Studied by 29 peopleWhat is sexual reproduction?
-Type of reproduction.
-Involves the production of gametes by meiosis.
-A gamete from each parent fuses to form a zygote.
-Genetic information from each gamete is mixed so the resulting zygote is unique.
What are gametes?
-Sex cells (sperm cells, egg cells).
-Haploid (half the number of chromosomes).
What is meiosis?
-Form of cell division involved in the formation of gametes.
-Chromosome number is halved.
-Involved two divisions.
What must occur prior to meiosis?
Interphase.
What happens during the first stage of meiosis?
-Chromosome pairs line up along the cell equator.
-The pair of chromosomes are separated and move to opposite poles of the cell.
-Chromosome number is halved.
What happens during the second stage of meiosis?
-Chromosomes line up along the cell equator.
-The chromatids are separated and move to opposite poles of the cell.
-Four unique haploid gametes are produced
Why is meiosis important for sexual reproduction?
-It increases genetic variation.
-It ensures that the resultant zygote is diploid.
What is the advantage of sexual reproduction?
It creates genetic variation, increasing the probability of a species adapting to and surviving environmental changes.
Describe the disadvantages of sexual reproduction.
-Two parents are required. This makes reproduction difficult in endangered populations or in species which exhibit solitary lifestyles.
-More time and energy is required so fewer offspring are produced.
What is asexual reproduction?
-Type of reproduction.
-Involves mitosis.
-Produces genetically identical offspring known as daughter cells.
Describe the advantages of asexual reproduction.
-Only one parent is required.
-Lots of offspring can be produced in a short period of time, enabling the rapid colonisation of an area and reducing competition from other species.
-Requires less energy.
What is the disadvantage of asexual reproduction?
No genetic variation, reducing the probability of a species being able to adapt to environmental change.
What is DNA?
A double-stranded polymer of a nucleotides, wound to form a double helix.
What are the monomers of DNA?
Nucleotides.
What are DNA nucleotides made up of?
-Common sugar.
-Phosphate group.
-One of four bases: A, T, C or G.
State the full names of the four bases found in nucleotides.
-Adenine.
-Thymine.
-Cytosine.
-Guanine.
Describe how nucleotides interact to form a molecule of DNA?
-Sugar and phosphate molecules join to form a sugar-phosphate backbone in each DNA strand.
-Base connected to each sugar.
-Complementary base pairs (A pairs with T, C pairs with G) joined by weak hydrogen bonds.
Define genome.
The entire genetic material of an organism.
What is a chromosome?
A long, coiled molecule of DNA that carries genetic information in the form of genes.
Define gene.
A section of DNA that codes for a specific sequence of amino acids which undergo polymerisation to form a protein.
Describe the method used to extract DNA from fruit.
1) Place a piece of fruit in a beaker and crush it.
2) Add detergent and salt, mix.
3) Filter the mixture and collect the liquid in a test tube.
4) Pour chilled ethanol into the test tube.
5) DNA precipitates forming a fibrous white solid.
6) Use a glass rod to collect the DNA sample.
Why is detergent added to the crushed fruit?
It disrupts the cell membranes, releasing DNA into solution.
Why is salt added to the crushed fruit?
Salt encourages the precipitation of DNA.
Why is chilled ethanol added rather than water?
DNA is insoluble in ethanol, encouraging its precipitation.
Explain how a gene codes for a protein.
-A sequence of three bases in a gene forms a triplet.
-Each triplet codes for an amino acid.
-The order of amino acids determines the structure (i.e. how it will fold) and function of protein formed.
Why is the ‘folding’ of amino acids important in proteins such as enzymes?
The folding of amino acids determines the shape of the active site which must be highly specific to the shape of its substrate.
What is protein synthesis?
The formation of a protein from a gene.
What are the two stages of protein synthesis?
1) Transcription.
2) Translation.
What does transcription involve?
The formation of mRNA from a DNA template.
Outline transcription.
1) DNA double helix unwinds.
2) RNA polymerase binds to a specific base sequence of non-coding DNA in front of a gene and moves along the DNA strand.
3) RNA polymerase joins free RNA nucleotides to complementary bases on the coding DNA strand.
4) mRNA formation complete. mRNA detaches and leaves the nucleus.
Describe the difference between mRNA and DNA.
-mRNA is single stranded whereas DNA is double stranded.
-mRNA uses U whereas DNA uses T.
Why is mRNA used in translation rather than DNA?
DNA is too large to leave the nucleus so cannot reach the ribosome.
What does translation involve?
A ribosome joins amino acids in a specific order dictated by mRNA to form a protein.
Outline translation.
1) mRNA attached to a ribosome.
2) Ribosome reads the mRNA bases in triplets. Each triplet codes for one amino acid which is brought to the ribosome by a tRNA molecule.
3) A polypeptide chain is formed from the sequence of amino acids which join together.
How is a tRNA molecule adapted to its function?
Each tRNA molecule has an anticodon which is specific to the codon of the amino acid that it carries.
What is mutation?
A random change in the base sequence of DNA which results in genetic variants.
Describe the effect of a gene mutation in coding DNA.
-If a mutation changes the amino acid sequence, protein structure and function may change.
-If a mutation does not change amino acid sequence, there is no effect on protein structure or function.
What in non-coding DNA?
DNA which does not code for a protein but instead controls gene expression.
Describe the effect of a gene mutation in non-coding DNA.
-A mutation may affect the ability of RNA polymerase to bind to non-coding DNA.
-This may result affect protein production and the resulting phenotype of the organism.
Outline how the work of Mendel helped scientists to develop their understanding of genetics.
-Mendel studies the inheritance of different phenotypes of pea plants.
-He established a correlation between parent and offspring phenotypes.
-He noted that inheritance was determined by ‘units’ passed on to descendants.
-Using gene crosses, he devised the terms ‘dominant’ and ‘recessive’.
Why was Mendel’s work initially overlooked?
Scientist’s didn’t understand Mendel’s work as there was no knowledge of genes or DNA at the time.
What is a chromosome?
A long, coiled molecule of DNA that carries genetic information in the form of genes.
Define gene.
A section of DNA that codes for a specific sequence of amino acids which undergo polymerisation to form a protein.
What are alleles?
Different versions of the same gene.
Define genotype.
An organism’s genetic composition, describes all alleles.
Define phenotype.
An organism’s observable characteristics due to interactions of the genotype and environment (which can modify the phenotype).
Define homozygous.
Having two identical alleles of a gene e.g. FF or ff.
Define heterozygous.
Having two different alleles of a gene e.g. Ff.
What is a dominant allele?
Describes an allele that is always expressed. Represented with a capital letter e.g. F.
What is a recessive allele?
An allele that is only expressed in the absence of a dominant allele. Represented with a small letter e.g. f.
What is monohybrid inheritance?
The inheritance of a single gene.
What is the problem with single gene crosses?
Most characteristics are controlled by multiple alleles rather than just one.
What are sex chromosomes?
A pair of chromosomes that determine sex:
-Males have an X and a Y chromosome.
-Females have two X chromosomes.
Why does the inheritance of a Y chromosome mean that an embryo develops into a male?
Testes development in an embryo is stimulated by a gene present on the Y chromosome.
Other than using a punnet square, how else can monohybrid inheritance be represented?
Using a family pedigree.
What is a sex-linked characteristic?
A characteristic that is coded for by an allele found on a sex chromosome.
Why are the majority of genes found on the X chromosome rather than the Y chromosome?
The X chromosome is bigger than the Y chromosome so more genes are carried on it.
Why are men more likely to show the phenotype for a recessive sex-linked trait than women?
-Many genes are found on the X chromosome that have no counterpart on the Y chromosome.
-Women (XX) have two alleles for each sex-linked gene whereas men (XY) often only have one allele therefore only one recessive allele is required to produce the recessive phenotype in males.
Name the four different blood groups.
A, B, AB, O.
What are codominant alleles?
Alleles that equally contribute to an organism’s phenotype. They are expressed to an equal extent.
What are the two causes of variation within a species?
-Genetics.
-Environment.
What is genetic variation?
-Variations in the genotypes of organisms of the same species due to the presence of different alleles.
-Creates differences in phenotypes.
What creates genetic variation in a species?
-Spontaneous mutations.
-Sexual reproduction.
What is mutation?
A random change to the base sequence in DNA which results in genetic variants.
State the three types of gene mutation.
-Insertion.
-Deletion.
-Substitution.
How may a gene mutation affect an organism’s phenotype?
-Neutral mutation does not change the sequence of amino acids. Protein structure and function same. No effect on phenotype.
-Mutation may cause a minor change in an organism’s phenotype e.g. change in eye colour.
-Mutation may completely change the sequence of amino acids. This may result in a non-functional protein. Severe changes to phenotype.
What is environmental variation?
-Variations in phenotype that are acquired during the lifespan of an organism.
-Due to environmental factors e.g. diet, lifestyle, climate, exposure to light, etc.
What is the Human Genome Project?
-Scientific research project involving thousands of scientists across the globe which successfully mapped the entire human genome.
-Scientists now aim to identify the function of every gene in the human genome.
How can the results of the Human Genome Project be applied to medicine?
-Enables scientists to understand how lifestyle factors interact with genes- identifying predisposition to disease and possible preventions.
-Disease-causing alleles identified more rapidly and the appropriate treatments prescribed earlier on.
-Scientists can predict an individual’s response to certain drugs. New drugs can be developed which are tailored to a specific allele.
What are the drawbacks associated with the discoveries of the Human Genome Project?
-Knowledge of predisposition to a disease can be stressful.
-Societal pressure influencing the decision to have children.
-Discrimination by employers, insurance firms, etc.
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