Heredity, Genetics and Protein synthesis

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Unit Tile: Heredity, Genetics and Protein Synthesis

TAQ 1

Assessment criterion 1.1

What are genes and chromosomes and what do they do?

A chromosome consists of hundreds or thousands of genes and a gene is the basic unit of inheritance and specialised parts that are thought to be important to the chromosome’s stability and function. The deoxyribonucleic acid DNA, which makes up the genes, is packaged with the aid of proteins to form a complex structure. Chromosomes also contain small amounts of ribonucleic acid RNA.

Every cell in our body has the same DNA but each person DNA is different and this is what makes each of us unique. DNA is made of two long-paired strands spiralled into famous double helix. Each strand contains millions of chemical building blocks called bases.

The genes are lying in long strands of DNA called chromosomes; it consists of a long combination of four different nucleotide bases chemical which are: A-adenine, C-cytosine, G-guanine and T-thymine which can determine our look, appearance, behaviour etc.

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Each human chromosome contains one very long DNA molecule which unravelled would measure about 4.8cm in length. The total length of DNA in the molecules of a human cell has been estimated to be about 2.2m. Chromosomal DNA is intricately folded and is tightly bound to protein molecules called histones. Histone are small proteins that are rich in the amino acid lysine and arginine.

TAQ 2

Assessment criterion 1.2

TAQ 2 (1)

Mendel was an Austrian monk who studied the inheritance of characteristics in garden peas (pisum sativum), which he grew in the vegetable garden of his monastery. He chose peas because they were easy to grow, they had a short life cycle, their pollination could be controlled, and they had easily observable characteristics. He studied seven characteristics, each of which has two contrasting alternatives. Mendel first had to establish pure-breeding plants which self fertilised produce identical offspring generation and generation. He then selected for his experiments two pure-breeding plants with alternative expressions of a particular characteristics example a tall plant and a dwarf plant. He crossed the plants by transferring pollen from one plant (the male parent) to the second plant (the female parent). Mendel made sure that the female plant could not be pollinated either by its pollen or by other by removing its anthers and covering its flowers with fine muslin bags. Mendel collected the seeds produced by the female parent and grew them the next year to give the first generation offspring F1 (first filial generation). He carefully recorded the characteristics of these plants and then crossed two plants from this generation. This type of cross involving plants of the same generation is called a self-cross. Again, the seed produced were collected and grown the following year to give the second generation offspring F2 (second filial generation), and the characteristics of each plant were recorded.

The most striking features of these results were:

  1. There were no plant of intermediate height
  2. There were no dwarf plants in the F1 generation, though they reappeared in the F2.

TAQ 2 (2)

T – Allele for tongue roller

t- Allele for non tongue roller

Mendel recognised that diploid organisms have two factors (alleles) for each character (gene), but only one factor is carried by each gamete. In cross between pure-breeding tongue roller and non tongue roller, all the children will be tongue roller because allele for tongue roller is dominant to the allele for non tongue roller. As the parent they must have been homozygous, therefore the genotypes can be represent as TT and tt. The male tongue roller produced gametes which each carried one dominant allele; the female gametes ach carried one recessive allele.

Punnett diagram

Parent phenotypes

Tongue roller

Non tongue roller

Parental genotypes

TT

Tt

Gamets

T

T

Offspring genotypes and phenotypes

Female gametes

""t

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Male gametes T

All tongue roller

In the diagram the genotype of the gametes to reinforce the idea that each allele is discrete and can potentially join with alleles in other gamete from sexual partner.

TAQ 2 (3)

The genetic explanation of the subsequent self cross between heterozygous is summarised as follows:

Parental genotypes

Tt

Tt

Gametes

T and t

T and t

Offspring genotypes and phenotypes

Female gametes

Tt

TT

Can taste

Tt

Can taste

Tt

Can taste

Tt

Cannot taste

T

Male gametes

t

Expected phenotype ration: 3 can taste, 1 cannot taste

The diagram show that there only three different possible genotypes in the offspring which are TT, Tt and tt. Assuming random fusion of gametes, the diagram also shows that the probability of each genotype occurring in the offspring as follows:

  • There is a 1 in 4 or 0.25 chances that a parent will have the genotype TT
  • There is a 2 in 4 or 0.5 chances that will be Tt
  • There is a 1 in 4 or 0.25 chances that will be tt

TAQ 2 (4)

In humans, a gene controls formations of muscles in the tongue that allow people those muscles to roll their tongue, while people who lack those muscles cannot roll their tongue. So the ability to roll one’s tongue is dominant over non-rolling tongue likewise the ability to taste certain substances are also generally controlled. For example, there is substance called phenylithiocabonate PTC which people can taste known as dominant while those who cannot taste known as recessive.

R- Represent tongue roller

r- Represent non tongue roller

T- Represent ability to taste (Taste PTC)

t- Represent inability to taste (Non taster)

RRtt × RrTt

RRTt

RRTt

RRTt

RRTt

RRtt

RRtt

RRtt

RRtt

RrTt

RrTt

RrTt

RrTt

Rrtt

Rrtt

Rrtt

Rrtt

RT

Rt

rT

rt

Genotype: 4RRTt, 4RRtt, 4RrTt, 4Rrtt

Phenotype: 8 tongue roller PTC taster, 8 tongue roller non- PTC taster.

TAQ 2 (5)

Each parent is Rr Tt and will be able to contribute the following allele combination to their offspring: RT, Rt, rT, rt. Since each child of two heterozygous parents has 1\2 chance of receiving a recessive trait from each parent, no matter what the first child phenotype the next child will have a 1\4 chance of being phenotypically recessive.

Parent genotype RrTt × RrTt

Gametes RT,Rt,tR, tr RT, Rt, rT, rt

Offspring genotypes and phenotypes

Gametes from one parent

RTRt rTrt

RRTT

RRTt

RrTT

RrTt

RRTt

RRtt

RrTt

Rrtt

RrTT

RrTt

rrTT

rrTt

RrTt

Rrtt

rrTt

rrtt

RT

Rt

rT

rt

Expected phenotype ratio:

  • 9 tongue rollers and taster
  • 3 tongue rollers and non-taster
  • 3 non-tongue rollers and tasting
  • 1 non-tongue roller and non-tasting.

TAQ 3

Assessment criterion 1.3

TAQ 3 (1)

Genes that are located on one or other of the sex chromosomes are said to be sex linked. Sex linkage results in an inheritance pattern different from that shown by genes carried on autocomes. Sex linkage genes may be on the X chromosomes (X linkage) or the Y chromosome (Y linkage). Because the X chromosome is much larger than the Y chromosome, most sex-link genes are concerned with the determination of female sex characteristics, the X chromosome also carries genes for non-sexual characteristic such as the ability to see particular colour and the ability to clot blood efficiently. In X linkage, a recessive allele is expressed more often in male than in female because male carry only one allele for an X-linked gene where as female have two alleles for the gene.

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Sex linkage refers to the tendency of certain inherited characteristic occurring more frequently in one sex than the other because the gene for the characteristics is carried on one or other of the sex chromosomes; a sex-linked characteristics may occur in both male and female although it tends to occur more frequently in one sex than the other, whereas sex limitation refers to characteristic that are usually expressed in only one or the other sex.

TAQ 3 (2)

Human sex is determined by a pair of sex chromosomes called X and Y, because these chromosomes do not look alike, they are sometimes called hetersomes and other chromosomes are called autosomes. Female have two X chromosomes (XX), male have one X and Y chromosomes (XY). Although the sex chromosomes determine the sex of an individual, it is important to realise that they do not carry all the genes responsible for the development of sexual characteristic. During meiosis, the sex chromosomes pair up and segregate into the daughter cells. Males are called the hererogametic sex because they produced different sperm approximately 50% contain an X chromosome and 50% have Y chromosome. Females are called the homogametic sex because all of their eggs contain only X chromosome.

In humans, the father’s sperm determines the sex of the baby: if a baby inherits a Y chromosome from its father it will be a boy; if it inherits an X chromosome from its father it will be a girl, so the sex of a baby depends on which sperm fertilises the egg cell.

TAQ 3 (3)

The alleles of different genes cannot assort independently if the genes are on the same chromosome. Such genes are said to be linked because the tent to inherited together. All the genes on the same chromosome are said to form linkage group.

During meiosis, at least one chiasma forms between two homologous chromosomes. However, it may not form between a particular pair of gene. The further apart the genes are more likely it is that crossing over will result in the formation of recombinants. The proportion of recombinants resulting from a dihydrid test cross is used to calculate the cross over value (COV), a measure of linkage and if linkage occur than the distance between genes

COV "" × 100 = "" = 10.8%

If the genes are not link, the expected phenotype ratio of such a cross is 1:1:1:1 and as there is a 50% chance that alleles on separate chromosomes will be inherited together, the expected COV is 50%. Linkage result in the COV being significantly less than 50%; the lower the value, the closer the gene. Thus the COV can be used to located the relative position of genes on chromosomes, a process called chromosome mapping or genetic mapping. By convention, one percent COV is equivalent to one map unit.

TAQ 4

Assessment criterion 2.1

The differences between individuals of the same species are called variation. These differences may be the result of genetic differences, the influence of the environment, or a combination of genetic and environmental influences. Variation is of two main types which are continuous and discontinuous variations.

In continuous variation, differences are slight and grade into each other. Characteristics such as human height and weight show these. Continuous variation, are usually determined by a large number of genes (they are polygenic) and considerable environmental influence.

In discontinuous variation, the different are discrete (separate) and clear cut, they do not merge into each other. Discontinuous variations are generally caused by different alleles of one, two or only a few genes.

Continuous variations are usually quantitative wish means they can be measured, whereas discontinuous variations are qualitative they tend to be defined subjectively in descriptive term.

Thus height in humans is continuous variations given a value in meters, whereas height in sweet peas is a discontinuous variations described as tall or dwarf.

TAQ 5 (1)

Assessment criterion 2.2

Mutation can be defined as a change in the amount of the chemical structure of DNA. If the information contained within the mutated DNA is expressed that is transcribed into mRNA and translated into a specific polypeptide chain, it can cause a change in the characteristics of an individual cell or organism. Mutation is the gametes of multicellular organisms that can be inherited by offspring. Mutation can happen spontaneously as a result of errors in DNA replication or errors during cell devision, or they can be induced by various environmental factors like x-ray, viral infection and many more. Factors that induce mutations are called mutagens.

TAQ 5 (2)

De novo mutation means new mutation which can occur in an egg or sperm cell, or those that occur just after fertilization. De novo mutation may explain genetic disorders in which an affected child has a mutation in every cell, but has no family history of the disorder, so I see it as a chromosome abnormality that occurred in the individual and was not inherited from the parents. Most mutations are harmful however, that is diploid organisms such as us, and mutations usually result in recessive alleles. Many mutations result in a change in the shape of a protain so that the protain cannot function properly. Mutation can occur in a non-coding part of DNA, occasionally, a mutation is beneficial, changing the phenotype so that an organism has a better chance of surviving and producing. Although beneficial mutations are very rare events, they are bound to happen sooner or later if there are a larger number of individuals in a population. These mutations are of immense importance because they are the ultimate source of all variation.

TAQ 5 (3)

Mosaicism is a condition in which cells with the same person have a different genetic makeup. This condition can affect any type of cell, including blood cell, egg and sperm cell, skin cell. This can be caused by an error in cell division very early in the development of the unborn baby. It may affect either the number or the structure of chromosomes; this is usually the result of error occurring during meiosis. A neuploidy is the loss or gain of single chromosome. It results in the formation of gametes with either one or two many or one too few chromosomes. One pair of chromosomes has failed to separate at anaphase and so the pair has moved to one pole of the cell, the phenomenon of non-disjunction. After fertilization a zygote formed with an extra chromosome is said to be trisomic, like Down syndrome where chromosome number 21 is trisomic. The errors in the production of ova rather than sperm and incidence of the mutation is related to the age ot the mother.

TAQ 5 (4)

Many species of organisms’ especially insect species have two or more adult body forms that are generally distinct from one another, but which are contained within the same interbreeding population. This condition is known as polymorphism.

Polymorphism is the term used to describe the presence of clear-cut, genetically determined differences between large groups in the same population where a population is split into two subpopulations it is the result of conditions that favour more than one phenotype within the population. If gene flow between the subpopulation is prevented, then ultimately each population may give rise to a new species. Examples of polymorphism are colour and banding grouping in humans.

TAQ 6 (1, 2 and 3)

Assessment criterion 3.1

Protein synthesis occurs in all cells at the ribosomes and involves the assembly of amino acids in the order into polypeptide chains as directed by the genetic code on the DNA. DNA needs to be kept intact as the permanent store of genetic information, so it is not used directly for protein synthesis. Instead, the information encoded in DNA, in the nucleus is transferred to smaller molecules of another nucleic acid called messenger RNA (mRNA). The two main stage of protein synthesis are transcription and translation.

Transcription is known as the transfer of information from DNA to mRNA. So starting transcription; the hydrogen bonds holding the double helix of DNA together are broken by an enzyme called helicase, exposing the bases in the transcribing DNA; A large enzyme called polymerase attaches to the strand at a particular base sequence, the promoter site initiating transcription and during transcripton, the enzymes move along the cistron in the 5’ → 3’ direction when the enzyme moves on to another region of the transcribing DNA strand, the double helix of the DNA reforms behind it, so on reaching a special stop, sequence called a terminator, the enzymes detaches and the mRNA molecule peels away from the DNA.

Translation described as the conversion of the information in MRNA to make polypeptides. It is the process by which information encoded within mRNA is used to make a specific polypeptide chain. This process happens on ribosomes in the cytoplasm. Each ribosome consists of two subunits made up of proteins and ribosomal RND (rRNA). The small subunit has a binding site for mRNA and the large subunit has binding sites for transfer RNA (tRNA).

Reference

Michael Kent (2000) Advanced Biology

Phil Bradfield, John Dodds and Norma Taylor (2001) AS Level Biology

Olubukola OkeyalePage 1