Genetic Information, Variation and Relationships

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Genes and the Genetic Code

Gene - a section of DNA that contains the coded information for making polypeptides and functional RNA

A gene is a section of DNA located at a particular position, called a locus, on a DNA molecule. The gene is a base sequence of DNA that codes for; the amino acid sequence of a polypeptide or a functional RNA, including rRNA and tRNA

Features of the Genetic Code

  • A few amino acids are coded for by only a single triplet
  • The code is known as a 'degenerate code' because most amino acids are coded for by more than one triplet
  • The code is non overlapping as the sequence of the bases is only read once e.g. 123456 is read as 123 and 456
  • The code is universal meaning all amino acids are coded by the same triplets

Only certain sequence code for an amino acid which are called exons

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DNA in Prokaryotic and Eukaryotic Cells

Prokaryotic Cells

  • DNA molecules are shorter
  • DNA is circular
  • DNA is not associated with any proteins
  • Prokaryotic cells therefore do not have chromosomes

Eukaryotic Cells

  • DNA molecules are longer
  • DNA is linear rather than circular
  • DNA is associated with the protein histones to form chromosomes
  • The mitochondria and chloroplast contain circular DNA not associated with proteins
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Chromosome Structure

1) DNA is wound around histones to fix it in position forming a DNA-histone complex which is then further coiled

2) The coil is then looped and further coiled before being packed into the chromosome allowing it to be condensed into a single chromosome

Homologous Chromosomes - one pair of maternal chromosomes and one pair of paternal chromosomes. A homologous pair is always two chromosomes that carry the same genes but not the same alleles of the genes e.g. blood group

Allele - one of a number of alternative forms of a gene. Each individual inherits one allele from each of its parents which can be the same or different so produces the same or different polypeptide chain. Any changes of the base sequence of a gene produces a new allele (mutation) which ocassionaly lead to the protein not functioning properly

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Structures of Ribonucleic Acid

Codon - the sequence of three bases on mRNA that codes for a single amino acid

Genome - the complete set of genes in a cell, including those in the mitochondria and chroloplasts

Proteome - the full range of proteins produced by the genome

RNA Structure - it forms a single strand in which each nucleotide is made up of the pentose sugar ribose, A, G, C and U and a phosphate group. mRNA and tRNA are important in protein synthesis

Messenger RNA (mRNA) - mRNA is a long strand arranged in a single helix.The base sequence is determined by the sequence of bases on DNA from transcription. Once formed it can leave the nucleud to the cytoplasm where it associates with ribosomes and acts as a template for protein synthesis

Transfer RNA (tRNA) - single stranded chain extending beyond the other allowing an amino acid to easily attached. At the opposite end  is a sequence of three base known as an anticodon

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Transcription and Splicing

Transciption - the process of making pre-mRNA using part of the DNA as a templete

1) An enzyme acts on a specific region of the DNA causing the two strands to separate and expose the nucleotide bases in that region

2) The nucleotide bases on one strand (template strand) pair with their complementary nucleotide from the nucleus. The enzyme RNA polymerase then moves along

3) The strand is then joined together to form a pre-mRNA molecule. The DNA strand jejoins behnd it so only 12 base pairs are exposed

Splicing of Pre-mRNA - the DNA of a gene is made up of sections called exons that codon for proteins and introns that do not. Therefore the introns would prevent the synthesise of a polypeptide. Therefore introns are removed and the functional exons are joined together during a process called splicing

Prokaryotic cells do not have introns so splicing of their DNA is unnecessary

mRNA then is attracted to ribosome which then attach so it is ready for translation

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Translation

1) A ribosome becomes attached to the starting codon at one end of the mRNA molecule

2) The tRNA molecule with the complementary anticodon sequence moves to the ribosome and pair up with the codon on the sequence of mRNA. This tRNA carries a specific amino acid

3) A tRNA with molecule with a complementary anticodon pairs with the next codon on the mRNA carrying another amino acid

4) The ribosome move along the mRNA, bringin together two tRNA molecules at any one time, each pairing up with the corresponding two codons on the mRNA

5) The two amino acids on the tRNA are joined by a peptide bond using an enzyme and ATP which is hydrolysed to provide the required energy

6) The first tRNA is released so it can collect another amino acid and the process continues as multiple ribosomes can pass immediately behind the first

7) The synthesis of a polypeptide continues until a ribosome reaches a stop codon where the polypeptide chain is complete

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Gene Mutation

Mutation - any change to the quantity or the base sequence of the DNA of an organism 

Gene Mutation - a change in the sequence of the bases in the DNA 

Substitution of Bases - a nucleotide in a DNA molecule is replaced by another nucleotide that has a different base. The significance of this will depend upon the precise role of the amino acid as they may determine the tertiary structure of the final protein. Therefore the protein may then be a different shape and therefore not function properly 

Deletion of Bases - when a nucleotide is lost from the normal DNA sequence. This changes the sequence of bases in DNA read in units of three bases. One deleted nucleotide causes all triplets in a sequence to be read differently 

Chromosome Mutations - changes to the structure or number of whole chromosomes. 

  • Changes in whole sets of chromosomes - when the organism has three or more sets of chromosomes rather than two 
  • Changes in the number of individual chromosomes - individual homologous pairs of chromosomes fail to separate leading to one more or one fewer chromosomes e.g. Down's syndrome
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Meiosis and Genetic Variation

Meiosis involves two nuclear divisions that normally occur immediately one after the other:

1) In meiosis 1 homologous chromosomes pair up and their chromatids wrap around each other.The chromatids may be exchanged by crossing over. By the end of this division the homologous pairs separate with one chromosome from each pair going into one of the daugther cells 

2) In meiosis 2 the chromatids move apart. At the end of meiosis 2, four cells have usaully been formed. Each of these cells contain 23 chromosomes 

Meiosis brings about genetic variation by:

  • Independent Segregation - during meiosis 1 the chromosomes line up with the homologous partner. Depending on the line up of the chromosomes depends on which pair goes into one daugther cell. Although pairs of homologous pairs have the same genes they have different alleles so produces new genetic combinations 
  • Crossing Over - as the chromatids become twistwed around each other portion of the chromosomes break offand rejoin with the other homologous pair. This creates variation as new combinations of maternal and paternal alleles are produced
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Genetic Diversity and Adaptions

Genetic Diversity - the total number of different alleles in a population

Population - a group of individuals of the same species that live in the same place so can interbreed 

The greater the genetic diversity, the more likely that some individuals in a population will survive an environmental change 

Reproductive Success and Allele Frequency 

  • Within any population of a species there will be a gene pool containing a wide variety of alleles
  • Random mutation of a new allele of a gene might give an individual an advantage. These individuals are more likely to gain more resources and reproduce successfully 
  • Therefore the new allele is passed to the next generation
  • Over many generation the number of the new allele will increase 
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Types of Selection

There are two types of selection:

  • Selection may favour individuals that vary in one direction from the mean of the population. This is called directional selection and changes the characteristics of the population. Therefore if the environmental conditions change, the phenotypes are best suited to the new conditions are most likely to survive e.g. bacteria is resistant to penicillin 
  • Selection may favour average individuals. This is called stabilising selection and preserves the characteristics of a population. Therefore if environmental conditions remain stabl, the individuals with phenotypes closet to the mean are favoured e.g. weight of babies 

Natural selection results in species that are better adapted to the environment that they live in. These adaptations may be:

  • Anatomical e.g. thicker fur for artic foxes 
  • Physiological e.g. kangaroos oxidising fats to produce water 
  • Behavioural e.g. migration of swallows from the UK to Africa 
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Species and Courtship Behaviour

Species - are capable of breeding to produce living, fertile offspring.

Organisms are identified by the binomial system. Its features are:

  • Universal system based on Latin or Greek names 
  • First name is the generic name so denotes the genus to which the organism belongs
  • The second name is the specific name denotes the species to which the organism belongs 

Courtship Behaviour 

  • Recognise members of their own species to ensure that mating only takes place between the same species to produce fertile offspring 
  • Identify a mate that is capable of breeding as both partners need to be sexually mature, fertile and receptive to mating 
  • Form a pair bond to lead to successful raising of offspring 
  • Synchronise mating 
  • Become able to breed
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Classification and Taxonomy

There are two main forms of biological classification:

  • Artificial classification divides organisms according to differences that are useful so are analogous characteristics where they have the same function but do not have the same evolutionary origins 
  • Phylogenetic classification - is based upon the evolutionary relationships between organisms and their ancestors, classifies species into groups using shared features derived from their ancestors and arranges the groups into a hierarchy in which the groups are contained within larger composite groups with no overlap

Homologous characteristics have similar evolutionary origins regardless of their functions in the adult of a species.

Bacteria are a group of single-celled prokaryotes with the following features; the absence of membrane-bounded organelles, unicellular, ribosomes are 70S etc 

Archaea are a groups of single-celled prokaryotes that were originally classified as bacteria which they resemble in appearance 

Eukarya are a group of organisms made up of one or more eukaryotic cells 

Classification - Domain, Kingdom, Phylum, Class, Order, Family, Genus and Species 

Phylogeny - the order of taxonomic ranks is based upon the supposed evolutionary line of descent of group members 

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Diversity Within a Community

Biodiversity - refers to the number and variety of living organisms in a particular area

Has three components:

  • Species diversity - refers to the number of different species and the number of individuals of each species within a community 
  • Genetic diversity - refers the variety of genes possessed by the individuals that make up a population of a species 
  • Ecosystem diversity - refers to the range of different habitats from a small local habitat to the whole of the Earth 

Measuring Diversity 

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Species Diversity and Human Activities

Impact of Agriculture - ecosytems are controlled by humans so as a result the number of species and the genetic variety or alleles they possess desired features. In addition pesticides are used to exclude these species because they compete for light, mineral ions, water and food required by the farmed species. The index of species diversity is therefore low in agricultural ecosystems 

The Balance between Conservation and Farming - certain practices have directly removed habitats and reduced species diversity e.g. 

  • Removal of hedgerows and grubbing 
  • Replacing natural meadows with crops 
  • Filling in ponds and draining marsh and other wetland 
  • Use of pesiticides and inorganic fertilisers 

Examples of conservation techniques include:

  • Maintaining hedgerows in an A shape 
  • Plant hedges rather than fences 
  • Use crop rotation 
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Investigation Diversity

Comparison of Observable Characteristics - measured by observing the characteristics or organisms 

Comparison of DNA Base Sequences - analysis of the patterns allows us to compare one species with another or one individual with another of the same species to determine how diverse they are. Species that are more closely related show more similarity in their DNA base sequences than species more distantly related

Comparison of the Base Sequence of mRNA - the base sequence of mRNA are complementary to those of the strand of DNA form which they are made

Comparison of Amino Acid Sequences in Proteins  - the degree of similarity in the amino acid sequence of the same protein in two species reflects how closely related the two species are 

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