Genetics: The code broken?

Topic 4: Genetics: The code broken?

A. The structure of a gene provides the code for a polypeptide

Describe the processes involved in the transfer of information from DNA through RNA to the production of a sequence of amino acids in a polypeptide

Proteins are the building materials for organisms; they may also function as hormones or enzymes. Polypeptides are the building blocks for proteins

Gene – is a section of the DNA that codes for the production of a specific polypeptide

Protein synthesis has two stages

1. Transcription

Occurs in the nucleus
The DNA unwinds to expose the base sequence on one strand (the template)
The sequence of the bases is transcripted (copied) by complimentary m-RNA nucleotides
The m-RNA only copies the coding part of the DNA sequence (exons), the ‘non-coding’ part (intron) is not copied by the m-RNA

2. Translation

The m-RNA molecule travels from the nucleus to the cytoplasm and attaches to a ribosome
T-RNA nucleotide in the cytoplasm reach the ribosome and one end of them translates the m-RNA code
Every three bases are read by an anticodon of the t-RNA nucleotide
On the other end of the t-RNA nucleotide is an amino acid
When the t-RNA anti-codon locks onto the m-RNA codon, the amino acid is held in place long enough for it to form a peptide bond with the next amino acid that is brought into place by the t-RNA nucleotide
When the amino acid has bonded with the one next to it the t-RNA nucleotide returns to the cytoplasm to pick up another amino acid to bring back
A chain of amino acids (polypeptide) is left

Choose equipment and resources to perform a first hand investigation to construct a model of DNA

Process information from secondary data to outline the current understanding of gene expression

Gene expression – the process of transferring the information encoded in a gene to its functional gene product (a protein) is called gene expression

A gene is considered ‘active’ or ‘expressed’ when its specific m-RNA and polypeptide is produced, and ‘inactive’ or ‘silent’ when the gene produces neither product
Sometimes newly formed gene products cannot be directly identified, however the gene action can be inferred indirectly by noting when the gene is expressed in the phenotype, this is much less accurate as there could be a long period before the effects of a missing or defective protein are expressed in the phenotype. This is common with many genetic diseases such as Alzheimer’s

B – Multiple alleles and polygenic inheritance provide further variability within a trait

Give examples of characteristics determined by multiple alleles in an organism other that humans

The gene is for the trait (eye colour) and the alleles are the form of that particular trait (blue/brown etc.)
Multiple alleles – means that in any population, there are more than two forms of a particular gene in the total gene pool
The wild type of allele produces the basic trait, whilst other alleles called mutant alleles arise from random mutations over a period of time, and produce variation in the phenotype

Example – 1, patterns on leaves in White Clover

The wild type of allele gives plain green leaf colour (V)
There are seven alleles possible, V, Vba, Vby, Vf, Vh, Vi
The seven alleles produce a total of 22 different phenotypes

Example – 2, eye colour in fruit flies

Twelve different alleles for eye colour
Producing phenotypes that range from dull red, through many different shades of red to pure white

Compare the inheritance of ABO and Rhesus blood groups

The ABO blood grouping in humans is an example ...

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Example – 1, patterns on leaves in White Clover

The wild type of allele gives plain green leaf colour (V)
There are seven alleles possible, V, Vba, Vby, Vf, Vh, Vi
The seven alleles produce a total of 22 different phenotypes

Example – 2, eye colour in fruit flies

Twelve different alleles for eye colour
Producing phenotypes that range from dull red, through many different shades of red to pure white

Compare the inheritance of ABO and Rhesus blood groups

The ABO blood grouping in humans is an example of multiple alleles
The letters ABO refer to the three different alleles possible in the human genome, they only inherit two alleles
there is only four possible phenotypes for blood type in human: A, AB, B, O

The letters A and B refer to two different carbohydrate molecules found on the surface of red blood cells, a persons red blood cells could have A, B, AB, or none of them O
The molecules of A and B on the surface of re blood cells can result in them acing as antigens, triggering an immune response, when antibodies act against the antigens on the surface it can cause the red blood calls to clump and for a clot. This can kill a person

Both A and B are dominant over O, and A and B are co-dominant
Co-dominant – means that they are both expressed at the same time, meaning that neither one is dominant
AB is known as the universal recipient
O is known as the universal donor

The Rhesus is the other best known blood type in humans, the two types are Rh+ (D) and Rh- (d), with Rh+ being dominant
Rh+ have an antigen D present on their red blood cells, Whilst Rh- doesn’t

A woman who is Rh- whose first baby is Rh+ is given an injection of anti-Rh antibodies to destroy any Rh+ blood cells that may have entered her blood stream during the birth process (passive immunity)
If she didn’t have the injection and her second baby was Rh+, the mothers blood would have B memory cells producing antibodies immediately at the beginning of her pregnancy, these would cross the placenta and damage the foetus

Solve problems to predict the inheritance of ABO blood groups and the Rhesus factor

Describe what is meant by polygenic inheritance and describe one example of polygenic inheritance in humans or another organism

Polygenic inheritance – occurs when many genes control a trait

Polygenic inheritance usually shows continuous variation which when graphed, produces a normal distribution pattern (a bell curve)

Height in humans is an example of a polygenic trait
As many as 10 genes control the trait, greatly increasing the # of phenotypes present, even assuming that each gene has only two alleles
Height can differ by less than a mm and this gradual increase from the very smallest for a certain age and gender, to the very tallest for a certain age and gender can be indicated by a bell curve

Outline the use of highly variable genes for DNA fingerprinting of forensic samples, for paternity testing and for determining the pedigree of animals

DNA identification depends on the existence of hyper variable (highly variable) regions of DNA

Natural variations are found in everyone’s DNA and are in the form of series of short sequence of base pairs (2-6 base pairs) which are repeated over and over again, these can be called short tandem repeats (STR), microsatalites or variable # tandem repeats
These STR’s are then used in DNA fingerprinting

DNA profiling in forensic investigations is carried out by specialist scientists in authorised labs
In forensic investigations samples are taken from suspects and then STR’s are matched between DNA from the crime scene and the suspects

Blood typing used to be the only method available for matching a child with its biological father in paternity cases, however, because someone who is blood type A or B could be heterozygous it is not very exact, DNA profiling is now used for more accurate results

DNA profiling can also be use to identify parentage in animals, this is important for breeders who sell animals priced to their bloodlines (horses etc.)

C – Studies of offspring reflect the inheritance of genes on different chromosomes and genes on the same chromosomes

Use the term Diploid and Haploid to describe somatic and gametic cells

Somatic cell – is any body cell that contain the full # of chromosomes for the certain species, they are also called ‘diploid’ cells because they contain chromosomes from two parents in a sexually reproducing organism (di=2)

Gamete cell – are either sperm or egg cells and contain half the # of chromosomes for the certain species, they are also called ‘haploid’ cells because they only contain chromosomes from one parent

Describe outcomes of dihybrid crosses involving simple dominance using Mendel’s explanations

Dihybrid cross – a cross where two traits are investigated

Mendel’s first law – the principle of segregation of alleles

During gamete formation the members of each pair of factors (Bb) separate into different gametes, with one factor per gamete

Mendel’s second law – the principle of independent assortment

When separating during meiosis one pair of factors behaves independently (not linked) of members of other pairs of factors

Predict the difference in inheritance patterns if two genes are linked

Gene linkage – is when genes are on the same chromosome
In dihybrid crosses the results change is genes are linked
This is because the normal number of variations in gametes is less, because the genes are assorting together on the same chromosome during meiosis

Process information from secondary source to analyse he outcome of dihybrid crosses when both traits are inherited independently and when they are linked

Explain how cross breeding experiments can identify the relative position of linked genes

When chromosomes cross over during meiosis, alleles are swapped from one homologous chromosome to another. This produces recombinations of alleles in the offspring, that are noted by different phenotypic ratios

In 1913 an American geneticist noticed that some genes were more tightly linked, some genes recombined quite frequently whilst others hardly at all
He proposed that crossing over occurred at random
the closer two genes were together the less likely they were to be separated by crossing over
if the genes are at opposite ends of the chromosome, separation by crossing over is almost a certainty
the % chance of crossing over could be used as a tool for mapping genes along a chromosome

relative position – is the position of the gene relative to another gene
absolute position – is the exact locus of the gene on a chromosome

Discuss the role of chromosome mapping in identifying relationships between species

the order of genes along a chromosome can be used to determine the relationship between species
there are a number of ways in which the genes can be rearranged, inversion is one – the reversal in order of a number of genes
chromosome maps provides a means of assessing how closely linked different species are and the order of evolution of a # of different species

D – The human genome project is attempting to identify the position of genes on chromosomes through whole genome sequencing

Discuss the benefits of the human genome project

Describe and explain the limitations of data obtained from the human genome project

the genome of an organism consists of the entire DNA in a cell

Genome project – international cooperative research project involving up to 18 countries it is publicly funded and by 2001 chromosome 22 was completed

Objectives

to map the genes of 24 human chromosomes (22 + X + Y) this means identifying which genes each chromosome carries
to locate the exact position of each gene on the chromosome
to determine the complete base sequence of each gene
identify the genetic variation in the human genome
identify and address the ethical, social and political issues arising from the HGP

Benefits (medical)

improved diagnosis and predisposition to disease by genetic testing e.g. cancer
better identification of disease carriers through genetic testing e.g. down syndrome
improved understanding of the developments of drug resistance in bacteria, leading to more effective drug treatment for them

Benefits (non-medical)

greater knowledge of family relationships through genetic testing e.g. paternity testing
improved knowledge of human evolution, including the relationship between humans and other species
improved ability to breed pest resistant plants and animals, this has positive cost effects on agriculture across the world and may provide solutions to 3rd world famine

Limitations

proteins do not exist as strait molecules, knowledge of base and a.a. sequence of a protein does not automatically reveal the function of the protein
whole sequences were not mapped, thus gaps exist in the map leading to misinterpretation of data
Potential discrimination against those with known genetic disorders, should insurance companies/employers (3rd parties) have access to medical information?
Knowledge of a genetic flaw does not mean effective treatment is available, is it better not to know?

Process information from secondary sources to access the reasons why the Human Genome Project could not be achieve by studying linkage maps

Linkage maps do not

Identify which chromosomes carry which genes
Give an exact locus of every gene on every human chromosome
Give the base sequence on every chromosome
Give the function of every gene on every chromosome

Outline the procedure to produce recombinant DNA

Recombinant DNA – is formed when DNA from one organism is inserted into another site

Making recombinant DNA

Firstly a gene is cut out of the chromosome of one species using restriction enzymes
A circular piece of DNA (plasmid) present in bacterial cells is removed from the cells and cut out with the same enzyme
Restriction enzymes are used to cut the desired gene from its DNA (the passenger DNA)
The same restriction enzyme is used to cut the DNA of the plasmid (the vector)
DNA ligase is used to fuse the gene into the plasmid at the site of sticky ends
The DNA has now been ‘recombined’ and there are two sources of DNA fused together
The plasmid is then placed back into the bacterial culture and it reproduces to make hundreds of copies of itself (gene cloning)

Some uses of recombinant DNA technology