Genetics Flashcards
What is a gene?
A heritable factor that consist of a length of DNA and influences a specific characteristic
What do most genes code for?
A polypeptide
What is the gene locus?
The specific position that the gene occupies on a specific chromosome
List the few trends for gene numbers
- Prokaryotes have fewer genes than eukaryotes
- Some animals have fewer genes than humans, others more
- Some plants have fewer genes than humans, but many have more
What are alleles?
Alternative forms of the same gene that differ in just one or very few bases.
What do alleles occupy?
The same gene locus and influence the same characteristic
What are the alleles for blood group?
IA, IB and i
How are new alleles formed?
By mutation and thus are a source of genetic variation necessary for evolution
What are mutations?
Random changes to the base sequence of a gene/DNA
What is base substitution?
A mutation which replaces one base with another
What can most mutations be classified as?
Harmful or neutral
What can mutations cause?
Cancer
If a harmful mutation occurs in a cell that will develop into a gamete, it will be passed on to offspring and may cause genetic disease
What can cause mutations?
They are RANDOM
- can be caused by errors made by DNA polymerase during DNA replication
- Mutagens that affect bases/nucleotides. Mutagens include high energy radiation and mutagenic chemicals
What are mutagens?
Factors that increase the mutation rate
What is sickle cell anemia?
A genetic disease that demonstrates how a single base substitution mutation can have significant consequences for the individual
Describe what people without sickle cell anaemia have
- Have the allele HBA which codes for the alpha globin polypeptide of haemoglobin
- 6th triplet is GAG
- mRNA has a codon of GAG
- glutamic acid is produced
Normal red blood cells, normal haemoglobin, efficient transport of oxygen, affected by malaria
Describe what people with sickle cell aneamia
- Base substitution mutation in the 6th triplet changes DNA from GAG to GTG
- Allele becomes HBS
- mRNA has a codon of GUG
- different tRNA minds to the codon and the 6th amino acid is valine
Sickle-shaped red blood cells, abnormal, less soluble haemoglobin, anaemia, resistant to malaria
What is the genome?
The whole of the genetic information of an organism
What is the genome size?
Total amount of DNA of an organism measured in millions of base pairs
What does the genome consist of in animals?
All the chromosomes in the nucleus and the mitochondria
What does the genome consist of in plants?
Chromosomes in the nucleus, mitochondria and chloroplasts
What does the genome consist of in prokaryotes?
The single chromosome plus any plasmids
What are the differences between prokaryotic and eukaryotic chromosomes?
Prokaryotes have circular DNA molecule whereas eukaryotes have a linear DNA molecule
Prokaryotes have naked chromosomes whereas eukaryotes have chromosomes associated with histones
Prokaryotes have one chromosome only whereas eukaryotes have two or more different chromosomes
What are the differences between prokaryotic and eukaryotic genomes?
- Prokaryotes have chromosomes with circular DNA, Eukaryotes have chromosomes with linear DNA.
- Prokaryotes have chromosomal DNA that is naked, Eukaryotes have chromosomal DNA associated with proteins called histones
- Prokaryotes have one chromosome only, eukaryotes have two or more
- Prokaryotes have plasmids present eukaryotes do not
- Prokaryotes have no mitochondrial or chloroplast DNA whereas eukaryotes have mitochondrial DNA and sometimes chloroplast DNA
- Prokaryotes have a smaller genome whereas eukaryotes have a larger genome
- Prokaryotes have a haploid genome whereas eukaryotes have a diploid genome
How do the chromosomes in eukaryotes differ?
They differ in the genes they carry, length and position of the centromeres
What does each chromosome carry?
Many genes in a specific sequence. When stained, each chromosome has a unique pattern of bands.
When is eukaryotic chromosomes visible?
During mitosis bc they condense by supercoiling and become shorter and fatter
What do sister chromatids have?
Identical base sequence and are held together by a centromere
What is the short arm and the long arm of the chromosome called
?
Short arm- p arm
Long arm- q arm
What are homologous chromosomes?
All the chromosomes of one particular type
Describe homologous chromosomes?
Have the same set of genes and in the same sequence but may have different alleles
What are haploid cells?
Contain one chromosome for each type
What are diploid cells?
Contain pairs of homologous chromosomes
Which cells have a diploid nucleus and which have a haploid cells?
Human somatic cells- diploid
Gametes- haploid
What are sex chromosomes?
The chromosomes that determine the sex
What are the types of the sex chromosomes and describe them?
X- large, has many genes which are essential for human viability and has a centromere towards the middle
Y- small, few genes which are not essential for human viability, centromere near the end
What causes the fetus to develop in a male?
The SRY gene in the Y chromosome. SRY codes for the protein TDF which initiates development of male features such as testis development.
What are autosomes?
The other 22 chromosomes that do not determine the sex
What does the number of chromosomes indicate?
The characteristic feature of members of a species
What is a karyotype?
The number and type of chromosomes present in a cell
What is a karyogram?
A photograph or diagram in which the chromosomes of a cell are shown in homologous pairs of decreasing length
How can you perform a karyogram?
Chromosomes at mitotic metaphase are stained and arranged in pairs according to their size, shape/centromere position and banding patter.
What are uses of a karyogram?
- Deducing the sex
2. Diagnosing chromosomal abnormalities
What are two ways to obtain cells from fetus to produce a karyogram?
- Amniocentesis- a needle is passed through the mother’s abdomen and used to withdraw amniotic fluid containing fetal cells
- Chorionic villus sampling- a tool is inserted through the mother’s abdomen or vagina and used to obtain cells from fetal tissues in the placenta called chorionic villi
What is genetic screening?
The testing of a fetus/individual’s DNA by looking for the presence of an allele or chromosome
What is autoradiography?
A technique used to visualise radioactively labelled substances
How did Cairns adapt autoradiography to produce images of E. coli chromosome?
- E coli cells were grown in a medium containing radioactively labelled thymine. DNA was therefore labelled- RNA doesn’t have thymine so it wasn’t labelled
- The cells were placed on a membrane and their cell walls were digested with the enzyme lysozyme to release the DNA on the membrane.
- The membrane was coated with a photographic film and left in the dark for two months. Some atoms in the DNA decayed and emitted high energy electrons.
- When the film was developed, lines of black dots appeared and each black dot is where a radioactive atom decayed–> DNA visualised
- DNA molecules in E. coli are circular and are 1100micrometers long.
What is genetic modification?
The transfer of genes from species to another
What are Transgenic organisms/Genetically modified organisms (GMOs)?
organisms that have genes from another organism
What is a transgene?
A foreign gene inserted in an organism
Why is the the amino acid sequence of the polypeptide translated by the GMO is unchanged?
The genetic code is universal
What is genetic engineering?
Genes can be transferred from one species to another by a variety of techniques
Describe the steps in gene transfer
- mRNA is extracted from donor cell
- A DNA copy of the mRNA (called cDNA) is made using the enzyme reverse transcriptase
- The mRNA, rather than the gene is extracted from donor cells because bacteria do not have introns and thus cannot process the introns of the human gene
- Plasmids are obtained from bacteria
- Plasmids are cut open with restriction enzymes (restriction endonucleases)
- If the donor DNA/gene and plasmid are cut with the same restriction enzyme, they have sticky ends that are complementary. Alternatively, sticky ends can be added to both the donor DNA and plasmid
- Donor DNA and plasmid are mixed. Their sticky ends link by complementary base pairing
- The enzyme DNA ligase splices the donor DNA and plasmid (seals the nicks by making sugar-phosphate bonds)
- The plasmid with the inserted gene, called the recombinant plasmid, is introduced into host cells
What is an example of gene transfer?
the transfer of the human
insulin gene, so that large quantities of the human insulin protein can be produced in
bacteria and used to treat diabetics
What is Bt maize?
A plant that has a gene from a bacteria
The gene codes for Bt toxin protein
Bt toxin kills insect pests feeding on the crop, e.g. corn borers
What are potential benefits of Bt maize?
Less insecticides are used, which are harmful to the farmers and the environment
Higher yields due to less pest damage
Less land used for crop production
What are potential harmful effects of Bt maize?
Non-target insects, such as the monarch butterfly (Danaus plexippus), may be killed as pollen from Bt maize is blown on plants that the monarch caterpillar eats
Cross-pollination might cause spread of the transgene to wild plants
Insect pests might develop resistance to the toxin
What are potential benefits of GM crops?
- less pesticides used (e.g. Bt maize)– good for environment and farmer’s health
- increased yields, thus more food available
- allow crops to be grown in areas that would normally not grow
- improvement of nutritional value e.g. higher vitamin content
- less cost of production
What are potential harmful effects of GM crops?
- spread of genes to wild plants by cross-pollination
- plants containing a toxin to kill pests might lead to spread of resistance to the toxin in the pests
- may affect food chains
- may affect consumers’ health, e.g. allergic reactions to proteins produced
- long-term effects not known
- transferred gene might mutate with unpredictable results that were not risk-assessed
- unfair to smaller farmers who cannot compete
What is gene therapy?
An experimental technique in which the normal allele of the gene must be inserted into affected cells to treat a disease
What are clones?
Groups of genetically identical organisms derived from a single parental cell
What is cloning?
Production of genetically identical organisms
What are natural methods of cloning?
- Asexual reproduction is a natural method of cloning. All prokaryotes reproduce asexually. Yeast reproduces asexually by budding. Many plant and some animal species use asexual reproduction. Plants clone themselves by growing extra bulbs, tubers, stolons (runners) etc. It is much less common in animals; but it happens in some invertebrates, e.g. in hydra and aphids.
- Rarely in nature, clones are produced by splitting of the embryo such as in human monozygotic/identical twins
Describe how artificial cloning of animals through breaking up the embryo into more than one group of cells
- called splitting or fragmentation
- possible because at this stage the cells are pluripotent, i.e. they can develop into all cell types/tissues
- thus, part of the embryo can develop into a separate individual
- happens rarely in nature, e.g. monozygotic twins can be regarded as clones by splitting
- artificially performed in livestock: in vitro fertilisation – embryo splitting - transfer
into surrogate mothers - usually performed at the eight-cell stage
Why is there little interest in cloning by breaking up the embryo into more than one group of cells?
- Limited number of clones
- At the embryo stage it is not possible to know whether the produced individual will have desirable characteristics
Describe artificial cloning using SOMATIC CELL NUCLEAR TRANSFER( SCNT)
- Dolly the sheep was the first mammal to be cloned from an adult somatic cell by Somatic Cell Nuclear Transfer (SCNT)
- Adult cells are taken from a sheep
- Cells are grown in the lab in a medium low in nutrients (this helps inactivating some genes so that the pattern of differentiation is lost)
- Unfertilised egg is removed from a second sheep
- The nucleus of the egg is removed
- One cultured adult cell is fused with one enucleated egg using an electric pulse
- Alternatively, the nucleus of the adult cell may be removed and fused with the enucleated egg
- The fused cells that develop into embryos are implanted into the uteri of sheep that act as surrogate mothers
- Dolly was born by a surrogate mother and was genetically Identical to the sheep that donated the adult cell/nucleus
- Cloned embryos are made by SCNT. If the scientists let the cloned embryo live, this is called reproductive cloning. If the cloned embryo is used for its stem cells it is called therapeutic cloning
What are methods to artificial clone plants?
- stem cuttings
2. Micropropagation
What are stem cuttings?
Short lengths of stem used for artificially cloning plants
Describe stem cuttings
If roots develop from the stem, the cutting will become an independent clone plant
-The stem is cut below a node (= where leaves are attached)
- leaves are removed from the lower half of the stem and often reduced in the upper half
- the lowest third of the cutting is placed into compost or water. Compost should be aerated, moisture-retentive and sterile
– a clear plastic bag with few holes is often used to cover the cutting and compost to prevent excessive water loss
– rooting usually takes a few weeks.
What is the locus of a gene?
Its particular position on homologous chromosomes
Describe what is meant by this:
1q25.3
The gene is found on chromosome 1
In the long arm
position 25.3
What does sexual reproduction require?
Meiosis to half the chromosome cumber and fusion of the haploid gametes.
What is fertilization?
The fusion of a female and male gamete to produce a zygote
When do chromosomes replicate?
During the S phase of interphase before meiosis I resulting in each chromosome consisting of two genetically identical sister chromatids joined together at the centromere.
Draw annotated diagrams of meiosis I and meiosis II
What happens in prophase I?
- Homologous chromosomes pair up and condense. A pair of homologous chromosomes at this stage is called a bivalent. Crossing over takes place
- Spindle microtubules grow from centrioles in each pole to the equator
- 3. The nuclear membrane breaks down at the end
What happens in metaphase I?
- Bivalents line up at the equator. The orientation of paternal and maternal chromosomes is random and independent of other pairs
- Spindle microtubules from the two poles attach to chromosomes. The two homologous chromosomes in a bivalent are attached to different poles. Each chromosome is attached to one pole only
What happens in anaphase I?
- Homologous chromosomes are pulled to opposite poles. The separation of chromosomes is called disjunction
- At the end of anaphase I, the cell membrane in the equator starts being pulled inwards
What happens in telophase I?
- Chromosomes have reached the poles and uncoil
- Nuclear membrane reforms
- two haploid cells are produced. The chromosomes in each cell have two chromatids, which will separate in meiosis II.
What happens in prophase II?
- Chromosomes condense
- New spindle microtubules grow from the poles to the equator
- The nuclear membrane breaks down
What happens in metaphase II?
Chromosomes line up on the equator and spindle microtubules attach to chromosomes
What happens in anaphase II?
- The centromere divides and the chromatids separate into chromosomes, which move to opposite poles
- the cell membrane starts being pulled inwards again to divide the cells
What happens in telophase II?
- Chromatids have reached opposite poles
- Nuclear envelope reforms
-cytokinesis takes place and four haploid cells are formed
What are the number of DNA molecules before the start of meiosis, at the end of meiosis I and at the end of meiosis II?
4
2
1
What are similarities between mitosis and meiosis?
- Both are types of division of the nucleus / cell division in eukaryotes
- Both start with a diploid parent cell
- Both consist of the stages prophase, metaphase, anaphase, telophase
- Both share the main events of each stage, e.g. chromosomes condense in prophase,
nuclear envelope breaks down at the end of prophase, microtubules attach to chromosomes in metaphase etc - In metaphase of mitosis and metaphase II of meiosis individual chromosomes line up in the equator
- In anaphase of mitosis and anaphase II of meiosis sister chromatids are separated to opposite poles
What are differences between mitosis and meiosis?
- Mitosis one division, meiosis two divisions
- Mitosis results in 2 daughter cells, meiosis results in 4 daughter cells
- Mitosis results in diploid nuclei, meiosis results in haploid nuclei
- Mitosis results in genetically identical cells, meiosis results in genetically different cells
- Mitosis Homologous chromosomes do not pair up, in meiosis they pair up
- In mitosis, crossing over doesn’t happen, in meiosis in does
- In mitosis, in metaphase individual chromosomes are lined up in the equator, in meiosis metaphase I pairs of chromosomes are lined up in the equator
- In mitosis, in anaphase sister chromatids are separated to opposite poles, in meiosis in anaphase I sister chromatids move together to the same pole
- In mitosis, DNA replication takes place prior to each division whereas in meiosis DNA replication takes place prior to meiosis I, but not meiosis II
- Mitosis Occurs in both sexually and asexually reproducing organisms, whereas meiosis only occurs in sexually reproducing organisms
What are three sources of genetic variation?
- Mutations- production of alleles
- Meiosis due to crossing over and random orientation of bivalents
- Sexual reproduction/ fertilisation
When does crossing over take place?
Prophase I of meiois
Describe crossing over
In prophase I homologous chromosomes pair up. This is called synapsis
- During synapsis, crossing over occurs.
- For this to happen, the DNA of non-sister chromatids is cut at exactly the same point (single strand break, SSB)
- The DNA of each chromatid is joined up to the DNA of the non-sister
chromatid. As a result, sections of DNA are swapped between homologous chromosomes - This produces new combinations of alleles on the chromosomes generating genetic variation
- The chromosomes that consist of DNA from both homologues are called recombinant chromosomes
- Where each cross-over has occurred, there is an X-shaped structure called a chiasma
- Crossing over occurs at random positions along the length of chromosomes and several crossing over events might occur in each bivalent
What is crossing over?
Crossing-over is the exchange of DNA material between non-sister
chromatids/ homologous chromosomes
Draw a diagram showing crossing- over
Describe how random orientation of bivalents in metaphase I leads to genetic variation
The orientation of each pair is random and independent of the orientation of other pairs
Therefore, when homologous chromosomes separate at anaphase I, different combinations of chromosomes (and thus of alleles) can be produced
The number of possible combinations of chromosomes produced by random orientation is 2^n
How does fertilisation cause genetic variation?
Fertilization is a random process- fertilization involves production of gametes by meiosis
What is non- disjuntion?
Non-separation of chromosomes during anaphase of meiosis
What does non-disjunction cause?
The gametes are produced with either one chromosome too many or too few. Gametes with one chromosome too few usually die and the one with too many sometimes survives.
What happens if an egg with two chromosomes is fertilised by a normal sperm?
The zygote has three chromosomes of one type
What causes Down- Syndrome?
Non- disjuntion at chromosome 21
What increases the chance of Down Syndrome?
Increasing age of parents, especially mother’s
What is trsiomy?
Individuals with one extra chromosome
What is monosomy?
Individuals with one chromosome less
