HISTORY
Forensic Genetic Genealogy
Forensic genetics is based on chromosomal DNA . Boveri and Sutton (both around 1902) named the chromosome theory of inheritance the Boveri–Sutton chromosome theory. Schleiden, Virchow, and Bütschliwere were among the first scientists to recognize the structures that are now so familiar to everyone as chromosomes. DNA was first isolated by Friedrich Miescherin in 1869. James Watson and Francis Crick identified molecular structures in 1953. The construction of models was steered by the X-ray diffraction data obtained by Rosalind.
CHROMOSOMES
A chromosome is a compact and structured entity housing most of an organism’s DNA. In eukaryotic cells, a complement of chromosomes (46 in humans) typically exists, carrying the genetic blueprint distributed across them. Of these, 22 pairs are autosomes, while one consists of the XY combination for males and XX for females.
During most of the cell cycle, a chromosome consists of one long double-helix DNA molecule (with associated proteins, such as histone protein). During the S phase, the chromosome is replicated, resulting in an X-shaped structure called the metaphase of the chromosome.
The original DNA and its newly copied version are both referred to as chromatids.The two “sister” chromatids are joined at a protein junction called a centromere (which forms the X-shaped structure).
Chromosomes are usually visible under a light microscope only when the cell undergoes mitosis (cell division). Even then, the entire chromosome containing both the joined sister chromatids are becomes visible only during a sequence of mitosis are known as metaphase.
STRUCTURE OF DNA
The two strands of DNA are known as polynucleotides because they are composed of smaller units called nucleotides. Each nucleotide consists of a nitrogen-containing base—cytosine (C), guanine (G), adenine (A), or thymine (T)—”Attached to a sugar molecule called deoxyribose and a phosphate group”. Adenine and guanine are classified as the purine bases, while cytosine and thymine are the pyrimidine bases.
These nucleotides link together through covalent bonds formed between the sugar of one nucleotide and the phosphate of the next, forming a backbone with a sugar-phosphate-sugar-phosphate pattern. The DNA strands run in opposite directions, referred to as antiparallel.
Each sugar molecule in the backbone carries one of the four types of nucleobases, or bases. Through hydrogen bonds, the bases from one strand pair with a complementary bases on the other strand according to specific rules: adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G), forming double-stranded DNA.
DNA REPLICATION
Step 1: Before DNA replication can commence, the double-stranded molecule undergoes unwinding, facilitated by the enzyme DNA helicase. This enzyme disrupts hydrogen bonds between base pairs, separating the strands into a Y-shaped replication fork, serving as the replication template. DNA exhibits directionality in both strands, denoted by 5′ and 3′ ends. The replication fork is bi-directional; one strand progresses in the 3′ to 5′ direction (leading strand), while the other proceeds 5′ to 3′ (lagging strand), necessitating distinct replication processes.
Step 2: Replicating the leading strand initiates with binding a short RNA segment, called a primer, to the 3′ end of the strand. This primer serves as the starting point for replication and is synthesized by the enzyme DNA primase.
Step 3: DNA polymerases, enzymes responsible for elongation, synthesize the new strand. Both bacteria and human cells possess five known types of DNA polymerases. Replication of the lagging strand involves binding multiple primers at intervals, with DNA polymerase synthesizing Okazaki fragments between them. This process is discontinuous, resulting in fragmented replication. While some polymerases are primary replication enzymes, others function in error checking and repair.
Step 4: Upon completing both continuous and fragmented strands, exonucleases remove RNA primers from the original strands, replacing them with appropriate bases. Another exonuclease conducts proofreading of the newly synthesized DNA, correcting any errors. DNA ligase enzyme joins Okazaki fragments, creating a continuous strand. Telomerase, a specialized DNA polymerase, catalyzes the synthesis of telomere sequences at DNA ends. The parent strand and its complementary DNA then coil into the characteristic double helix structure.
PRINCIPLES OF PROTEIN SYNTHESIS
The arrangement of nucleobases along the DNA backbone encodes biological information. RNA strands are synthesized using DNA as a template in transcription. Following the genetic code, these RNA strands are translated to determine the sequence of a amino acids within the proteins in a process known as translation.
RNA TYPES AND STRUCTURE
RNA molecules are the single-stranded nucleic acids composed of nucleotides.
• RNA plays a significant role in the synthesis of proteins. as it is also involved in the transcription, decoding, and translation of the genetic code used to produce proteins.
• RNA stands for the ribonucleic acid, and like DNA, RNA nucleotides contain three components:
q A Nitrogenous Base
q A Five-Carbon Sugar
q A Phosphate Group
• RNA nitrogenous bases include adenine (A), guanine (G), cytosine (C), and uracil (U) instead of thymine
FUNCTIONS OF RNA
Messenger RNA (mRNA) is critical in DNA transcription and is integral to protein synthesis. Transcription involves copying the genetic information from DNA into an RNA message.
Transfer RNA (tRNA) is essential in the translation phase of protein synthesis. tRNA adopts a cloverleaf-like structure with three hairpin loops, featuring an amino acid attachment site and a specialized section known as the anticodon site within its middle loop. The anticodon on tRNA pairs with specific codons on mRNA, which are sequences of three nucleotides coding for amino acids or signaling the termination of translation.
Translation is the process of decoding the nucleotide sequences of mRNA into specific amino acid sequences. These sequences are then linked together to construct a protein.
Ribosomal RNA (rRNA) is the fundamental component of a ribosomes, which consist of large and small subunits. The large subunit harbors mRNA binding sites and tRNA binding sites. Acting akin to a conveyor belt in a factory, ribosomes facilitate the movement of tRNA along mRNA, enabling the synthesis of proteins by coordinating the incorporation of amino acids specified by mRNA codons.
GENE
The genetic code refers to the arrangement of nucleotide bases within nucleic acids (DNA and RNA), which dictates the formation of amino acid chains in proteins.
A codon is a set of three consecutive nucleotide bases that encode for either amino acid or signify the initiation or termination of protein synthesis.
These triplet codons are blueprints for assembling amino acids during protein production.
Ultimately, amino acids are connected sequentially to construct proteins.
GENETIC CODE: CODONS
RNA codons assign specific amino acids, with the sequence of bases determining the quantity of amino acid produced. Each of the four nucleotides in RNA can occupy one of three codon positions, resulting in 64 possible codon combinations. Among these, 61 codons correspond to amino acids, while three (UAA, UAG, UGA) act as termination signals, marking the end of protein synthesis.
The codon AUG is the initiation signal for translation and coding for the amino acid methionine. It signifies the start of protein synthesis. Additionally, multiple codons can specify the same amino acid, providing redundancy in the genetic code.
PROTEIN SYNTHESIS
TRANSCRIPTION
DNA transcription is the process of protein synthesis that involves a transcribing genetic information from DNA to RNA. There are four main steps to the process of DNA transcription:
• UNWINDING– During transcription, specific proteins called transcription factors unwind the DNA strand
• BINDINGS– RNA Polymerase binds to DNA. Genes that encode proteins are transcribed by RNA polymerase II. Genes encoding ribosomal RNAs are transcribed by RNA polymerase I, and genes encoding transfer RNAs are transcribed by RNA polymerase III.
• ELONGATION– The strand that serves as the template is called the antisense strand. The strand that is not transcribed is called the sense strand. However, the length of transcribed mRNA may be elongated to various degree
• TERMINATION– RNA polymerase moves along the DNA until it reaches a terminator sequence.. Then, RNA polymerase releases the mRNA polymer and detaches from the DNA.
TRANSLATION
Translation occurs in the cytoplasm following mRNA’s departure from the nucleus. Before being translated, mRNA undergoes various modifications. These modifications include the removal of non-coding sections known as introns, the addition of a poly-A tail at one end consisting of adenine bases, and a guanosine triphosphate cap at the other end. These adjustments eliminate unnecessary sections and protect mRNA ends, preparing them for translation.
In the cytoplasm, mRNA, rRNA, and tRNA work together to translate the transcribed message into chains of amino acids. During translation initiation, a small ribosomal subunit binds to the mRNA, while an initiator tRNA recognizes and attaches to a specific codon sequence on the mRNA. Afterward, a large ribosomal subunit joins the complex, initiating translation.
Both ribosomal subunits traverse along mRNA during elongation, translating its codons into a polypeptide chain. Ribosomal RNA catalyzes the formation of peptide bonds between amino acids, facilitating chain elongation. The process continues until a termination codon is encountered on mRNA.
Translation ceases upon reaching a termination codon, and the polypeptide chain is released from tRNA. The ribosome dissociates into large and small subunits. Each RNA codon is interpreted throughout translation, and the corresponding amino acid is added to the growing polypeptide chain. The resulting polypeptide undergoes further modifications before becoming a fully functional protein.
In summary, translation comprises three main stages: initiation, elongation, and termination. During these stages, ribosomal subunits bind to mRNA; amino acids are linked by tRNA, and protein synthesis concludes at a stop codon.
AMINO ACIDS
• Ala: Alanine
• Asp: Aspartic acid
• Glu: Glutamic acid
• Cys: Cysteine
• Phe: Phenylalanine
• Gly: Glycine
• His: Histidine
• Ile: Isoleucine
• Lys: Lysine
• Leu: Leucine
• Met: Methionine
• Asn: Asparagine
• Pro: Proline
• Gln: Glutamine
• Arg: Arginine
• Ser: Serine
• Thr: Threonine
• Val: Valine
• Trp: Tryptophan
• Tyr: Tyrosine
POSTTRANSLATIONAL MODIFICATION
Posttranslational modification (PTM) refers to the enzymatic alteration of proteins after their synthesis. Following protein biosynthesis, this process occurs when ribosomes translate mRNA into polypeptide chains. These polypeptides then undergo PTM to attain their mature functional form. PTMs play crucial roles in cell signaling.
These modifications can occur on amino acid side chains or at the protein’s C- or N-termini. They expand the chemical diversity of the 20 standard amino acids by altering existing functional groups or introducing new ones, such as phosphate groups.
Eukaryotic proteins often undergo glycosylation, where carbohydrate molecules are attached. Glycosylation aids in protein folding, enhances stability, and serves regulatory functions.
Lipidation, the attachment of lipid molecules, often targets proteins or specific protein regions associated with cell membranes.
Other forms of PTM include cleaving peptide bonds, such as converting a propeptide to its mature form or removing the initiator methionine residue. For example, insulin, a peptide hormone, undergoes two cleavage events after disulfide bond formation, resulting in a protein consisting of the two polypeptide chains connected with disulfide bonds.
