DNA, genes and chromosomes — University of Leicester
Genes and Chromosomes and Fundamentals - Learn about from the MSD Manuals - Medical Consumer Version. Your genes are part of what makes you the person you are. You are different from everyone alive now and everyone who has ever lived. DNA. But your genes. The diagram shows the relationship between the cell, its nucleus, chromosomes in the nucleus, and genes. chromosome, showing gene as section of DNA.
Chromosomes Chromosome proteins, called histones, tightly bind to the DNA double helix. This binding compresses the long DNA molecules so that they fit within a cell. Humans contain 23 pairs of chromosomes, and if you unwound all the DNA from a human cell and placed it end to end, it would exceed six feet in length. During cell division, a cell replicates its complement of chromosomes so that each daughter sell gets the full diploid set.
What Is the Relationship Between a Chromosome & an Allele? | Sciencing
Sciencing Video Vault Genes and Alleles Genes appear throughout the length of each chromosome, and each chromosome pair has a unique set of genes. You can only recognize genes from their information content -- the sequence of nucleotide bases. Otherwise, genes are indistinguishable from the rest of the chromosome. The site of a gene on a chromosome is its locus. You can designate a locus by counting the number of bases from the beginning of the chromosome to the start of the gene.
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.
Cell division - AQA
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.
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.DNA, Chromosomes, Genes, and Traits: An Intro to Heredity
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 Absence of a nonsex chromosome is fatal to the fetus. Large areas on a chromosome may be abnormal, usually because a whole section was left out called a deletion or mistakenly placed in another chromosome called translocation.
For example, chronic myelogenous leukemia is sometimes caused by translocation of part of chromosome 9 onto chromosome This abnormality can be inherited or be the result of a new mutation.
Mitochondrial chromosomes Mitochondria are tiny structures inside cells that synthesize molecules used for energy. Unlike other structures inside cells, each mitochondrion contains its own circular chromosome. This chromosome contains DNA mitochondrial DNA that codes for some, but not all, of the proteins that make up that mitochondrion. Mitochondria from the sperm usually do not become part of the developing embryo.
Traits A trait is any gene-determined characteristic. Many traits are determined by the function of more than one gene. For example, a person's height is likely to be determined by many genes, including those affecting growth, appetite, muscle mass, and activity level.
However, some traits are determined by the function of a single gene. Variation in some traits, such as eye color or blood type, is considered normal. Other variations, such as albinismMarfan syndromeand Huntington diseaseharm body structure or function and are considered disorders. However, not all such gene abnormalities are uniformly harmful. For example, one copy of the sickle cell gene can provide protection against malaria, but two copies of the gene cause sickle cell anemia.
Genetic Disorders A genetic disorder is a detrimental trait caused by an abnormal gene. The abnormal gene may be inherited or may arise spontaneously as a result of a mutation. Gene abnormalities are fairly common. Humans carry an average of to abnormal genes.
However, most of the time the corresponding gene on the other chromosome in the pair is normal and prevents any harmful effects. In the general population, the chance of a person having two copies of the same abnormal gene and hence a disorder is very small. A always pairs with T, and C always pairs with G. The DNA molecule has two important properties.
It can make copies of itself. If you pull the two strands apart, each can be used to make the other one and a new DNA molecule.
DNA, genes and chromosomes
It can carry information. The order of the bases along a strand is a code - a code for making proteins. Genes A gene is a length of DNA that codes for a specific protein. So, for example, one gene will code for the protein insulin, which is important role in helping your body to control the amount of sugar in your blood. Genes are the basic unit of genetics.
Human beings have 20, to 25, genes. These genes account for only about 3 per cent of our DNA. The function of the remaining 97 per cent is still not clear, although scientists think it may have something to do with controlling the genes.