Genetics Flashcards
Gene and Chromosome
Gene- DNA sequence that defines a certain heritable characteristic
Number of genes in an organism does not correlate with the complexity of that organism
Chromosome- DNA molecule that carries genes
Prokaryote & Eukaryote chromosomes
Prokaryote
- Consists of a circular DNA molecule
- Naked- No associated proteins
- Plasmids often present
- One chromosome
Eukaryote
- Liner DNA molecule
- Has histone proteins
- No plasmids
- 2 or more different chromosomes
Chromatids
Chromosomes in prophase and metaphase of mitosis have sister chromatids,
Each has a DNA molecule that was produced by replication during interphase.
Held by a centromere
Autoradiography
John Cairns grew E.coli in a medium containing radioactively labeled thymine, DNA become labeled and not RNA. Placed cells over the membrane, Coated the membrane with a photographic film, left them in the dark fro 2 months. When film was developed, lines of black dots showed position of DNA molecules from E.coli. Discovered DNA molecules are circular and longer that the E.coli cell
Research used the same method to investigate eukaryote chromosome, showed eukaryote chromosomes contain 1 long DNA molecule rather than a number of shorter molecules.
Genome
The whole genetic information of an organism.
The total amount of DNA in one set of chromosomes in a species.
The size of the genome of a species is not related to the complexity of the organism
Prokaryotic DNA
- Bacteria have one circular DNA
molecule, not associated with
proteins. - Extra genetic information is stored on plasmids and can be shared between bacteria.
- Antibiotic resistance genes are often found on plasmid DNA
Eukaryotic DNA
- DNA is associated with proteins
called histones. - Histones are used to wrap DNA
around them to protect from damage & control expression of certain genes. - A pair of identical chromosomes is called a homologous pair, they carry the same genes- possibly different alleles
- A complete set of chromosomes a diploid number of chromosomes.
- Sex cells contain half the number of chromosomes (one from each pair),haploid, to conserve the species’ number of chromosomes after fertilization .
Sex Chromosomes
- 23 pair of chromosomes determines the baby’s gender,
X- larger. Y- Smaller
If two X are present the child is female
If XY are present the baby is male
all other chromosomes are called
autologou).
Karyotype
is the characteristic pattern of chromosomes of an organism,
referring to their size, shape and banding pattern and number.
Karyogram
- Image of all chromosomes of an organism’s cell, Shown in decreasing size of the homologous pairs
- Help determine the sex of the organism
- And also possible chromosomal irregularities that might be disease-causing.
Irregularity in humans is trisomy 21, an extra chromosome in the 21st pair. Cause Down syndrome.
Down Syndrome and Karyogram
Irregularity in humans is trisomy 21, extra chromosome in the 21st pair. Cause Down syndrome.
How Kyragroms can be made
Amniotic fluid sampling- Amniocentesis
- A hypodermic needle is inserted through the abdomen of the mother into the amniotic sack.
- The embryo swims in the amniotic fluid which contains cells the embryo sheds.
- The karyogram is obtained by collecting the DNA from these cells.
Chorionic villus sampling
- Chorionic villi makes up the embryonic side of the placenta.
- These villi are of the embryonic tissue origin, they have the same
cells as the child.
- Sampling the chorionic villi ( needle), child’s cells can be obtained, karyogram constructed.
Fertilization
An event where male and female gamete fuse together to produce a zygote
Meiosis
Type of cell division in which one cell with a diploid nucleus divides into 4 genetically distinct cells with haploid nuclei.
Process where gametes (sex cells) are made.
Meiosis I
Prophase I
- DNA has already been duplicated, the cell contains a double number of chromosomes
- DNA supercoils, chromosomes shorten.
- Nuclear membrane breaks down
- Centrioles move towards the pole
Metaphase I
- Homologous chromosomes pair up at the equator
- Spidle microtubules attach to one chromosome from each pair
Anaphase I
- Spidle microtubule pull chromosomes to opposite poles( They shorten)
Telophase I
- Nuclear membrane developed around each set of chromosomes
- Cell divide into 2 with haploid number of nuclei
-Chromosomes partly uncoil
Cell proceeds to Meoisis II
Meiosis II
Meiosis II phases are identical
to mitosis, but starting number of
chromosomes are halved!
Prophase II
- Chromosomes supercoil again & become shorter.
- Centrioles move to opposite poles
- Nuclear membrane breaks down.
Metaphase II
- Chromosomes line up at the metaphase plate, next to each other across the equator.
- Spindle microtubules attach to the
centromeres of the chromosomes
Anaphase II
- Spindle microtubules pull the sister chromatids apart,
- Each pole receives one DNA
copy of each chromosome
Telophase II
- Each pole of the cell contains half the number of chromatids compared to the beginning of meiosis I, the same number of chromosomes
- Nuclear membrane forms, cell divides into two cells.
- 4 cells are yielded in total, each has half the number of chromosomes
Crossing over
The process through which the non-sister chromatids within a homologous pair, exchange genetic material during prophase I
Recombinants
Exchange of genetic material between non-sister chromosomes of a homologous pair, gametes end up with chromosomes with new gene combinations that were not
present before
Random Orientation
Refers to the fact that the positioning of homologous chromosomes at metaphase plate during metaphase I random
Independent assortment
The orientation of one pair of homologous chromosomes in meiosis 1 is independent of
the orientation of any other pair,
Gene variation and Meiosis
2 processes in meiosis promote genetic variation
- Random orientation of pairs of homologous chromosomes in metaphase I- Random orientation
- Crossing over during prophase 1- Crossing over
Allele
Variation of a gene that differs from another allele by a few bases only. Results in variations of a characteristic in an organism.
A gene can have none as well as several alleles.
Genotype
Combination of alleles of one or more genes (Capital and
lower case letters in Punnett grids). Remember that diploid organisms
have two copies of each chromosome, which allows two to have two alleles per gene.
Genotypic ration- Proportions of the various genotypes produced by the cross
Phenotype
Physical trait that is expressed by a certain genotype (what you can see with your eyes, like eye colour etc.)
Phenotypic ratio- Proportions of the various phenotypes
Homozygous
two of the same alleles
Heterozygous
Two different copies of an allele
Dominant Allele
Allele that is expressed both in homozygous and
heterozygous combinations.
Recessive allele
Dominant alleles that are both expressed when present, since
neither of them overpowers the other ones.
Locus
A fixed, physical location on a chromosome where genes can be
found.
Mendel’s Law of inheritance
Gregor Mendel- Father of genetics
Discovered basic laws of inheritance by cross-fertilizing pea plants with different traits.
- Observed “Hidden” traits that resurfaced after several generations- Recessive alleles
Was not the first biologist to discover this but repeated and held multiple trials to ensure he was right
3:1 Ration
Mendel crossed 2 varieties of peas and found that the offspring (F1 generation) had the same characteristic as one parent.
Plants are self-fertilized, producing offsprings, this generation(F2 generation) had both of the parental types 3:1 ratio
- Each pea plant has 2 alleles that affect the character.
- Parents are homozygous as they have 2 similar alleles
- F1 plants are heterozygous, 2 different allele, masks the effect of other parents recessive allele.
- 1/4 of F2 generation have 2 recessive allele.
Punnet grid
Represents the maternal alleles on one side, and paternal alleles on the other side of the grid.
First step- One can determine possible alleles parent’s gametes can have
Second step- Combining all possible parent alleles in the grid can determine possible offspring combinations.
- Parental generation is P1
- First offspring generation F0, all other offspring generations are numbered F1, F2 etc.
- Dominant alleles determine the letter used to describe the trait (Brown eyes are dominant to green eyes, trait will be defined by letter B)
- Dominant alleles written in capital letter, and recessive alleles in the lower case
Co-Dominant alleles
Co-dominant alleles are those where neither of the alleles over-rules the others. All the present alleles are expressed.
- The letter used to represent these alleles is I(capital).
- For the specific alleles, ie ABO blood group, blood group A
is labeled as capital I with a superscript A, so IA and blood group B as IB.
- If another allele is recessive to both the co-dominant alleles,Its
labeled as a lower case letter i with no superscript.
Blood groups
-Each individual has one of the four possible blood groups, A, B,
AB or O. Surface molecules carried on the red blood cells (RBCs) help the body distinguish between self and non-self.
- An individual may have A, B, and AB molecules or no molecules (O).
- Depending on the blood group the individual will have antibodies or an immune reaction to the blood group they don’t have
- Person with blood group A cannot receive blood from someone with B - Everyone can receive from O-type donors
- Blood group O can receive only from O donors.
- If blood not received from the right donor it leads to one’s immune system attacking their own blood.
Sex Linkage- Rules of Punnett grids
The inheritance of genes that are located on the sex chromosomes (Association of characteristics with the sex of the individual)
X chromosomes carry more genes than Y chromosomes, males will often lack one copy of the sex-linked genes.
So many sex-linked diseases affect males more than females. Females are the only carries od such diseases
Rules of Punnett grids
- Letters assigned to the traits are either X or a Y, depending on
the gender.
- The trait is labelled as a superscript on the X or Y.
- Dominant allele - capital letter, recessive - lower case
Red-Green Color-Blindness
- Sex-linked disorder carried on the X chromosome. Affected individuals cannot distinguish between red and green colours.
- Recessive disorder, only individuals with no dominant alleles are affected.
-Males have a disadvantage since they have only one chromosome (carry the healthy or the affected gene.)
-Females carry two X chromosomes, so two alleles for colour vision. Males carry one X chromosome with the gene for colour vision, Y doesn’t have. - If a female has one healthy and one affected gene, she is a carrier & healthy. She can pass on her affected X chromosome
to her son, would be affected by the disease.
-Females can be affected
but need to have a carrier mother and an affected father. Still a 50% chance they will have the disease.
Recessive Genetic Disorder
- Mendel’s laws can be observed in Humans.
- Recessive genetic disorders mean only individuals with 2 affected alleles get the disorder. Recessive alleles= autosomal genes
Cystic Fibrosis
Disease caused by a recessive allele of a gene coding for a chloride channel
- The most common genetic disease in parts of western Europe
- Parents don’t tend to have it but are carriers.
- Carriers are not affected as they carry a recessive allele and a dominant allele is also resent
Dominant Genetic Disorders
One affected allele is enough for the individual to have the disease
Small number of genetic disease are due to dominate alleles of autosomal genes
Huntington’s Disease
Neurodegenerative disease is caused by dominant alleles of the gene coding for huntingtin, a protein with unknown function
- Develops during adulthood, when they have kids
- Mostly only 1 parent has it, so unlikely for a child to have 2 dominant allele
- A 50% of a parent passing it on to a child
Pedigree Charts
Pedigree charts are a way to represent the inheritance of certain traits in a form of a family tree, where the oldest individuals are set at the top, and their offspring follow
downwards.
Rules of Pedigree charts
- The female is always labelled as a circle, Male as a square .
- Most pedigree charts show affected individuals as colored figures and healthy and carrier individuals as transparent.
- Helps determine the possible genotypes of individuals and chances for affected offspring in the future.
Exam tips for Pedigree charts
- Charts where most males are affected usually represent a sex-linked trait (more than 90% of the affected individuals are males).
- Two healthy individuals cannot have a child with a dominant disorder.
- In recessive disorders, two healthy parents can have an affected child.
- Two affected parents cannot have a healthy offspring
Dihybrid crosses
Two traits can be followed in a Punnett grid to predict the possible combinations of traits.
we assume that each trait is located on its own chromosome,
and that they are inherited separately.
9:3:3:1 Ration
- 9:3:3:1 ratio is often observed in a dihybrid cross of non-linked genes (traits in different chromosomes), involving to heterozygous parents for both traits.
- This ratio is disturbed if the parents are not heterozygous for both traits, some of the alleles are co-dominant or the genes are sex-linked.
Linked Genes
Refers to genes that are located on the same autosomal (non-sex)
chromosome.
- Two genes are on the same chromosome, which means that the alleles of those two genes are inherited together.
-Alleles PR and alleles pr will be transmitted together, - Even though this parent is heterozygous for both genes, it will only give gametes with two
possible combinations. - Each chromosome contains many genes, the combinations of alleles of these genes would always be same, leading to little variation between individuals
Crossing over and linked genes
Crossing over allows for genetic material to be shared.
-Due to this, the new parental gamete combinations are PR and pr (combinations from the original chromosomes), and Pr and pR (the recombinants- new combinations not in the parent).
- Crossing over does not happen in the same locus in every gamete. So,
the two original combinations are more common than the
recombinants.
- So a majority of children will have the original parental combinations , while a minority will have the recombinant genes (pR and Pr).
- The further the two genes are in the chromosome, the more chances
there are for crossing over to separate the alleles and create recombinants.
Polygenic inheritance
o the phenomenon where one trait is controlled by
multiple genes. When a trait is controlled by more than one gene, the result is often a
wide range of variations between the individuals.
Discrete variation occurs when a trait is controlled by one or only a few genes. In traits
that show discrete variations, individuals either have the trait or don’t. An example of
this could be Huntington’s disease, where you either carry the dominant allele for the
disease, and you have it, or you don’t, and you’re healthy.
In continuous variation, the trait is controlled by many genes, and they are often
co-dominant. In such a case, the genes have a cumulative effect. An example of such
variation is skin colour, or height.
Polygenic inheritance of skin colour
Skin colour is controlled by multiple genes, each determining the presence of the
melanin pigment (the pigment that gives skin a dark colour) in the skin cell. Each
gene has two possible alleles, expression of melanin, or no melanin expression. If an
individual has a set of genes where the majority have the allele for melanin
expression, the colour of that person’s skin will be dark.
The more the alleles for melanin production, the more the pigment this person will
have in their cells. If many genes control a certain characteristic, the distribution of
such characteristic will be close to the normal distribution.
In the example of human height, many genes control height, so there is a continuous
variation of human heights. Still, most people will be of average height, and some
people will be very tall or very short.
Discrete variation
Occurs when a trait is controlled by one or only a few genes. Individuals either have the trait or don’t. An example of
this could be Huntington’s disease.
Continuous Variation
The trait is controlled by many genes, and they are often co-dominant. The genes have a cumulative effect. An example of such variation is skin color, or height.
Polygenic inheritance of skin color
- Skin color is controlled by multiple genes, each determining the presence of the melanin pigment in the skin cell.
- Each gene has two possible alleles, expression of melanin, or no melanin expression.
- If an individual has a set of genes with the majority of the allele having a melanin expression, skin color would be dark.
Polygenic inheritance and human height
- If many genes control a certain characteristic, the distribution of
such characteristics will be close to the normal distribution. - Human height, many genes control height, so there is a continuous
variation in human heights. - Still, most people will have an average height, and few would be very tall or very short.
Mutations
Random change to the base sequence of a gene
- Mutations that replace 1 base of a gene with Another= base substitution
- Mutations are a source of genetic variation, necessary for evolution to occur.
- Few mutations prove to be beneficial, some cause diseases- cancer
Mutation rate is increased…
High energy radiation- X-rays, short-medium UV, gamma rays and alpha particles from radioactive isotopes
Mutagenic chemicals- Nitrosamines in tobacco, mustard gas used for chemical weapons and the solvent, benzene
Nuclear Accident at Chernobyl
1986 accident in Chernobly, Ykrani cause explosions and a fire in the core nuclear reactor.
- Radioactive iodine-131, caesium-134 and cesium-137 were spread over large parts of Europe.
- Approx 6 tonnes of radioactive metals from the reactor were broken up into small particles and escaped
- 28 workers died, increased leukemia in other workers exposed to high radiation
- Radioactive iodine concentration in the environment increased. Iodine in water and milk increased.
- Thyroid cancer increased
- Horses and cattle near the plant died from damage to their thyroid gland
- Bioaccumulation caused radioactive caesium in fish in Scandinavia, Germany and wales
- Lamb consumption was banned for many years in some areas, long half-life of caesium-137
- Increased risk of cancer and genetic disease, hard to prove
- 4km2 of pine forest changed color and died due to high radiation.
- Less Human in wildlife caused lynx and wild boars to trive
Sickle-cell Anemia
Heritable disease caused by a mutation of a gene coding for the
haemoglobin molecule.
- A base substitution mutation causes an adenine base in the GAG
triplet to be substituted by thymine (GTG). - GAG codes for glutamic acid which becomes substituted by valine (GTG).
- The difference is it changes the shape of the haaemoglobin protein, leading to a less functional molecule.
- Individuals with sickle cell anaemia have moon shaped red blood cells with lower oxygen-carrying capacity.
- Malaria parasite is less likely to infect the sickle cells.
Using Databases
- Database have been developed since 1960s to help store complex information
- Ideal suited to storing the large amounts of base sequence data generated from genome research
- Great improvements to make it easy for user to search on databases and extract data
- Development of the internet opened up access to databases worldwide.
- Data can now be shared easily
Gene Loci and Protein Products
- The gene locus has its own particular position on homologous chromosome
- The loci of human gene can be founding using the OMIM website
Base sequence of genes
- Changes in base sequence occur over time. If species are separated and become different species.
- Differences in base sequences change over time
- Number of difference can detected how long ago different species arose from a common ancestor
- Useful to compare base sequences of genes to find these
- The GenBank can help with this
Polymerase Chain Reaction- PCR
- Method were a molecule of DNA is copied multiple times to create a high number of identical DNA molecules.
PCR steps
Denaturation- DNA is heated to 95 degrees celcius, DNA separates into 2 strands.
Annealing- Temp is reduced to 53 degrees celsius, allowing the primer to bind to the 3’ ends of the strands
Elongation- Temp is increased to 73 degrees celsius, Taq DNA polymerase replicates both strands, starts with primer.
Produces 2 double-stranded compies of the original DNA
Gel Electrophoresis
Method of separating mixtures of proteins or fragments based on size and charge
Used to determine the paternity of a child. Comparing fragments of the child and potential fathers
Process of Gel Electrophoresis
- Mixture of DNA fragments is placed on a thin sheet of agarose gel, acts as a molecular sieve
- Electric field generated by attaching electrodes to both ends
- Depending on the charge of the particles they move towards 1 of the electrodes
- How fast they move depends on the size of the molecule. Small- Fast, Big- Slow
- Results in pattern bans that can be seen under UV light and compared between cells or individuals
DNA profiling
- In human DNA, Loci in the chromosome in place of the gene, containing shorter sequence of 3,4 and 5 bases repeated, Sort tandem repeats- STR
- At the STR, many possibilities of alleles, varying in the n.o of repeats.
DNA Profiling
STR alleles can be used in DNA profiling/fingering. Used in forensic investigation
- A sample DNA is obtained
- DNA from selected STR loci is copied by PCR
- Copies of STR alleles are made by PCR, from 1 person’s DNA
E.coli and the production of insulin
Bacteria can be genetically modified to produce a human protein of interest.
Example is the production of insulin through E. coli.
- Messenger RNA is extracted from human cells producing insulin and converting that mRNA
- This piece of DNA could now be ligated into a plasmid using restriction enzymes,
- Once the plasmid with the gene of interest is made, it can be incubated with the bacteria which will start- transcribing and translating the newly acquired
gene.
- Bacteria are now called recombinant bacteria.
- Results in the production of the protein, which can then be collected as a part
of the bacterial secretions.
Gene Transfer using Plasmids
Genetic modification transfer of genes from 1 species to another
- Organism with gene transferred to them are genetically modified organisms- GMO)
Hoe genes are trasnfered
- Gene transfer between species using a vector
- Example, the vector can be a plasmid( DNA loop small)
- 2 enzymes are used to insert genes into the plasmids
- Restriction endonucleases cut DNA molecules at specific base sequences
- DNA ligase makes sugar-phosphate bonds to link nucleotides together
- Recombinant plasmid (Plasmid with gene from another species) is received bya host cell
Benefits of Genetic Modifications
- Reduced pest damage, higher crop yield, fewer food shortages
- Better yield, less land needed for more crop production, unused land can be conserved for wildlife
- No need for pesticides that damage other organisms in the field
Risks of Genetic Modifications
- Long term effects of GM foods have not been determined
- Pollen from modified crops can be blown and kill other organisms, not related to the plant
- GM plants have evolutionary advantage, random cross-pollination can create imbalance to the ecosystem
Bt Maize- Advantages
Maize can be modified to express the bacterial gene for Bt toxin, which pests usually attack. So crops become resistant to pest infestation
- Higher crop yields more food
- Less land- More for wildlife
- Less use of insecticide sprays, Expensive and harmful
Bt maize- Disadvantages
- Maize pollen contain toxin if blown away can hurt other organisms
. Monarch butterflies & caterpillars - Leaves and stems still contain toxins, Harmful to insect detritivores in the soil & stream
- Transfer gene, leads to cross-pollination, which toxic to insects feeding on them.
- Insects can form resistance to the Bt toxin
Clone & Cloning
An organism that is genetically identical to its parent organism
Cloning is a technique of producing genetically identical cells, tissue or organisms.
Cloning in animals is difficult
Artificial Cloning Animals
- Simple way to clone animals is to break up an embryo at an early stage(consisting of embryonic stem cells)
- Each cell develops into a genetically identical individual
Setback is at the embryo stage the characteristics of animals are unknown
Somatic-cell Nuclear Transfer
- Nucleus is removed from an egg cell.
- Its replaced by a nucleus from a differentiated somatic(body) cell
- Method used to produce Dolly
Dolly the sheep
First cloned animal
- Egg cell is taken from a sheep, nucleus removes. It contains half the genetic material, the nucleus cannot be used
- Udder cells(somatic) are taken from another sheep.
- Grown in a derived environment, they switch off their genes
- Its nuclues is isolated(contains all chromosome) and fused with the egg cell without the nucleus
- Fused cell is inserted into another sheep, it develops as a newly formed embryo,]
