Abbreviated as G-A-T-C, these four-building blocks are like a recipe book which holds the instructions for making all the proteins in the body of an organism. By reading the DNA genetic code from these 4 chemical structures, a cell can produce proteins to perform various functions.
INTRODUCTION TO GENETICS AND BREEDING
Genetics is the study of how heritable traits are transmitted from parents to offspring Genetics can be defined as the application of the effects of genetic variation and selection in promoting and sustaining valuable trait combinations in animal and crop breeding programmes.
Genetic improvement is the use of genetically superior animals to parent the next generation. The definition of genetic superiority will be a function of many variables, including potential profitability of the genotype for the given production environment and market being supplied. Not all genetic superiority needs to be based on profit but in commercial livestock enterprises, profitability is likely to be most important.
Genetic improvement should be continuous, and the most important variable that will limit continuous improvement is inbreeding. A sustainable programme must manage inbreeding.
It must be designed in conjunction with culling decisions that influence the current herd and the existing herd management
The expression of genes in an organism can be influenced by the environment and some environmental exposures can directly damage genetic material. It is difficult to imagine how environmental factors such as temperature or trauma can affect a DNA molecule. In order to understand this, we have to accept that the environment is a continuum that spans from the external world up to the molecular level.
The science of genetics
Genetics is the study of how common heritable traits from previous generations are transferred to future generations. Erythrocytes (Red blood cells) are the exception to this.
What is a chromosome?
Chromosomes are bundles of tightly coiled DNA located within the nucleus of almost every cell in our body.
In animal cells, DNA is tightly packed into thread-like structures called chromosomes. A single length of DNA string is wrapped many times around histones (complex protein structures) to form nucleosomes. The nucleosomes coil up tightly to create chromatin loops which wrap around each other to make up a full chromosome.
Each chromosome has two short arms (p- arms), two longer arms (q- arms), and a centromere holding it together at the centre.
At the ends of each of our chromosomes are sections of DNA called telomeres. Telomeres protect the ends of the chromosomes during DNA replication: The process by which a cell makes an identical copy of its genome before it divides. Telomeres can be thought of as an Aglet – the piece of plastic coating that keeps shoelaces from unravelling. Similarly, telomers prevent chromosome from unravelling and losing integrity with each replication.
Chromosome counts in different species
Humans have 23 pairs of chromosomes (46 in total): one set from the father and a set from the mother. One set (or pair) of the 23 chromosomes are the socalled sex chromosomes, to differentiate between male or female and are expressed as XY for male and XX for female.
The other 22 pairs are non-sex chromosomes (or autosomes) and look the same in males and females.
What is DNA?
Nucleic acids are the molecules use to store, transfer and express genetic information. DNA is the abbreviation for deoxyribonucleic acid. It is the molecule that stores genetic information in an organism. The nucleotide is the most basic sub-unit of DNA. Nucleic acids are biopolymers, or small biomolecules, essential to all known forms of life.
We need to understand the chemistry behind DNA to fully appreciate the importance and function of the molecule.
DNA represents a double helix (ladder) that is formed by base pairs attached to a sugar-phosphate backbone. The sides of the ladder (molecule) are made up of a 5- sugar molecule (deoxyribose), with a chemical formula as follows:
DNA contains four basic building blocks or “bases”: (A base refers to the basic unit of genetic instructions).
These are:
Abbreviated as G-A-T-C, these four-building blocks are like a recipe book which holds the instructions for making all the proteins in the body of an organism. By reading the DNA genetic code from these 4 chemical structures, a cell can produce proteins to perform various functions.
Refer to the protein process and different types of systems that develop from different types of proteins below.
The basis (building blocks GATC) can further be categorised in two single groups:
Pyrimidine group: thymine and cytosine
The chemistry of these nitrogen-containing basis forms the key to the function of DNA as it allows for complementary base pairing.
Complementary base pairing
Cytosine forms 3 hydrogen bonds with Guanine Adenine forms 2 hydrogen bonds with Thymine
It can be expressed as:
and
A = T (A bonds with T)
These bindings are called complementary base pairing because each base can only pair with a complementary base partner. Due to the chemical structure, C will only bond with G, and A will only bond with Tnin DNA.
Because of complementary base pairing, the hydrogen-bonded nitrogenous bases are often referred to as base pairs. The bases on one strand of the DNA molecule pair together with complementary bases on the opposite strand of DNA to form the ‘rungs’ of the DNA ladder.
Each base pair is joined together by hydrogen bonds. Each strand of DNA has a beginning and an end, called 5’ (five prime) and 3’ (three prime) respectively. The two strands run in the opposite direction (antiparallel) to each other so that one runs 5’ to 3’ and one runs 3’ to 5’ – they are called the sense strand and the antisense strand, respectively.
DNA replication
Replication is the process by which a cell makes an identical copy of its genome before it divides. During DNA replication, the double helix strand is unwound and each strand acts as a template for the next strand. Bases are matched to synthesize the new partner strands
DNA is a long molecule that contains our unique genetic code. Traits (characteristics) are described by the genetic information carried by a DNA molecule. DNA is the hereditary material in all living organisms (animals, plants, microorganisms, insects, humans, etc.) and is packed and compressed into chromosomes.
A genome is an organism’s complete set of genetic instructions. Each genome contains all the information needed to build that organism and allow it to grow and develop. A human genome is approximately 3,000,000,000 base pairs long and is packaged into 23 pairs of chromosomes, to make up the chromosome count of 46.
A genome is made up of a chemical called deoxyribonucleic acid or DNA for short.
The DNA making up each of our chromosomes contains thousands of genes.
Most DNA is located in the cell nucleus (where it is called nuclear DNA), but a small amount of DNA can also be found in the mitochondria (where it is called mitochondrial DNA).
The DNA between different species are distinguished by the following factors:
- The number of A=T and G=C pairs on the DNA
- The sequence in which these base pairs occur
- The variation in base pair structures e.g. A=T; T=A; C=G; G=C
- The length of the DNA molecule determined by the number of base pairs What is RNA?
We understand that DNA is like a blueprint of biological guidelines that a living organism must follow to exist and remain functional. RNA is synthesised from DNA when needed to carry the self-replicating DNA code from the cell nucleus, outside the cell, via the cytoplasm to the ribosomes. As DNA is found in the cell nucleus and in mitochondria, RNA molecules are found in the nucleus, cytoplasm and ribosomes.
RNA, or ribonucleic acid, helps to carry out this blueprint’s guidelines. RNA is more versatile than DNA, capable of performing numerous, diverse tasks in an organism, but DNA is more stable and holds more complex information for longer periods of time. Unlike the double helix structure of DNA, RNA is a single stranded structure, consisting of Cytosine, Guanine, Adenine and Uracil.
The base pair structures in RNA allows for adenine to bind to uracil (A=U), and cytosine to bind to guanine (C=G).
The information that is found in DNA determines which traits are to be created, activated or deactivated. The various forms of RNA transfer this code from the cell nucleus to the cell ribosomes in order to form the different proteins which are the building blocks of different types of tissues, organs and body parts.
RNA is also composed of a phosphate group, five-carbon sugar (the less stable ribose), and four nitrogen-containing nucleobases: adenine, uracil (not thymine), guanine, and cytosine.
RNA Structure
Similar to DNA, RNA is also a nucleic acid. Nucleic acids are long biological macromolecules that consist of smaller molecules called nucleotides. Again, as in DNA, these nucleotides in RNA also contain four nucleobases — sometimes called nitrogenous bases or simply bases — two purine and pyrimidine bases each.
RNA Transcription
DNA provides a simple mechanism for protein synthesis. In transcription, or RNA synthesis, the codons of a gene are copied into messenger RNA by RNA polymerase. As opposed to DNA replication, transcription results in an RNA complement that includes uracil (U) in all instances where thymine (T) would have occurred in a DNA complement.
During transcription, RNA, a single-stranded, linear molecule, is formed. It is complementary to DNA, helping to carry out the tasks that DNA lists for it to do.
In both the molecules, the nucleobases attach to their sugar-phosphate backbone. Each nucleobase on a nucleotide strand of DNA attaches to its partner nucleobase on a second strand (A links to T, and C links to G). This linking causes DNA’s two strands to twist and wind around each other, forming a variety of shapes, such as the famous double helix (DNA’s “relaxed” form), circles, and supercoils.
In RNA, adenine (A) links to uracil U) (not thymine), and cytosine (C) links to guanine (G). As a single stranded molecule, RNA folds in on itself to link up its nucleobases, though not all become partnered. These subsequent threedimensional shapes, the most common of which is the hairpin loop, help determine the role of the RNA molecule: messenger RNA (mRNA), transfer RNA (tRNA) or ribosomal RNA (rRNA).
Functions of RNA
RNA has several different functions that, though all interconnected, vary slightly depending on the type. There are three main types of RNA:
Messenger RNA (mRNA) transcribes genetic information from the DNA found in a cell’s nucleus and carries this information to the cell’s cytoplasm and ribosome.
Transfer RNA (tRNA) is found in a cell’s cytoplasm and is closely related to mRNA as its helper. tRNA literally transfers amino acids, the core components of proteins, to the mRNA in a ribosome.
Ribosomal RNA (rRNA) is found in a cell’s cytoplasm. In the ribosome, it takes mRNA and tRNA and translates the information they provide. From this information, it “learns” whether it should create, or synthesize, a polypeptide or protein.
Genetic Code
The codon is the basic unit of gene coding. The genetic code is the set of rules by which information encoded in genetic material (DNA or RNA sequences) is translated into proteins (amino acid sequences) by living cells
The genetic code by which DNA stores the genetic information consists of “codons” of three nucleotides. The functional segments of DNA which code for the transfer of genetic information are called genes. With four possible bases, the three nucleotides can give 4x4x4 = 64 different possibilities, and these combinations are used to specify the 20 different amino acids used by living organisms.
The genetic code expressing 64 different possibilities
The ribonucleic acid (RNA) that is directly involved in the transcription of the pattern of bases from the DNA to provide a blueprint for the construction of proteins is called messenger RNA or typically mRNA. The pattern for protein synthesis is then read and translated into the language of amino acids for protein construction with the help of transfer RNA or tRNA.
The sequence of bases in DNA operates as a true code in that it contains the information necessary to build a protein expressed in a four-letter alphabet of bases which is transcribed to mRNA and then translated to the twenty-aminoacid alphabet necessary to build the protein. Saying that it is a true code involves the idea that the code is free and unconstrained; any of the four bases can be placed in any of the positions in the sequence of bases. Their sequence is not determined by the chemical bonding. There are hydrogen bonds between the base pairs and each base is bonded to the sugar phosphate backbone, but there are no bonds along the longitudinal axis of DNA. The bases occur in the complementary base pairs A-T and G-C, but along the sequence on one side the bases can occur in any order, like the letters of a language used to compose words and sentences.
The most important properties of the genetic code:
The code is a triplet codon
The logic is that the nucleotide code must be able to specify the placement of 20 amino acids. Since there are only four nucleotides, a code of single nucleotides would only represent four amino acids, such that A, C, G and U could be translated to encode amino acids. A doublet code could code for 16 amino acids (4 x 4). A triplet code could make a genetic code for 64 different combinations (4 X 4 X 4) genetic code and provide plenty of information in the DNA molecule to specify the placement of all 20 amino acids.
The code is non-overlapping
Successive triplets of the code are read in order. It begins with the start codon and ends with the stop codon, and it is non-overlapping because of the single reading frame and only reading one triplet at a time. Each codon specifies a particular amino acid to build the protein.
The code is written without commas
The genetic code is written using no internal punctuation (like commas and semi-colons).
The code is non-ambiguous
Each codon specifies a particular amino acid, and only one amino acid. In other words, the codon ACG codes for the amino acid threonine, and nothing else.
- The code has polarity
DNA is a two-stranded molecule. Each strand is a polynucleotide composed of A (adenine), T (thymine), C (cytosine), and G (guanine) residues polymerized by “dehydration” synthesis in linear chains with specific sequences. Each strand has polarity, such that the 5′-hydroxyl (or 5′-phospho) group of the first nucleotide begins the strand and the 3′-hydroxyl group of the final nucleotide ends the strand
- The code is degenerate
Degeneracy just means that some amino acids have more than one nucleotide codon specifying them during protein synthesis. There are many instances in which different codons specify the same amino acid.
- Some codes have start codons
A start codon is the message that starts a certain protein being synthesized. For all proteins this is the nucleotide sequence “AUG” and this codes for the amino acid methionine. (an easy way to remember AUG is the question: “Are You Going?”)
- Some codes act as stop codons
The stop codons are the messages that inform the cell that the protein is made (complete?) and to stop adding more amino acids to the polypeptide. The stop codons do not have amino acids.
They are:
- UAA (Remember as: “You Are Annoying”)
- UAG (Remember as “You Are Gross”)
- UGA (Remember as “You Go Away”)
The code is universal
Almost all organisms in nature (from bacteria to humans) use exactly the same genetic code. The rare exceptions include some changes in the code in mitochondria, and in a few protozoan species.