Lab 2 Genetics PDF - Titu Maiorescu University

Titu Maiorescu University Faculty of Dental Medicine Specialization: Dental Medicine Genetics, year 2 The chemistry of HEREDITY  There are two classes of genetic materials that are responsible for the transfer of information from one generation to ano...

Titu Maiorescu University Faculty of Dental Medicine Specialization: Dental Medicine Genetics, year 2 The chemistry of HEREDITY  There are two classes of genetic materials that are responsible for the transfer of information from one generation to another:  DNA or deoxyribonucleic acid  RNA or ribonucleic acid  It is in the DNA or RNA sequences that biological information is stored and passed on.  Most organisms contain DNA except some viruses which contain RNA as their genetic material. The Nucleus  … is the largest and most prominent of a cell’s organelles; .... is considered the control center of the cell because it stores all the genetic instructions for manufacturing proteins.  Inside the nucleus lies the blueprint that dictates everything a cell will do and all of the products it will make.  This information is stored within DNA.  Each cell in your body (except for germ cells) contains the complete set of your DNA.  When a cell divides, the DNA must be duplicated so that each new cell receives a full complement of DNA. Organisation of DNA  The genetic instructions that are used to build and maintain an organism are arranged in an orderly manner in strands of DNA. Within the nucleus are threads of chromatin composed of DNA and associated proteins.  Along the chromatin threads, the DNA is wrapped around a set of histone proteins. A nucleosome is a single, wrapped DNA-histone complex.  Multiple nucleosomes along the entire molecule of DNA appear like a beaded necklace, in which the string is the DNA and the beads are the associated histones.  When a cell is in the process of division, the chromatin condenses into chromosomes, so that the DNA can be safely transported to the “daughter cells.”  The chromosome is composed of DNA and proteins; it is the condensed form of chromatin. Titu Maiorescu University Faculty of Dental Medicine Specialization: Dental Medicine Genetics, year 2  It is estimated that humans have almost 22,000 genes distributed on 46 chromosomes. DNA Structure and Functions  According to Watson and Crick, the basic building block of DNA is the nucleotide, which consists of three parts: a sugar molecule called deoxyribose, a phosphate group and a nitrogenous base.  Deoxyribose: It is a five-carbon sugar molecule that forms the backbone of DNA strands. The deoxyribose sugar molecules are connected through a phosphodiester bond, creating a sugar-phosphate backbone.  Phosphate Group: It is attached to the 5’ carbon of the deoxyribose sugar. The phosphate group is responsible for linking adjacent nucleotides together through phosphodiester bonds. The phosphate group gives the DNA a negative charge.  Nitrogenous Bases: In a cell, there are four types of nitrogenous bases: adenine (A), thymine (T), cytosine (C), and guanine (G). These bases Titu Maiorescu University Faculty of Dental Medicine Specialization: Dental Medicine Genetics, year 2 project from the sugar-phosphate backbone into the interior of the helix and are involved in complementary base pairing.  The two DNA strands (polymers) wound around each other in a right-handed double helix that looks like a twisted ladder or a spiral-shaped staircase.  The 5’ carbon of one nucleotide is connected to the 3’ carbon of another nucleotide by phosphodiester bonds. There is a covalent bonding between the oxygen of the 3’ carbon of the first nucleotide and the phosphorous in the phosphate group of the 5’ carbon of the second nucleotide. Thus, these are called 3’-5’ phosphodiester bonds.  The two strands run in opposite directions, known as antiparallel orientation. One strand has a 5’ end with a free phosphate group and a 3’ end with a free hydroxyl group (-OH). The other strand is oriented in the opposite direction having its 5’ end facing the 3’ end of the first strand and vice versa. Some critical features of a DNA strand are: 1. Complementary Base Pairing: The nitrogenous bases in DNA form pairs through noncovalent hydrogen bonding, ensuring specificity in base pairing. Adenine (A) always pairs with thymine (T) through two hydrogen bonds, while cytosine (C) always pairs with guanine (G) through three hydrogen bonds. This complementary base pairing is the foundation of DNA’s ability to replicate accurately and transmit genetic information. Titu Maiorescu University Faculty of Dental Medicine Specialization: Dental Medicine Genetics, year 2 2. Base Stacking: The flat, planar structure of the nitrogenous bases allows them to stack on top of each other within the double helix. This base-stacking interaction stabilizes the DNA molecule and contributes to its structure and shape. 3. Major and Minor Grooves: The base pairing along the helix forms grooves on the surface of the DNA double helix. These grooves are known as the major groove and the minor groove. They are critical in interactions with proteins that bind to specific DNA sequences and regulate cellular processes. What is the Function of DNA?  DNA is necessary because it contains the instructions for an organism to grow and reproduce. Here are the essential functions of DNA: 1. Store and Transmit Genetic Information (Inheritance) - DNA contains the instructions that determine an organism’s characteristics, such as its physical traits, behavior, and metabolic processes. These genetic instructions are passed from parents to offspring during reproduction, ensuring species continuity and trait inheritance 2. Helps to Synthesize Proteins - DNA serves as a template for synthesizing proteins through a two-step process: transcription and translation. 3. Helps to Copy DNA - Before a cell divides, it must duplicate its genetic material to ensure that each daughter cell receives an identical copy of the DNA (DNA replication). 4. Bringing Genetic Diversity within the Population - While DNA replication is remarkably accurate, errors can occasionally occur, leading to changes in the DNA sequence known as mutations. Some modifications can be advantageous, offering new adaptations that confer survival benefits in specific environments. 5. Regulating Gene Functions - Not all genes in a cell are active all the time. Instead, they are precisely regulated to respond to specific cues and signals. This regulation allows cells to differentiate into various types during development and adapt their functions to environmental changes. DNA Replication - Helps maintain the exact copies generation after generation.  DNA replication is the copying of DNA that occurs before cell division can take place.  DNA replication faithfully duplicates the entire genome of the cell. During DNA replication, several different enzymes work together to pull apart the two strands so each strand can be used as a template to synthesize new complementary strands. Titu Maiorescu University Faculty of Dental Medicine Specialization: Dental Medicine Genetics, year 2  The two new daughter DNA molecules each contain one preexisting strand and one newly synthesized strand. Thus, DNA replication is said to be “semiconservative.” STAGE 1: INITIATION. The two complementary strands are separated, much like unzipping a zipper. Special enzymes, including helicase, untwist and separate the two strands of DNA. STAGE 2: ELONGATION. Each strand becomes a template along which a new complementary strand is built. DNA polymerase brings in the correct bases to complement the template strand, synthesizing a new strand base by base. A DNA polymerase is an enzyme that adds free nucleotides to the end of a chain of DNA, making a new double strand. This growing strand continues to be built until it has fully complemented the template strand. STAGE 3: TERMINATION. Once the two original strands are bound to their own, finished, complementary strands, DNA replication is stopped and the two new identical DNA molecules are complete.  Mistakes made during DNA replication, such as the accidental addition of an inappropriate nucleotide, have the potential to render a gene dysfunctional or useless. Fortunately, there are mechanisms in place to minimize such mistakes. A DNA proofreading process enlists the help of special enzymes that scan the newly synthesized molecule for mistakes and correct them. Titu Maiorescu University Faculty of Dental Medicine Specialization: Dental Medicine Genetics, year 2 Gene expression – Protein synthesis  DNA is housed within the nucleus, and protein synthesis takes place in the cytoplasm, thus there must be some sort of intermediate messenger that leaves the nucleus and manages protein synthesis.  This intermediate messenger is messenger RNA (mRNA), a single-stranded nucleic acid that carries a copy of the genetic code for a single gene out of the nucleus and into the cytoplasm where it is used to produce proteins.  Gene expression begins with the process called transcription, which is the synthesis of a strand of mRNA that is complementary to the gene of interest.  This process is called transcription because the mRNA is like a transcript, or copy, of the gene’s DNA code.  The triplets within the gene on this section of the DNA molecule are used as the template to transcribe the complementary strand of RNA.  A codon is a three-base sequence of mRNA, so-called because they directly encode amino acids. Stage 1: INITIATION. A region at the beginning of the gene called a promoter—a particular sequence of nucleotides—triggers the start of transcription. Stage 2: ELONGATION. Transcription starts when RNA polymerase unwinds the DNA segment. One strand, referred to as the coding strand, becomes the template with the genes to be coded. The polymerase then aligns the correct nucleic acid (A,C, G, or U) with its complementary base on the coding strand of DNA. RNA polymerase is an enzyme that adds new nucleotides to a growing strand of RNA. This process builds a strand of mRNA. Stage 3: TERMINATION. When the polymerase has reached the end of the gene, one of three specific triplets (UAA, UAG, or UGA) codes a “stop” signal, which triggers the enzymes to terminate transcription and release the mRNA transcript.  Before the mRNA molecule leaves the nucleus and proceeds to protein synthesis, it is modified in several ways. The process called splicing removes the non-coding regions from the pre- mRNA transcript.  A spliceosome—a structure composed of various proteins and other molecules—attaches to the mRNA and “splices” or cuts out the non-coding regions. Titu Maiorescu University Faculty of Dental Medicine Specialization: Dental Medicine Genetics, year 2  The removed segment of the transcript is called an intron. The remaining exons are pasted together. An exon is a segment of RNA that remains after splicing.  Translation requires two major aids: first, a “translator,” the molecule that will conduct the translation, and second, a substrate on which the mRNA strand is translated into a new protein  Both requirements are fulfilled by other types of RNA. The substrate on which translation takes place is the ribosome.  Ribosomal RNA (rRNA) is a type of RNA that, together with proteins, composes the structure of the ribosome.  Transfer RNA (tRNA) is a type of RNA that ferries the appropriate corresponding amino acids to the ribosome and attaches each new amino acid to the last, building the polypeptide chain one-by-one.  The tRNA molecules must be able to recognize the codons on mRNA and match them with the correct amino acid.  An anticodon is a trinucleotide sequence located at one end of a transfer RNA (tRNA) molecule, which is complementary to a corresponding codon in a messenger RNA (mRNA) sequence. Titu Maiorescu University Faculty of Dental Medicine Specialization: Dental Medicine Genetics, year 2  Translation consists of three main stages: 1. INITIATION - takes place with the binding of a ribosome to an mRNA transcript. 2. ELONGATION - involves the recognition of a tRNA anticodon with the next mRNA codon in the sequence. Once the anticodon and codon sequences are bound, the tRNA presents its amino acid cargo and the growing polypeptide strand is attached to this next amino acid. This attachment takes place with the assistance of various enzymes and requires energy. 3. TERMINATION – occurs when the final codon on the mRNA is reached which provides a “stop” message that signals termination of translation and triggers the release of the complete, newly synthesized protein.