part to another. This can be seen in the condition known as antennapedia – where the antennae of Drosophila look more like legs.

outline how apoptosis (programmed cell death) can act as a mechanism to change body plans

Apoptosis – programmed cell death that occurs in multicellular organisms. Cells should undergo about 50 mitotic divisions (the Hayflick constant) and then undergo a series of biochemical events that leads to an orderly and tidy cell death. This is in contrast to cell necrosis, an untidy and damaging cell death that occurs after trauma and releases hydrolytic enzymes. The apoptosis process occurs very quickly.

Enzymes break down the cell cytoskeleton.
The cytoplasm becomes dense, with organelles tightly packed.
The cell surface membrane changes and small bits called blebs
Chromatin condenses and the nuclear envelope breaks. DNA breaks into fragments.
The cell breaks into vesicles that are taken by phagocytosis. The cellular debris is disposed of and does not damage any other cells or tissues.

Apoptosis is controlled by a diverse range of cell signals, including cytokines made by cells of the immune system, hormones, growth factors and nitric oxide. Nitric oxide can induce apoptosis by making the inner mitochondrial membrane more permeable to hydrogen ions and dissipating the proton gradient. Protons are released into the cytosol. These proteins bind to apoptosis inhibitor proteins and allow the process to take place.

Apoptosis is an integral part of plant and animal tissue development. The excess cells shrink, fragment and are phagocytosed so that the components are reused and no harmful hydrolytic enzymes are released into the surrounded tissue. Apoptosis is tightly regulated during development, and different tissues use different signals for inducing it. It weeds out ineffective or harmful T lymphocytes during the development of the immune system.

During limb development apoptosis causes the digits (fingers and toes) to separate from each other.

Children between the ages of 8-14 years, 20-30 billion cells per day undergo apoptosis. In adults, 50-70 million cells per day undergo apoptosis. If the rates are not balanced:

Not enough apoptosis leads to the formation of tumours.
Too much apoptosis leads to cell loss and degeneration.

Cell signalling plays a crucial role in maintaining the right balance.