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What are the four nucleotides found in DNA
Adenine, Thymine, Cytosine, Guanine
How are nucleotides paired
A purine pairs with a pyrimidine. Purines are double ringed (adenine and guanine), Pyrimidines are single ringed (thymine and cytosine). adenine pairs with thymine, guanine pairs with cytosine
What is the structure of a nucleotide?
phosphate group, deoxyribose sugar, nitrogenous base
What is DNA
deoxyribonucleic acid is a double-helix shaped molecule that carries the instructions for proteins which are required for the cell and organism to survivce
What is a gene
a gene is a section of DNA that codes for a specific protein
How is DNA structured
nucleotide sugars and phosphates link together to form the sugar phosphate strands, weak hydrogen bonds between two base pairs link two nucleotide strands, two DNA strands bond and spiral around each other to form the double helix
Antiparallel DNA strands
the carbon atoms of the sugar are numbered 1-5. C 1 bonds to a base (A,T,G,C), C 5 bonds to a phosphate group. DNA strands are antiparallel, one runs in a 5’ to 3’ direction, the other in a 3’ to 5’ direction
Why does each person have two copies of each gene?
one is inherited from their mother and one inherited from the father
What is a genome
the complete set of DNA contained within an organism’s chromosomes
What is an allele
An alternative form of a gene that accounts for the variation between individuals
What is the difference between an allele and a gene
Genes encode for the general characteristics, e.g. eye colour, alleles encode for specific alterations, e.g. blue eyes
What is a chromosome (structure)
A DNA molecule tightly coiled many times around histone (protein), that has been further coiled into supercoils called nucleosomes. Made up of two sister chromatids.
What is a chromosome (function)
they carry genetic information in the form of genes
What are some feature of homologous chromosomes
the same structural features (size, banding patterns, centromere positions), the same genes at the same loci positions (while the genes are the same the alleles may be different)
What are unlinked genes
Genes located on different chromosomes
What are linked genes
A set of genes located closely together on the same chromosomes that except for crossing over tend to be inherite dtogether
What is an autosome
Any chromosome that is not a sex chromosome
How many pairs of autosomes do humans have
22
What sex chromosomes does a female have?
2 X chromosomes
What sex chromosomes does a male have?
1 X and 1 Y chromosome
what is aneuploidy
when a cell or an organism varies in the usual number of chromosomes in its genome by the addition or loss of a chromosome
what is trisomy
the addition of a whole chromosome (3 copies of a particular chromosome)
what are some examples of trisomy
down syndrome, trisomy 21, edwards syndrome, trisomy 18
what is monosomy
the deletion of 1 chromosome of an autosomal homologous pair
what is an example of monosomy
there are limited example because it usually causes embryonic death, but not in sex chromosomes, e.g. turners syndrome
What is a somatic cell
somatic cells are diploid (2n) they are non-sex cells
what is a gamete
haploid sex cells, one chromosomes of each pair is inherited from the father (sperm), one is inherited from the mother (ovum) egg (n) + sperm (n) = zygote (2n)
where are gametes produced from
germline cells in the gonads via meiosis
what are the phases of meiosis
meiosis 1: prophase I, metaphase I, anaphase I, telophase I
meiosis 2: prophase II, metaphase II, anaphase II, telophase II
what is the outcome of meiosis 1
meiosis 1 is where homologous pairs split, it produces 2 haploid cells and chromosomes are double stranded
what is the outcome of meiosis 2
meiosis 2 is like mitosis, it creates 2 haploid cells, chromosomes are single stranded
what happens in interphase
distinct chromosomes are not visible
what happens in prophase I (meiosis)
Nucleoli disappear,, nuclear membrane disappears, chromatin condenses and is visible as double stranded chromosomes, homologous chromosomes synapse, crossing over may occur
what happens in metaphase I (meiosis)
homologous chromosomes line up in pairs along the equator of the cell, each pair of homologous chromosomes becomes attached to a different spindle fibre
what happens in anaphase I (meiosis)
the centromeres do not split, chromosomes of homologous pairs begin to migrate to opposite poles, each chromosome still consists of two chromatids
what happens in telophase I and cytokinesis (meiosis)
a chromosome from each homologous pair will be at opposite poles of the cell, the nuclear membranes will reform, a cleavage furrow will appear, two cells will have been produced from the original cell, each cell will have half the number of chromosomes of the original cell
what happens in metaphase II (meiosis)
chromosomes line up along the equator, each chromosome attaches to a different spindle fibre by its centromere
what happens in anaphase II (meiosis)
the centromeres split, one chromatid from each chromosome moves towards each pole
what happens in prophase II (meiosis)
the chromosomes become visible as double stranded chromosomes, spindle fibres appear, the chromosomes start to move towards the equator of the cell
what happens in telophase II and cytokineses
the chromatids unwind and de-condense, the nuclear membranes and nucleoli reappear. Four gametes are produced, each with a haploid number of chromosomes. in humans, four sperm of equal sizes are produced, or one large egg and three smaller cells are produced
what is independent assortment of homologous pairs
the random orientation of homologous chromosomes along the metaphase plate during metaphase I
Crossing over of chromosomes
occurs between homologous chromosomes where part of one chromosome exchanges with the corresponding part of its matching partner during prophase I. this produces gametes with new genetic combinations that are different from each other and the precursor cell
Recombination is due to:
crossing over of homologous chromosomes at prophase I , and independent assortment of non-homologous chromosomes during metaphase I
what process is the major contributor to genetic biodiversity
meiosis
what are some error in meiosis
nondisjunction - where chromosomes do not seperate
aneuploidy - an abnormal number of chromosomes in a cell
polyploidy: a whole set of chromosomes fail to disjunct
explain homozygous, heterozygous, and hemizygous chromosomes
if the maternal and paternal alleles are the same, the offspring is said to be homozygous. if the maternal and paternal allele are different, the offspring is said to be heterozygous. males old have one allele for each gene located on a sex chromosome and are said to be hemizygous
how do you represent alleles?
the convention is to capitalise the dominant allele and use a lower case letter for the recessive allele e.g. BB, Bb, bb
what is a phenotype
the physical or biochemical characteristics of an organism that are the result of gene expression (or set of genes) and the environment
what is a genotype
the genetic composition of an organism at one particular gene locus, as represented using letter symbols
explain complete dominance
complete dominance is where one allele is expressed over the other. the dominant allele will mask the recessive allele when in a heterozygous state. homozygous dominant and heterozygous forms will be phenotypically indistinguishable
explain co-dominance
co-dominance occurs when pairs of alleles are both expressed equally in the phenotype of a heterozygous individual. heterozygotes therefore have an altered phenotype as the alleles are having a joint effect
how do you represent co-dominance in writing?
When representing alleles, the convention is to use superscripts for the different codominant alleles (recessive still lower case) e.g. black ( C^B) and white (C^w) feathers create a speckled coat (C^BC^w).
how do you represent blood type in writing?
the letter I is used to represent the different antigenic forms
why are blood transfusions not compatible between certain blood groups?
because humans produce antibodies against foreign antigens.
explain incomplete dominance
a pattern of dominance where neither allele from the genotype of a heterozygous individual is dominant and both are expressed in an intermediate phenotype
explain sex-linked genes
sex-linked genes are genes that are located on a sex chromosomes. as human females have two X chromosomes (and therefore 2 alleles) they can either be homozygous or heterozygous. therefore X-linked dominant traits are more common in females, human males have only one X are are hemizygous for X-linked traits, X-linked recessive traits are more common in males because it can’t be masked by a second allele
what trends are always true for X-linked conditions
only females can be carries (a heterozygote for a recessive disease conditions), males cannot be heterozygous carriers, males will always inherit an X-linked trait from their mother, a female cannot inherit an X-linked condition from an unaffected father (must receive his dominant allele)
what are some genetic factors which influence variation of phenotypes?
sexual reproduction, alleles, mutation and single polymorphisms
what is a trait
phenotypic characteristics are an observable feature such as colour, shape, or biochemical property. a trait is a variant of that characteristic, for example, brown eyes, notched ear shape, or AB blood type
what is the influence of the environment on the phenotype
the environment are the conditions and resources external to an organisms with which that organisms typically interacts with. environmental features such as temperature, light, nutrition, and the presence of other organisms influence how gene are expressed. the environment modifies the function of the proteins produced rather than the creation of the proteins themselves.
explain proportionate (influences on) heritability
the amount of variation in phenotypes can be explained by genes in a given population or the extent to which variation can be attributed to genes. Because heritability is a proportion, its numeral value will range from 0.0 (genes do not contribute at all to phenotypic differences) to 1.0 (genes are the only reason for individual differences)
what is epigenetics
the study of changes in phenotypes as a result of variations in gene expression. Molecular interactions increase or decrease the expression of genes, altering how much or little of a protein is produced (DNA is not changed)
What are the two types of modifications that occur in epigenetics?
DNA methylation and histone modifcation
explain DNA methylation
methyl groups (small hydrocarbon molecules) attach to particular nucleotides making DNA more coiled and reducing transcriptions. Epigenetics can cause more or less methylation
explain histone modification
adding a methyl group to histone tails (methylation), making DNA more coiled and reducing transcription and therefor gene expression. Epigenetics can cause more or less histone modifcation
Why is epigenetics important
epigenetics plays an important role in cell differentiation and providing mechanisms for developing organisms to respond to their environment
what does a monohybrid cross do
a monohybrid cross determines the allele combinations for potential offspring for one gene only
what does a monohybrid test cross do
a monohybrid test cross involves mating an unknown genotypic individual with a known homozygous recessive. this is because recessive alleles will always be masked by the presence of a dominant allele, hence the phenotype of any offspring will reflect the genotype of the unknown parent
in a monohybrid test cross, is all offspring express the dominant phenotype the unknown parent is…
homozygous dominant
in a monohybrid test cross is half the offspring are dominant and half are recessive the unknown parent is….
heterozygous
in a monohybrid test cross if all the offspring express the recessive phenotype the unknown parent is…
homozygous recessive
explain lethal genes
some genotypes can result in death and do not appear in the offspring of a cross
what does a dihybrid cross do
a dihybrid cross determines the genotypic and phenotypic combinations of offspring for two particular genes
what is the expected phenotypic result from a dihybrid cross of two heterozygous organisms
9:3:3:1
how do you detect linked genes in a dihybrid cross
you perform a test cross of a heterozygous with a homozygous recess. if two genes are not linked, the genes will assort independently and the outcome will be four classes of offspring in equal proportions. if the two genes are linked, there will be four classes of offspring, but they will not be equal, e.g. there will be an excess of offspring from parental gametes and a deficiency of offspring from recombination gametes
What is an unaffected male in a pedigree
a blank square
what is an unaffected female in a pedigree
a blank circle
what is an affected male in a pedigree
a coloured square
what is an affected female in a pedigree
a coloured circle
How do you determine if a trait is autosomal dominant in a pedigree
both males and females are affected, all affected individuals have at least one affected parent, two affected parents can have an unaffected child, the trait cannot skip a generation
how do you determine if a trait is autosomal recessive in a pedigree
both males and females are affected, two unaffected parents can have an affected child, all children of two affected parents will have the trait
How do you determine if a trait is x-linked dominant in a pedigree
males with the trait pass it onto all his daughters and none of his sons, a female with the trait may pass it onto her daughters and sons
How do you determine is a trait is x-linked recessive in a pedigree
if a female is affected her father must be, if a female is affected her sons must be, if a male is affected his mother may be, the trait can skip a generation
how do you determine if the trait is y-linked recessive in a pedigree
all sons of a female with the trait are affected, all affected females have a father with the trait, non of the sons of an affected male can inherit the condition from the father
how do you determine if the trait is a y-linked pattern in a pedigree
only males show the trait, affected males will pass the allele to his sons, cannot pass the trait onto his daughter
sexual reproduction
the fusion of two distinct haploid gametes to produce a single diploid zygote composed of two sets of chromosomes
why is sexual reproduction important
for the longevity of the species and maintaining genetic diversity. The larger the gene pool, the greater the resilience of a population
advantages of asexual reproduction
leads to genetic variation which provides long-term evolutionary potential, populations are better able to adapt to and survive environmental changes with greater genetic variations, unfavourable genetic variation is eliminated from a population more efficiently
disadvantages of sexual reproduction
slower rate of reproduction than asexual, fewer offspring are produced over a longer time, energetically costly (gamete formation, courting, parental care), finding a mate may be difficult in scattered populations, recombination may introduce unfavourable variation in populations
asexual reproduction
producing offspring without the fusion of gametes
what is budding (asexual reproduction)
formation of a bud due to increased cell growth. Bud then breaks away to develop into seperate organism. E.g. yeast, sponges, jellyfish, coral, worms
what is fragmentation (asexual reproduction)
parent breaking into seperate fragments. Each fragment can become seperate organisms. E.g. worms, sea stars, many plants
what is sporogenesis (asexual reproduction)
formation of spores (small haploid units) that are dispersed into surroundings, where they grow into multicellular haploid organisms E.g. many plants, fungi, algae, mould
what is parthenogenesis (asexual reproduction)
often called a ‘virgin birth’ where an embryo can develop from a female gamete alone, without the need for a male gamete. Eggs that are produced via mitosis develop into a new organism. E.g. bees, wasps, ants, aphids, some fish, some lizards
what are cuttings/vegetative propagation (asexual reproduction)
plants can reproduce without the need for seeds, various sections of the plant may break away from the parent plant E.g. plants
advantages of asexual reproduction
populations grow faster than sexually reproducing populations, offspring are identical clones of the parents this is especially important for organisms that have adapted a phenotype that is fine-tuned to survive in a particular environment, does not require organisms to find a mate to reproduce, requires very little parental investment
disadvantages of asexual reproduction
genetic diversity is low
what is a clone
a genetically identical copy of a gene, cell, or organism
what is a natural example of cloning
plants, fungi, and bacteria all produce clones of themselves naturally
explain what happens in embryo splitting
a process where the early stage embryo are naturally or artificially divided, possible up until the 16-cell stage because all cells are undifferentiated stem cells, increases the number of offspring born in each breeding season
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