Genetics A2 OCR Biology

Genes

A gene is a length of DNA, it is a sequence of nucleotide bases that code for one or more polypeptides.

There are around 25000 genes in the human genome. A few are found in the mitochondria and are situated on chromosomes within the nucleus.

Each gene is located on a specific place called a locus.

Genetic Code

The sequence of nucleotide bases on a gene provides a code , with instructions for the constuction of a protein.

Characteristics:

• Triplet code - 3 bases codes for one amino acid
• Degenerate code - All amino acids have more than one code
• Widespread but not universal

Transcription

First stage of protein synthesis

• Gene unwinds and unzips by dipping into the nucleolus. Hydrogen bonds break.
• RNA nucleotides bind to the exposed complementary bases. Catalysed by RNA polymerase.
• U with A, G with C, and A with T.
• The extra phosphoryl groups are released, the energy is used to bond the nucleotides together.
• mRNA is produced and is released from the DNA and passes out of the nucleus, via a nuclear pore, to a ribosome

Translation

Second stage of protein synthesis

• Transfer RNA is made in the nucleus and passes into the cytoplasm
• A molecule of mRNA binds to a ribosome
• 2 codons attach to the small subunit of the ribosome.
• Using ATP energy and an enzyme the tRNA binds with the first codon usually AUG to UAC. Hydrogen bonds are formed.
• A second tRNA binds to the 2nd codon with complementary anticodon.
• Peptide bond forms between the 2 adjacent amino acids. Enzyme catalyses this reaction.
• The ribosome moves along the mRNA, reading the next codon.
• A 2rd tRNA brings another amino acid and a peptide bond forms.
• The first tRNA leaves and is able to collect and bring another of its amino acids
• The chain grows until a stop codon is attached.

Types of Mutations

Mutation - is a change in the amount of, or arrangement of, the genetic material in a cell.

Chromosome mutation- involve changes to parts of or whole chromosomes

DNA mutations - are changes to genes due to changes in nucleotide base sequences

2 types:

• Point mutations - in which base pairs replace another. (substitutions)
• Insertion/deletion mutations - in which one or more base pairs are inserted or deleted from a length of DNA. May cause a frameshift.

Neutral effects

If a gene is altered by a change to its base sequence, it becomes another version of the same gene. It is an allele of the gene.

It may produce no change to the organism if:

• Mutation if in a non-coding region of the DNA
• It is a silent mutation, although it has changed it still codes for the same amino acid. Protein is unchanged.

The altering could produce a change to the structure of the protein. A different characteristic will occur but this characteristic does not give any advantage or disadvantage to the organism.

Harmful or beneficial effects

Example: Skin Colour and vitamin D.

Lac Operon

Enzyme induction - inducible enzymes are synthesised at varying constant rate, according to the cells circumstances. Bacteria adapt to their environment by producing enzymes to metabolise differe nutrients only when they are present.

Operon - is a length of DNA, made up of structural genes and control sites.

Structural genes - Code for proteins such as enzymes. Consists of a sequences of base pars that can be transcribed into a length of mRNA.

Operator region - is a length of DNA. It switches on and off the structural gene.

Promoter region - is a length of DNA. Binding site of an enzyme to befin the transcription of the structural genes.

Method when nutrient is absent

• Regulator gene is expressed and the repressor protein is synthesised.
• Repressor protein has 2 binding sites; one that binds to the nutrient and one that binds to the operator region.
• Repressor protein binds to the operator region and covers the promoter region.
• RNA polymerase can not bind to the promoter region so the structural genes can not be transcribed into mRNA.
• Without mRNA these genes cannot be translated and the enzymes cannot be synthesised.

Method when nutrient is present

• Nutrient molecules bind to the other site on the repressor protein. This cause the repressor protein to change shape so it cannot bind with the operator region.
• The repressor protein breaks away from the operator region.
• This leaves the promoter region unblocked. RNA polymerase can now bind to it and initiate the transcription of mRNA for the structural genes.
• The operator repressor inducer system acts as a molecular switch. It allows transcription and subsequent translation of the structural genes into the lac enzymes.
• As a result the bacteria can use the enzyme to take up the nutrient from the medium into their cells. They can use these nutrients for growth and energy.

Body plans

Drosphila development:

• A multinucleate syncytium is formed
• After the 8th division, the 256 nuclei migrate to the other part
• By the 11th division the nuclei form an outer layer around a central, yolk-filled core.
• The division rate slows, and the nuclear genes switch from replicating to transcribing.
• The plasma membrane invaginates around the 6000 nuclei and the resulting cells form a single outer layer.
• After 2-3 hours the embryo divides into a series of segments
• These correspond to the body plan.
• 3 segments merge to form the head, 2 thoracic segments and 8 abdominal segments.

Genetic control

The development is genetically mediated by homeobox genes.

Some genes determine the embryo's polarity

Other genes called segmentation genes, specify the polarity of each segment

Homeotic selector genes specify the identity of each segment and direct the development of individula body segments.

There are 2 gene families:

• Complex that regulates development of thorax and abdomen segments
• Complex that regulate development of head and throax segments

Homeobox genes - Contain a sequence of 180 base pairs and this sequence produces plypeptides of about 60 amino acids. Homeobox genes are arranged in clusters known as Hox clusters.

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Retinoic acid - Derivative of vitamin A. It activates homeobox genes. It is Morphogen, a substance that governs the pattern of tissue development

Apoptosis

Apoptosis is programmed cell death that occurs in multicellular organisms.

Apoptosis is an orderly and tidy cell dealth where as necrosis is an untidy and damaging cell death that occurs after trauma and releases hydrolytic enzymes.

Sequence of events

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

Controlling Apoptosis

The process is controlled by a diverse range of cell signals, some of which come from inside te cells and some from outside.

The signals include 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

Proteins are released into the the cytosol. These proteins bind to apoptosis inhibitor proteins and allow the process to take place.

Apoptosis and development

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 to seperate from each other.

The rate of cells dying should balance the rate of cells produced by mitosis

If the rates are not balanced:

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

Meiosis

Meiosis - is a reduction division. The resulting daughter cells have half the original number of chromosomes. They are haploid and can be used for sexual reproduction.