Topic 3 - genetics Flashcards
What 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
What is meiosis?
A form of cell division involved in the formation of gametes
Chromosome number is halved
Involves 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 chomatids are separated and move to opposite poles of the cell
Four unique haploid gametes are produced
Why is meiosis important for sexual reproduction?
In increases genetic variation
In ensured 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 environment changes
Describe the disadvantages of sexual reproduction
More time and energy is required so fewer offspring are produced
Two parents are required
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
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 nucleotides, would 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 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
Place a piece of fruit in beaker and crush
Add detergent and salt, mix
Filter the mixture and collect the liquid in a test tube
Pour chilled ethanol into the test tube
DNA precipitates forming a fibrous white solid
Use a glass rod to collect the DNA sample
Why is detergent added to the crushed fruit?
It distrusts cell membranes, releasing DNA into solution
Why is salt added to the crushed fruit?
It encourages the precipitation of DNA
Why is chilled ethanol added rather than water?
It 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 and function of protein formed
Why is the ‘folding’ of amino acids important in proteins such as enzymes?
It 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?
Transcription
Translation
What does transcription involve?
The formation of mRNA from a DNA template
Outline transcription
DNA double helix unwinds
RNA polymerase binds to a specific base sequence of non-coding DNA in front of a gene and moves along the DNA strand
RNA polymerase joins free RNA nucleotides to complementary bases on the coding DNA strand
mRNA formation complete so it detaches and leaves the nucleus
Describe the differences 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
mRNA attaches to ribosome
Ribosome reads the mRNA bases in triplets. Each triplet codes for one amino acid which is brought to the ribosome by a tRNA molecule
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 a 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 of function
What is non-coding DNA?
DNA which does not code for a protein but instead controls gene expression
Describe the effect of a gene mutation in con-coding DNA
A mutation may affect the ability of RNA polymerase to bind to non-coding DNA
This may affect protein production and the resulting phenotype of the organism
Outline how the work of Mendel helped scientists to develop their understanding of genetics
He studied the inheritance of different phenotypes of pea plants
Established a correlation between parent and offspring phenotypes
Noted that inheritance was determined by ‘units’ passed on to descendants
Using gene crossed, he devised the terms ‘dominant’ and recessive’
Why was Mendel’s work initially overlooked?
Scientists didn’t understand Mendel’s work as there was no knowledge of genes or DNA at the time
What are alleles?
Different versions of the same gene
Define genotype
An organisms genetic composition, describes all alleles
Define phenotype
An organisms observable characteristics due to interactions of the genotype and environment
Define homozygous
Having two identical alleles of a gene
Define heterozygous
Having two different alleles of a gene
What is a dominant allele?
An allele that is always expressed
What is a recessive allele?
An allele that is only expressed in the absence of a dominant allele
What is mono hybrid 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 in the Y chromosome
Other than using a punnet square, how else can mono hybrid 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 chromosomes one rather than the Y chromosome?
The X chromosome is bigger
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 have two alleles for each sex-linked gene whereas men often only have one allele
Only one recessive allele is required to produce the recessive phenotype in males
Give an example of a characteristic that is determined by more than one allele
Blood group is determined by three
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
Describe codominance in blood groups
I^A and I^B are codominant
I^AI^B gives the blood group AB
Why does I^A I^O give blood group A?
I^O is recessive to I^A
What are the possible genotypes for blood group B?
I^BI^O
I^BI^B
What is the genotype for blood group O?
I^O I^O
What are the two causes of variation within a species?
Genetics
Environment
What is genetic variation?
Variation 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?
Sexual reproduction
Spontaneous mutations
State the three types of gene mutation
Insertion
Deletion
Substitution
How may a gene mutation affect an organism’s phenotype?
May cause a minor change
May completely change the sequence of amino acids which severely changes the phenotype
It can have no effect in phenotype
What is environmental variation?
Variations in phenotype that are acquired during the lifespan of an organism
Due to environmental factors
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
Disease causing alleles identified more rapidly and the appropriate treatments prescribed earlier on
Can predict a certain individuals response to certain drugs
What are 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
