Genetics



Genetics





Genetics is the study of genes, genetic variation, and heredity in organisms including for human beings. Each individual contains genes that provide instructions regarding physical, biochemical, and functional inherited traits. The Human Genome Project estimates each individual has 20,000 to 25,000 genes, which instruct protein makeup in the human body. Genes are made up of deoxyribonucleic acid (DNA), which transmits sequencing instructions that encode for protein structures. Most of an individual’s genetic makeup includes similar genes while a small portion of genes, less than 1%, are somewhat different. These differences are determined by alleles, which provide small differences in DNA sequences that determine characteristics as normally inherited while mutations or abnormalities can occur within the individual as genetic-related disorders or conditions.


GENETIC COMPONENTS

Genes are the instructions for building proteins for the human body. Genetic information carried in genes includes hereditary material as codes formed together in DNA as four bases, adenine (A), thymine (T), cytosine (C), and guanine (G) represented as base pairs in a double helix. Each pair is bound together as A with T and C and G to form a base pair as ladder rungs in the DNA molecule, along with a sugar and phosphate molecule that shape the backbone called a nucleotide. A nucleotide pair is represented as two strands to form a double helix in the DNA. A portion of the nucleotides within a gene instructs the coding of the protein while the gene itself provides other sequencing instructions about the protein makeup itself. DNA is located within the cell nucleus (nuclear DNA) including a small amount in the mitochondria (mitochondrial DNA).

The base pairs of the DNA instruct the function of each individual with over 3 billion bases found in the human body, of which 99% are similar. DNA is responsible for replication and duplication of these base codes to make copies of translated instructions that form an exact copy for the new cells. The human-coding gene can range between 500 letters and over 2.3 million while the average gene is 3,000 letters long. The human genome contains approximately 21,000 protein coding genes, which make up less than 2% of the nucleotides that also code for RNA molecules. The average individual has one to three base pair differences (include over 1.4 million differences) based on shape, function, and sequencing protein differences that make each person unique.

Chromosomes are threadlike structures contained within the nucleus of the DNA molecule. Histones are proteins made up of tightly coiled DNA that support the structure of the chromosome. The chromosome can be described by its constriction point known as the centromere. Each arm of the chromosome is called a chromatid, with the short arm called the “p arm” and long arm called the “q arm.” Telomeres are sections of DNA found at the end of each chromosome that provide distinct structures containing the same sequence of bases repeated over and over again, which play a role in organization, protection, and replication of the chromosome. Chromosomes are only visible under the microscope during cell division where visualization of the location of the centromere describes the location of specific genes. An illustration of the chromosome structure is provided below.


Each individual inherits one copy of his or her gene from each of the two parents. Every normal human cell (except reproductive cells) has 46 chromosomes, 22 paired chromosomes called autosomes that look the same in males and females and the 23rd chromosome, known as the sex chromosome, a pair of XX in the female and an X and a Y in the male. A person’s individual set of 23 pairs of chromosomes is organized as a profile in what is known as a karyotype.

Chromosomes are arranged in the karyotype based on their size, with chromosome 1 as the largest spanning 249 million base pairs with chromosome 21 as the smallest spanning approximately 48 million base pairs (DNA building blocks). The location of a gene within a chromosome is identified using a diagram called an ideogram. Staining is performed to determine size, location, and banding patterns as done in the karyotype to look for abnormalities in the chromosome such as for Down’s syndrome in chromosome 21. Staining is also used for the ideogram to describe each gene on the chromosome as seen in below illustration.






CHROMOSOME FORMATION

Chromosomal formation occurs through the processes of mitosis and meiosis as the two types of cell division. Chromosome formation occurs during mitosis and meiosis during cellular replication and division for autosomal and sex chromosomes.


Mitosis

Chromosomes divide and replicate through the process of mitosis. The cellular process of mitosis replicates chromosomes and prepares two identical nuclei as the fertilized ovum, known as the zygote, undergoes a kind of cell division. During this process, the double helix of DNA separates into two chains; each chain serves as a template for constructing a new chain. Individual DNA nucleotides are linked into new strands with bases complementary to those in the originals. In this way, two identical double helices are formed, each containing one of the original strands and a newly formed complementary strand. These double helices are duplicates of the original DNA chain.

Mitotic cell division occurs in five phases: interphase, prophase, metaphase, anaphase, and telophase. At interphase, one diploid cell of 46 chromosomes (2n) begins DNA replication of chromosomes as two sister chromatids attached at the centromere of the chromosome. At prophase, the nuclear membrane and nucleolus begin to break down as DNA condenses to form chromosomes while the mitotic proteins called microtubules form and attach to the kinetochores of each chromosome. In metaphase, DNA condenses the chromosomes that align at the center of the cell through the action of the microtubules fibers with DNA. At anaphase, the microtubules fibers shorten, pulling the chromatids of each chromosome to opposite poles of the cell while unattached fibers further elongate the cell. During telophase, the chromatids (called chromosomes) reach the poles of the cell as nuclear membranes reform as nucleoli, as chromosomes relax and DNA unwinds. Cytokinesis then occurs as division of the cytoplasm forms two daughter cells with 46 (2n) chromosomes in each of these cells genetically identical to the original and to each other.



MEIOSIS

Meiosis is the process of formation of the egg cell found in the ovary for the female and for the sperm cell found in the testis for the male. As germ cells, these cells are diploid (2n) having two sets of chromosomes that will undergo meiosis to become a haploid cell (1n) or one set of chromosomes. During fertilization, haploid cells fuse to form a diploid offspring (female or male). Meiosis involves two divisions, as Meiosis I and Meiosis II, which result in four haploid cells (1n). In Meiosis I, beginning at Prophase I, identical sister chromatids are joined at their centromeres including two homologous chromosomes lined up next to each other that undergo crossing-over at the site of the centromere to exchange DNA. After crossing over, the sister chromatids of one chromosome are no longer identical to one another. In Metaphase I, homologous chromosomes line up randomly along the equator of the cell, known as the process of independent assortment where gametes have different combinations of parental chromosomes with spindle fibers that attach to each centromere. In Anaphase I, chromosomes move apart from one another along the spindle fibers to the opposite end of the cell, with each chromosome double-stranded with two sister chromatids while separating each homologous pair as haploid for the formation of two different new cells. In Telophase I, cytokinesis occurs with cell division to form two cells created with half the number of chromosomes of the original cell. Meiosis II starts similar to division in mitosis at Prophase II where two cells with two sets of chromosomes each include spindle fibers with formation at the poles of the cell. In Metaphase II, the chromosomes line up along the equator, as each cell has only one of each homologous chromosome. Then in Anaphase II, sister chromatids move away from each other. Cytokinesis occurs in Telophase II forming four genetically different (1n) haploid cells.


GENETIC VARIATIONS


Numerical Abnormalities from Nondisjunction

Nondisjunction occurs with chromosomes that fail to disjoin during mitosis or meiosis. There are three forms of nondisjunction that include failure of sister chromatids separating during mitosis, failure of a pair of homologous chromosomes to separate in Meiosis I, and failure of sister chromatids to separate in Anaphase II in Meiosis II. Monosomy refers to a nondisjunction in which an individual is missing one of a pair of chromosomes, as 2n – 1. An example of monosomy is Turner’s syndrome where the female has only one X chromosome. Trisomy refers to a nondisjunction in which an individual has an extra chromosome, as 2n + 1. Down’s syndrome, or trisomy 21, refers to an individual with 3 chromosomes found at chromosome 21 instead of having one pair of chromosomes.


Structural Abnormalities

Alteration of the structure of the chromosome can occur through a deletion, duplication, translocation (to another chromosome), inversion, or ring formation in the
chromosome. A de novo abnormality is a noninherited chromosomal condition, while maternal age and environment can also be factors for chromosomal abnormalities. Mosaicism is another abnormality referred to as an abnormal cell division in two or more cells creating different numbers of chromosomes.



Genetic Alterations

Genetic alterations related to cancer include aneuploidy, which refers to an abnormal chromosome number due to malignancy. Amplification occurs when overexpression of a gene results from an increase number of genes found in certain cancers. The four active stages in the cell cycle (G1, S, G2, and M) can also be altered when loss of checkpoints in one or more of these stages can also lead to genetic instability and cell damage.


DNA SEQUENCING

As mentioned previously, each of the two strands of DNA in a chromosome consists of thousands of combinations of four nucleotides: adenine (A), thymine (T), cytosine (C), and guanine (G), arranged in complementary triplet pairs (called codons), each of which represents an amino acid; a specific sequence of triplets represents a gene. The strands are held together loosely by chemical bonds between adenine and thymine or between cytosine and guanine. The looseness of the bonds allows the strands to separate easily during DNA replication.

The genes carry a code for each trait a person inherits, from blood type to eye color to body shape and a myriad of other traits.

DNA ultimately controls the formation of essential substances throughout the life of every cell in the body. It does this through the genetic code, the precise sequence of AT and CG pairs on the DNA molecule. Genes control not only hereditary traits, transmitted from parents to offspring, but also cell reproduction and the daily functions of all cells. Genes control cell function by controlling the structures and chemicals that are synthesized within the cell.

A multistep process occurs to encode genetic information. The first step involves transcription of DNA sequencing where proteins attach to the DNA strand to make copies of the DNA sequencing in the form of messenger RNA. The next step includes messenger RNA or mRNA, by a sequence of bases, transferring encoded genetic information across the cell nucleus to the outside of the cell. The genetic information is then used through the work of ribosomes to translate and synthesize proteins.


PATTERNS OF INHERITANCE

Autosomal disorders, sex-linked disorders, and multifactorial disorders result from changes to genes or chromosomes. Some defects arise spontaneously, whereas others may be caused by environmental agents, including mutagens, teratogens, and carcinogens.



Trait Predominance

Each parent contributes one set of chromosomes (and therefore one set of genes) so that every offspring has two genes for every locus on the autosomes. Some characteristics, or traits, such as eye color, are determined by one gene that may have many variants (alleles). Others, called polygenic traits, require the interaction of two or more genes. In addition, environmental factors may affect how genes are expressed.

Variations in a particular gene — such as brown, blue, or green eye color — are called alleles. A person who has identical genes on each member of the chromosome pair is homozygous for that gene; if the alleles are different, the person is said to be heterozygous.


Autosomal Inheritance

On autosomes, one allele may be more influential than another in determining a specific trait. The more powerful, or dominant, gene product is more likely to be exhibited in the offspring than the less influential, or recessive, gene product. Offspring will exhibit a dominant allele when one or both chromosomes in a pair carry it. A recessive allele will not be exhibited unless both chromosomes carry identical copies of the allele. For example, a child may receive a gene for brown eyes from one parent and a gene for blue eyes from the other parent. The gene for brown eyes is dominant, and the gene for blue eyes is recessive. Because the dominant allele is more likely to mask the recessive allele, the child is more likely to have brown eyes.

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Sep 22, 2018 | Posted by in ANATOMY | Comments Off on Genetics
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