Biology- Chapter 4

The DNA in eukarytotes is associated with proteins which help to keep the DNA organized.

Gene: a heritable factor that controls a specific characteristic.

Allele: one specific form of a gene, differing from other alleles by one or a few bases.
 

Genome: the complete set of an organism’s base sequences. 

                      Gene mutation: a random, rare change in genetic material. A mistake made by nature.

The consequence of a base substitution mutation in relation to the process of translation or transcription, using the example of sickle-cell anaemia, is that the codon GAG becomes GTG. This causes that a different amino acid, valine, is added to the polypeptide. As a result, the molecule of haemoglobin, has a different shape.

 Sickle-cell anaemia symptoms are i.e. weakness, fatigue and shortness of breath. The oxygen cannot be transported through the blood as efficiently, and the effected red blood cells can get stuck in capillaries, causing slower blood flow.

Meiosis is a reduction division of a diploid nucleus (46 chromosomes) to form haploid nuclei (23 chromosomes).

                        Homologous chromosomes: the pairs of chromosomes. They are similar in shape and size, and carry the same genes. There are two, one from the father and one from the mother. However, they are not identical because the alleles for the genes from each parent could be different.

Meiosis is a step-by-step process which results in four haploid daughter cells. In order to produce these, the parent cell must divide twice.
Prophase 1:
- DNA becomes more compact and visible
- Homologous chromosomes pair up (father, mother)
- Crossing-over occurs; an exchange of genetic material between non-sister chromatids à forming spindle fibres.
Metaphase 1:
- The pairs of homologous chromosomes line up across the cell’s equator.
- Nuclear membrane disintegrates.
Anaphase 1:
- Spindle fibres from the poles attach to chromosomes and pull them to opposite poles of the cell.
Telophase 1:
- Spindles and spindle fibres disintegrates.
- The chromosomes uncoil, and new nuclear membranes form (many plants do not have this phase)
- Cytokinesis happens (cells splitting into two haploid cells)

Prophase 2:
- DNA condenses into visible chromosomes again.
- New spindle fibres are produced.
Metaphase 2:
- Nuclear membranes disintegrates.
- Chromosomes line up along the equator in each cell.
- Spindle fibres from opposite poles attach to each of the sister chromatids at the centromeres.
Anaphase 2:
- Centromeres split, releasing the sister chromatids as an individual chromosome.
- Spindle fibres pull individual chromatids to opposite ends of the cell. (Animal cell – cell membranes pinch off in the middle. Plant cell – new cell plates form to divide the cell into sections).
Telophase 2:
- Chromosomes unwind their strands of DNA.
- Nuclear envelopes form around each of the four haploid cells, preparing them for cytokinesis.

In a non-disjunction, where two or more homologous chromosomes stick together instead of separating, the result is an unequal distribution of chromosomes (in humans – an egg or sperm cell might have 24 instead of 23 chromosomes). When the non-disjunction happens in the 21^st pair, it is known as Down’s syndrome.

In karyotyping, chromosomes are arranged in pairs according to their size and structure. The structure depends on the position of the centromere.

Karyotyping is performed using cells collected by chorionic villus sampling or amniocentesis, for pre-natal diagnosis of chromosome abnormalities. That is i.e. if the mother of the child is over 35 years of age. This is done to check the number of pairs of chromosomes.

    A human karyotype: 
To determine the gender we must examine the last pair of chromosomes, and in this case we have two X chromosomes (**=female). A non-disjunction has occurred if there is more or fewer than two chromosomes in any pair. This female has three chromosomes in her 21^st pair à Down’s syndrome. However, we cannot see how severely she will be affected.

          Genotype: the symbolic representation of pair of alleles possessed by an organism, typically presented by two letters.
Phenotype: the characteristics or traits of an organism.
Dominant allele: an allele that has the same effect on the phenotype, whether it is paired with the same allele or a different one. Recessive allele: an allele that has an effect on the phenotype, only if it is paired with the same allele.
Co-dominant alleles: pairs of different alleles when both affect the phenotype.
Locus: an allele’s particular position on homologous chromosomes of a gene.
Homozygous: having two identical alleles of a gene.
Heterozygous: having two different alleles of a gene.
Carrier: an individual who has a recessive allele of a gene that have no impact on the phenotype.
Test cross: testing a suspected heterozygote plant or animal by crossing it with a known homozygous recessive.

A monohybrid cross, using a Punnett grid:

(A = brown eyes, a = blue eyes)

50% of the offspring will have blue eyes.
50% of the offspring will have brown eyes.

Some genes have more than two alleles (multiple alleles). An example is the blood types in humans, ABO.

The blood type in humans, ABO, has three alleles for the same gene, multiple alleles. A & B are co-dominant, meaning that both affect the phenotype.

The sex chromosomes determine whether the person will be a male or a female. In human females there are two X-chromosomes. When women produce gametes, each egg will contain one X-chromosome. Human males have one X and one Y-chromosome. When males produce sperm cells, half of them contain one X-chromosome and half contain one Y-chromosome. As a result when an egg cell meets a sperm cell during fertilization there is a 50/50% chance that the child will be a girl/boy, since the Y-chromosome is dominant.

Some genes are present on the X-chromosome and absent from the shorter Y-chromosome, because the X-chromosome is bigger and has more loci.

Sex linkage: any genetic trait whose allele has its locus on the X- or the Y-chromosome is said to be sex-linked.

Color blindness and haemophilia are examples of sex linkage. Here, the locus for that specific allele is present only on the X-chromosome. This makes it more likely for boys to suffer from these diseases.

(The girl will only be a carrier, whereas the boy will suffer from the disease).

A human female can be homozygous or heterozygous with respect to sex-linked genes. There are three possible genotypes for females, but only two possibilities for males. Only women can be heterozygous, and as a result, they are the only ones who can be carriers

                       Female carriers are heterozygous for X-linked recessive alleles. A homozygous female would either be infected or non-infected. In order for her to be carrier she has to be heterozygous.

B = brown eyes, b = blue eyes

(All offspring will have brown eyes but 50% will carry the allele for blue eyes).

A pedigree chart showing gender, carriers and affected offspring:

Polymerase Chain Reaction (PCR) is a lab technique which takes a very small quantity of DNA and copies all the nucleic acids in it to make millions of copies of the DNA. The technique is used to get enough DNA to be able to analyze it.

In gel electrophoresis, fragments of DNA move in an electric field and are separated according to their size. They travel through a gel where the smallest particles travel most rapidly.

Gel electrophoresis of DNA is used in DNA profiling, which is the process of matching an unknown sample of DNA with a known sample.

DNA profiling can be used in paternity suits when the identity of someone’s biological father must be known for legal reasons. Forensic specialists collect DNA samples and use gel electrophoresis to compare the collected DNA. If the patterns are similar, the two individuals are most probably related.

When analyzing DNA profiles, one looks at the similarities and differences in DNA. Related individuals have similar DNA, and in a crime scene, specialists look for DNA identical to the DNA found from the criminal.

Three outcomes of the sequencing of the complete human genome are:
1. Doctors can find out where to look for a disease-carrying allele.
2. Production of new medications.
3. Reveal details about ancestries and migration.

When genes are transferred between species, the amino acid sequence of polypeptides translated from them is unchanged because the genetic code is universal. This means that A, T, C and G always code for the same amino acids, which makes gene transfer possible.

A basic technique used for gene transfer:
1. The bacterium E.coli is used as a host cell.
2. A plasmid from the host cell is removed and cut open by a restriction endonuclease.
3. The gene to be copied is placed inside the open plasmid, and pasted into the plasmid using DNA ligase.
4. The plasmid is now called a recombinant plasmid, and used as a vector for introducing the new gene.
5. The vector is placed inside the host bacterium which copies and express the gene.

Two examples of the current uses of genetically modified crops or animals:
- Tomatoes tolerant to regions of high salinity.
- The sheep producing factor IX in her milk (preventing haemophilia)

Clone: A group of genetically identical organisms or a group of cells derived form a single parent cell

A technique for cloning (Dolly):
1. From the donor, a non-gamete cell from the other was collected and cultured. The nucleus was removed.
2. An unfertilized egg was collected from another sheep and the nucleus was removed.
3. Using a zap of electrical current, the egg cell and the nucleus from the donor were fused together.
4. The new cell developed in-vitro in a similar way to a zygote, and started to form an embryo which developed normally.
5. Dolly was born.

   Ethical issues of therapeutic cloning in humans:
- Is it acceptable to generate a new human embryo for the sole purpose of medical research?
- It can benefit the child, but it is not to take for granted (playing with the life of a human).