Genes and Chromosomes - Fundamentals - Merck Manuals Consumer Version
Explain the relationship between the following words – cells, genes, chromosomes, tissues, DNA, proteins. Tissues are integrated groups of cells with a common. tissues are made of cells that contain chromosomes made of DNA with regions called genes that code for proteins. Conclusion Questions DNA Detectives. Explain the relationship between the following words – cells, genes, chromosomes, tissues, DNA, proteins. Explain why scientists probably used PCR when they prepared the DNA for your.
Each molecule of tRNA brings one amino acid to be incorporated into the growing chain of protein, which is folded into a complex three-dimensional structure under the influence of nearby molecules called chaperone molecules. These cells look and act differently and produce very different chemical substances. However, every cell is the descendant of a single fertilized egg cell and as such contains essentially the same DNA.
Cells acquire their very different appearances and functions because different genes are expressed in different cells and at different times in the same cell. The information about when a gene should be expressed is also coded in the DNA. Gene expression depends on the type of tissue, the age of the person, the presence of specific chemical signals, and numerous other factors and mechanisms.
Knowledge of these other factors and mechanisms that control gene expression is growing rapidly, but many of these factors and mechanisms are still poorly understood. The mechanisms by which genes control each other are very complicated. Genes have markers to indicate where transcription should begin and end. Various chemical substances such as histones in and around the DNA block or permit transcription.
Replication Cells reproduce by splitting in two. Because each new cell requires a complete set of DNA molecules, the DNA molecules in the original cell must reproduce replicate themselves during cell division. Replication happens in a manner similar to transcription, except that the entire double-strand DNA molecule unwinds and splits in two. After splitting, bases on each strand bind to complementary bases A with T, and G with C floating nearby.
Brain Basics: Genes At Work In The Brain | National Institute of Neurological Disorders and Stroke
When this process is complete, two identical double-strand DNA molecules exist. There are also chemical mechanisms to repair DNA that was not copied properly. However, because of the billions of base pairs involved in, and the complexity of, the protein synthesis process, mistakes can happen. Such mistakes can occur for numerous reasons including exposure to radiation, drugs, or viruses or for no apparent reason.
Minor variations in DNA are very common and occur in most people. Most variations do not affect subsequent copies of the gene. Mistakes that are duplicated in subsequent copies are called mutations. Inherited mutations are those that may be passed on to offspring. Mutations can be inherited only when they affect the reproductive cells sperm or egg. Mutations that do not affect reproductive cells affect the descendants of the mutated cell for example, becoming a cancer but are not passed on to offspring.
Mutations may be unique to an individual or family, and most mutations are rare. Mutations may involve small or large segments of DNA. Depending on its size and location, the mutation may have no apparent effect or it may alter the amino acid sequence in a protein or decrease the amount of protein produced.
If the protein has a different amino acid sequence, it may function differently or not at all. An absent or nonfunctioning protein is often harmful or fatal.
For example, in phenylketonuriaa mutation results in the deficiency or absence of the enzyme phenylalanine hydroxylase. This deficiency allows the amino acid phenylalanine absorbed from the diet to accumulate in the body, ultimately causing severe intellectual disability. In rare cases, a mutation introduces a change that is advantageous. For example, in the case of the sickle cell gene, when a person inherits two copies of the abnormal gene, the person will develop sickle cell disease.
However, when a person inherits only one copy of the sickle cell gene called a carrierthe person develops some protection against malaria a blood infection.
Although the protection against malaria can help a carrier survive, sickle cell disease in a person who has two copies of the gene causes symptoms and complications that may shorten life span. Natural selection refers to the concept that mutations that impair survival in a given environment are less likely to be passed on to offspring and thus become less common in the populationwhereas mutations that improve survival progressively become more common.
Thus, beneficial mutations, although initially rare, eventually become common. The slow changes that occur over time caused by mutations and natural selection in an interbreeding population collectively are called evolution. Not all gene abnormalities are harmful. For example, the gene that causes sickle cell disease also provides protection against malaria. Chromosomes A chromosome is made of a very long strand of DNA and contains many genes hundreds to thousands.
The genes on each chromosome are arranged in a particular sequence, and each gene has a particular location on the chromosome called its locus. In addition to DNA, chromosomes contain other chemical components that influence gene function. Pairing Except for certain cells for example, sperm and egg cells or red blood cellsthe nucleus of every human cell contains 23 pairs of chromosomes, for a total of 46 chromosomes.
Normally, each pair consists of one chromosome from the mother and one from the father. There are 22 pairs of nonsex autosomal chromosomes and one pair of sex chromosomes. Paired nonsex chromosomes are, for practical purposes, identical in size, shape, and position and number of genes. Because each member of a pair of nonsex chromosomes contains one of each corresponding gene, there is in a sense a backup for the genes on those chromosomes.
The 23rd pair is the sex chromosomes X and Y. Sex chromosomes The pair of sex chromosomes determines whether a fetus becomes male or female. Males have one X and one Y chromosome. Females have two X chromosomes, one from the mother and one from the father. In certain ways, sex chromosomes function differently than nonsex chromosomes. The smaller Y chromosome carries the genes that determine male sex as well as a few other genes.
The X chromosome contains many more genes than the Y chromosome, many of which have functions besides determining sex and have no counterpart on the Y chromosome. In males, because there is no second X chromosome, these extra genes on the X chromosome are not paired and virtually all of them are expressed. Genes on the X chromosome are referred to as sex-linked, or X-linked, genes.
Normally, in the nonsex chromosomes, the genes on both of the pairs of chromosomes are capable of being fully expressed. However, in females, most of the genes on one of the two X chromosomes are turned off through a process called X inactivation except in the eggs in the ovaries.From DNA to protein - 3D
X inactivation occurs early in the life of the fetus. In some cells, the X from the father becomes inactive, and in other cells, the X from the mother becomes inactive. Because of X inactivation, the absence of one X chromosome usually results in relatively minor abnormalities such as Turner syndrome. Thus, missing an X chromosome is far less harmful than missing a nonsex chromosome see Overview of Sex Chromosome Abnormalities.
If a female has a disorder in which she has more than two X chromosomes, the extra chromosomes tend to be inactive. Thus, having one or more extra X chromosomes causes far fewer developmental abnormalities than having one or more extra nonsex chromosomes.
For example, women with three X chromosomes triple X syndrome are often physically and mentally normal. Males who have more than one Y chromosome see XYY Syndrome may have physical and mental abnormalities. Chromosome abnormalities There are several types of chromosome abnormalities.
A person may have an abnormal number of chromosomes or have abnormal areas on one or more chromosomes. Many such abnormalities can be diagnosed before birth see Testing for chromosome and gene abnormalities. Abnormal numbers of nonsex chromosomes usually result in severe abnormalities. For example, receiving an extra nonsex chromosome can be fatal to a fetus or can lead to abnormalities such as Down syndromewhich commonly results from a person having three copies of chromosome In his book, Evolving BrainsJohn Allman explains how the code of base pairs is read.
I have added bold emphasis to his explanation. The code [of base pairs] is read from one direction in one strand. Three-letter sequences, triplets, specify each amino acid, and the sequence of triplets in turn specifies the chain of amino acids that makes up a protein.
Thus some amino acids are specified by more than one triplet, although no triplet specifies more than one amino acid. The other three triplets are stop codons that signal the end of a particular protein.
The complete sequence of triplets that encodes a protein is a gene. Genes are grouped together in large volumes as chromosomes which are large enough to be seen under a microscope. The image below is a light microscopic presentation of a normal, human male chromosome set karyogram from the German Mental Retardation Network image links to source. Other than germ cells, all humans' cells normally contain 46 chromosomes: In each pair of autosomes, one chromosome is inherited from an individual's father and one from his or her mother.
When contributions of sex chromosomes proceed normally, the mother contributes an X chromosome and the father contributes either an X or a Y chromosome.
Neil Shubin, in Your Inner Fish: A Journey Into the 3. This peeling also takes place when eggs and sperm are made, and this is how genes are passed to offspring. I have put his words into a block quotation here. Our body is made up of hundreds of different kinds of cells. This cellular diversity gives our tissues and organs their distinct shapes and functions. The cells that make our bones, nerves, guts, and so on look and behave entirely differently. Despite these differences, there is a deep similarity among every cell inside our bodies: If DNA contains the information to build our bodies, tissues, and organs, how is it that cells as different as those found in muscle, nerve, and bone contain the same DNA?
The answer lies in understanding what pieces of DNA the genes are actually turned on in every cell. A skin cell is different from a neuron because different genes are active in each cell. When a gene is turned on, it makes a protein that can affect what the cell looks like and how it behaves.
Brain Basics: Genes At Work In The Brain
Therefore, to understand what makes a cell in the eye different from a cell in the bones of the hand, we need to know about the genetic switches that control the activity of genes in each cell and tissue. Here's the important fact: At conception, we start as a single cell that contains all the DNA needed to build our body.
The plan for that entire body unfolds via the instructions contained in this single microscopic cell. To go from this generalized egg cell to a complete human, with trillions of specialized cells organized in just the right way, whole batteries of genes need to be turned on and off at just the right stages of development.