Genetics 1 Flashcards
Differences between Prokaryote and Eukaryote chromosomes:
Eukaryote chromosomes: Linear DNA molecules Many Associated with proteins histones No plasmids
Prokaryote chromosomes Circular DNA molecule Only one Naked Plasmids usually present
Structure of chromosomes
Each chromosome is composed of two identical molecules of DNA, after DNA replication. These molecules are called sister chromatids and are held together by the centromere
The number of chromosomes is a characteristic feature of members of one species. Somatic cells of all sexually reproducing organisms carry two sets of chromosomes called homologous chromosomes (mother -father). Homologous chromosomes have the same size and the same type of genes located in corresponding position. These cells are called diploids and are symbolized as 2n (where n=single set of chromosomes).
The chromosome number and size vary from species to species. For example: HUMAN=46
GENOME
is the entire genetic information of an organism. It can be measured in millions of base pairs (bp) of DNA. Genome sizes vary a lot. Usually less evolved organisms contain smaller genomes than more evolved
What is meiosis
Meiosis involves one division of the chromosomes followed by two divisions of the nucleus and the cell. The result is that the number of chromosomes in each cell is reduced by half.
Meiosis occurs only in cells destined to become gametes, that is sperm and oocytes. The diploid (2n) parent cell gives rise to four haploid (n) daughter cells.
Example: Human diploid cells contain 46 chromosomes (2n=46 were n=23). This means that the genetic material is arranged in 23 pairs of HOMOLOGOUS CHROMOSOMES.
MEIOSIS I
1.Early prophase I:
During this stage homologous chromosomes pair up in a process called synapsis. The pair of homologous chromosomes are called bivalents. The nuclear envelope disintegrates. Spindle is formed.
2.Late prophase I:
Exchange of material (pieces of chromosomes) takes place between non-sister
chromatids of the homologous pair. This process is called crossing-over.
3.Metaphase I
Pairs of homologous chromosomes align in the spindle equator.
4.Anaphase I
Homologous chromosomes separate, each chromosome moves towards opposite poles sliding on spindle.
5.Telophase I and cytokinesis
The two nuclei are visible and the cell divides in two. Each new cell contains HALF the number of chromosomes than the original cell.
MEIOSIS II
It follows (sometimes not immediately after the first-it may be followed after a long period of time), as a simple mitosis (see diagram). No DNA replication takes places between the two divisions.
Significance of meiosis
• Production of gametes which have a role in sexual reproduction. With gamete fusion a zygote is formed and the original number of chromosomes is restored.
• Creation of genetic variation: this happens by two ways:
1. RANDOM DISTRIBUTION OF HOMOLOGOUS CHROMOSOMES DURING METAPHASE I. By this way there are possible gametic combinations which arise due to random distribution=2n (where n=haploid number of chromosomes)
2. CROSSING-OVER OF PIECES OF CHROMOSOMES between non-sister chromatids.
GENE
A heritable factor of the DNA, located on a chromosome, that controls the formation of a polypeptide (an enzyme)-and thus a characteristic.
ALLELE
Alternative forms of a gene occupying corresponding positions on homologous chromosomes. Alleles of the same gene differ by one or a few bases.
GENETIC LOCUS
The position on a chromosome occupied by a given gene.
HOMOZYGOUS INDIVIDUAL:
An individual possessing a pair of identical alleles for and particular locus
HETEROZYGOUS INDIVIDUAL:
an individual possessing a pair of unlike alleles for a given locus
DOMINANT ALLELE:
The allele that is always expressed and shown when present either in a homozygous or in a heterozygous condition.
RECESSIVE ALLELE:
the allele that is only expressed when found in a homozygous condition.
PHENOTYPE
The physical or chemical expression of an organism’s genes (the external appearance of an organism).
GENOTYPE
The genetic composition of an organism, in terms of alleles.
1st law the LAW OF SEGREGATION:
it states that each individual possesses characteristics which are found in pairs (alleles found on homologous chromosomes), but gametic cells possess only one characteristic of the pair since alleles segregate (separate) during meiosis I.
CODOMINANT ALLELES
When two different alleles found in a heterozygous individual, they have an EQUAL effect on the phenotype for the given characteristic.
MULTIPLE ALLELES
Usually a gene has two alternative alleles. In some situations, however, one gene may have more than two alleles for the same genetic locus. Any time only TWO of the possible are found on the locus. This is called multiple alleles.
Blood ex
Finding blood groups in important for blood transfusions. The presence of either alleles A (IA) or B (IB) indicates a different change in the basic glycoprotein found on the membrane of red blood cells. The presence of the i allele indicates NO change in the basic glycoprotein on the red blood cells.
SEX DETERMINATION
in humans there are 23 pairs of chromosomes. Of these, 22 pairs are identical in both sexes. The 23rd pair, however, is different in the male and the female.
The 22 pairs are called autosomes, while the 23rd pair is called sex chromosomes. In females these two chromosomes are identical XX, however, in males one X is present and the other is a different, smaller one, the Y chromosome.
SEX LINKAGE
Sex linkage refers to the carrying of GENES on the sex chromosomes. Some of these genes refer to body characteristics and have nothing to do with sex.
The X chromosome is larger than the Y and therefore carries more genes than the Y. These characters which are carried on the X chromosome and have no corresponding allele on the Y chromosome, are called sex-linked characteristics (traits).
Haemophilia
a disease where the sufferers cannot produce a protein factor responsible for blood clotting. A sex-linked recessive allele is responsible for the disease.
XH=normal Xh=hemophiliac allele
Possible genotypes: XHXH=normal female
XHXh=normal female but CARRIER
XhXh=diseased female (rare) XHY=normal male XhY=diseased male
An example of a dominant trait in humans: Huntington’s disease.
This is a rare genetic disorder due to the presence of a dominant allele on the 4th chromosome. The presence of this allele causes degenerative changes to the brain, with a late onset, after the age of 30 or 40 years old. Because of the late onset, people diagnosed of the disease already have children. There is a genetic test to diagnose if a person will develop the disease before symptoms appear. In a cross between a heterozygous parent and a normal one there is 50% chance for a child to be affected. Hh(diseased parent) x hh (normal parent)
An example of a recessive trait in humans: Cystic fibrosis
This is the commonest genetic disease in parts of Europe and US. It is due to the presence of a recessive allele on the 7th chromosome. The product of the normal gene is a chloride ion channel that is involved in the secretion of sweat, mucous and digestive juices. In homozygous individuals for the recessive allele the protein channel malfunctions causing the mucous secreted from the lungs to be very sticky. This becomes a good environment for lung infections to appear. In the general population there is a 1/400 chance for both parents to be carriers for the disease. If person is heterozygous for a recessive allele he is not suffering, but can pass the allele to the offspring. He is called carrier.
These do not show symptoms but can have an affected child in a 1⁄4 probability: Ff(normal carrier parent) x Ff (normal carrier parent)
MUTATIONS
A mutation is a random change to the base sequence of a gene. There are two factors which can increase the mutation rate:
RADIATION: Gamma rays, X-rays, short-wave UV radiation, emissions from radioactive isotopes, are all mutagenic agents.
CHEMICALS: benzopyrene and nitrosamines found in tobacco smoke can act as cancinogens and thus mutagens.
Mutations are random- there is no mechanism under which they occur.
They are, however, sources of variation-necessary for evolution.
If a mutation occurs in somatic cells these will be eliminated with the death of the organism. Mutations, however, which occur in cells destined to become gametes, will be passed on to the next generation.
Mutations in chromosome number
hanges in the normal number of chromosomes regarded as chromosomal mutations and may lead to serious abnormalities.
DOWN’S SYNDROME OR TRISOMY 21: Individuals suffering from this syndrome have one extra chromosome in all their somatic cells. This chromosome is in the 21st pair and this is why the syndrome is called trisomy 21 (three chromosomes in in the 21st pair).
This comes from a mistake during separation of chromosomes in anaphase I or anaphase II of meiosis. Not normal separation of the pair 21, is called NON- DISJUNCTION. This may lead to the formation of a gamete which contains both 21 chromosomes. When this gamete (an ovum) fuses with a normal sperm, the zygote produced will contain 3 chromosomes of the 21st pair (trisomy 21).
Non-disjunction increases with the age of the mother and therefore older women (above the age of 36) have higher risks of obtaining children with Down’s syndrome.
Karyotyping: A Method to check (screen) whether a fetus suffers from a syndrome or not
Procedures for obtaining fetal tissue
1
1) Amniocentesis
It refers to the procedure of removing a sample of amniotic fluid through the abdomen by a syringe. The amniotic fluid contains cells of fetal origin that can be cultured for diagnostic tests. Ultrasound scanning facilitates the procedure by outlining the position of the fetus and the placenta. Amniocentesis is usually performed at about the 16th week of pregnancy or earlier. The main complication is a small risk of miscarriage (0.5%).
2Chorionic villus
2) Chorionic villus sampling
In CVS fetal tissue from the villi of the trophoblast is taken through the vagina and the cervix of the uterus as early as the 9th week of pregnancy. Rate of fetal loss slightly higher than in amniocentesis (1%). In addition, success of chromosome analysis appears to be lower than with amniocentesis.
Karyotyping is the method where a KARYOGRAM is produced
The sample taken from either method is processed as follows:
1. The fetal cells are incubated with chemicals that stimulate them to divide by mitosis
2. Colchicine is used to stop mitosis in the metaphase. This chemical destroys the fibers of the spindle. Chromosomes, therefore, are most easily visible in metaphase.\
3. A hypotonic fluid is used to burst the cells and spread out the chromosomes.
4. The burst cells are examined under a light microscope and a photograph is
taken from one cell.
Gene mutations
A change in the structure of a gene. This might be due to:
• Deletion of a base
• Insertion of a new base
• Substitution of one base by another
These mutations cause changes in the reading frame of the genetic code and thus a possible change in the product-the protein.
There are however, some alleles which persist in the populations and this is because they offer an advantage to individuals. One example of such an allele is sickle-cell anaemia.
Sickle-cell anaemia: a base substitution mutation.
Haemoglobin is the blood protein responsible for transferring oxygen to tissues. It possesses 4 polypeptide chains: two alpha chains and two beta chains. Each α chain contains 141 amino acids and each β chain contains 146 amino acids.
A base substitution mutation on the gene which codes for the beta globin (A→T) causes a change in the codon on the mRNA and thus, a different amino acid is produced in the mutant polypeptide.
