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Nucleic acids:
DNA = Deoxyribonucleic acid
A double helix made from two strands, DNA is a polymer, where each monomer is known as a nucleotide. A nucleotide has a pentose sugar, a phosphate group and one of four nitrogenous bases (A:T/C:G). Bases join together by complementary base pairing, forming hydrogen bonds between the bases. Three bases in a row code for an amino acid.
RNA = Ribonucleic acid
A single strand version of DNA, where the pentose sugar is ribose not deoxyribose. RNA is used as an intermediate molecule for protein synthesis. RNA has the base U (Uracil) instead of T (Thymine).
The central dogma:
DNA (replicates)
Nucleus
Transcription
mRNA
Translation
RER/Cytoplasm
PROTEIN
Dominant vs recessive alleles:
-Dominant alleles only require one copy of the allele for the phenotype to be expressed.
-Therefore both heterozygous dominant and homozygous dominant both express the dominant phenotype. Huntington's disease (HD) is an example of this.
-Recessive alleles require two copies of the recessive allele in order for the phenotype to be expressed. CF (cystic fibrosis) is an example of this.
Probability of inheritance:
-You may be expected to apply mathematics to this idea of inheritance. For example, how many children would show the cystic fibrosis phenotype if both parents were carriers for the condition and the parents had 8 children?
-This can be taken a step further and you can be expected to draw your own Punnet square (right) from a different parental cross and then again play with the probability of inheritance. For example, a cross between homozygous dominant/heterozygous for a dominant disease such as HD? What are the genotypes of the offspring and how many are affected?
Evolution by way of natural selection:
-Natural selection describes how within a given population, some individuals may be better suited to their environment.
-This variation arises due to random mutation, and may mean a certain individual(s) will be more likely to survive and reproduce.
-The mutation/variation may lead to specific advantageous traits e.g. a longer neck in a giraffe so that it can reach taller trees.
-Since these individuals are more likely to survive/reproduce, they increase the allele frequency for the advantageous form of the gene. Over time, this advantageous allele is passed through generations and over time leads to evolution; the idea that the allele will be more frequent throughout the population.
*This idea differs to Lamarcks's theory of evolution as he stipulated that an organisms trait develops during the course of its lifetime if it uses it more.
Important definitions:
-Gamete: A given organisms reproductive cell e.g. sperm.
-Genome: The DNA of an organism.
-Chromosome: Structure of tightly coiled DNA in nucleus.
-Gene: A section of DNA that codes for a specific protein.
-Allele: An alternative form of a gene.
-Dominant: One copy of allele required for phenotype.
-Recessive: Two copies of allele required for phenotype.
-Homozygous: Both inherited alleles are the same.
-Heterozygous: Two different alleles inherited.
-Genotype: The combination of alleles e.g. Dd.
-Phenotype: The characteristic expressed by the genotype.
-Codominance: Both alleles expressed in the phenotype.
Genetic variation and mutation:
Variation is defined as the genetic differences between organisms. It arises due to a number of different factors:
-Random fertilisation of gametes (each gamete is genetically different. Since only one sperm can fertilise one egg, genetic variation arises here).
-Crossing over (when homologous chromosomes overlap and exchange equivalent portions of genetic material between the non-sister chromatids)
-Independent segregation of chromosomes. Each haploid cell will receive half the amount of genetic material as the parent (haploid).
-Mutation (random changes made to the base DNA code which cause different proteins to be coded for by genes) can be stimulated by mutagenic agents e.g UV light/benzopyrene.
Sexual vs asexual reproduction:
-Asexual reproduction differs from sexual reproduction because it only requires one parent. An example of asexual reproduction is bacteria, which divide by binary fission to form two copies of themselves.
-Since each daughter cell is genetically identical, asexual reproduction produces less variation than sexual reproduction.
-Asexual reproduction is therefore a target for gene projects such as making insulin, which is achieved using bacteria dividing in this way.
-An advantage of asexual reproduction is that it takes less energy and is faster than sexual reproduction.
DOMINANT
RECESSIVE
D
d
D
d
DD
Dd
Dd
dd
F
F
f
f
FF
Ff
Ff
ff
Since a dominant allele occurs in 75% of offspring, this is the likelihood of one of the offspring displaying the phenotype for HD
Since a recessive allele occurs in only 25% of offspring, this is the likelihood of one of the offspring displaying the phenotype for CF
Speciation and evolution:
-Speciation is the idea that one species can form two different species over a long period of time due to things such as geographical isolation etc.
-Geographical isolation can mean that different selection pressures act on each isolated population, meaning that different alleles will be advantageous in each population that develops over time.
-The key idea here is that what defines two different species is the idea that they cannot breed successfully to produce fertile offspring, so when the two species are brought back together and a male and a female is taken from each population, the gametes will not be able to fuse and grow into a fertile offspring, and therefore they are different species.
Evolution through natural selection can be seen when studying the effect of antibiotics on bacteria. Because the antibiotic causes the bacteria to die, it acts as a selection pressure for mutational resistance. This is how dangerous microbes such as MRSA can arise.
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