Nucleus contains how many chromosomes

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 phenylketonuria Phenylketonuria PKU Phenylketonuria is a disorder of amino acid metabolism that occurs in infants born without the ability to normally break down an amino acid called phenylalanine.

Phenylalanine, which is toxic 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 Sickle Cell Disease Sickle cell disease is an inherited genetic abnormality of hemoglobin the oxygen-carrying protein found in red blood cells characterized by sickle crescent -shaped red blood cells and chronic However, when a person inherits only one copy of the sickle cell gene called a carrier , the person develops some protection against malaria Malaria Malaria is infection of red blood cells with one of five species of Plasmodium, a protozoan.

Malaria causes fever, chills, sweating, a general feeling of illness malaise , and sometimes diarrhea 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 population , whereas 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. A chromosome is made of a very long strand of DNA and contains many genes Genes Genes are segments of deoxyribonucleic acid DNA that contain the code for a specific protein that functions in one or more types of cells in the body.

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. Except for certain cells for example, sperm and egg cells or red blood cells , the nucleus of every normal 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 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.

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 Turner Syndrome Turner syndrome is a sex chromosome abnormality in which girls are born with one of their two X chromosomes partially or completely missing. Turner syndrome is caused by the deletion of part Thus, missing an X chromosome is far less harmful than missing a nonsex chromosome see Overview of Sex Chromosome Abnormalities Overview of Sex Chromosome Abnormalities Sex chromosome abnormalities may be caused by full or partial deletions or duplications of sex chromosomes.

Chromosomes are structures within cells that contain DNA and many genes. A gene is 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. Genes contain However, in meiosis, the cycle occurs twice meiosis I and meiosis II before the four haploid daughter cells are produced.

Another difference between the stages of mitosis and meiosis is that in meiosis, homologous chromosomes pair up during metaphase instead of chromatids. In a homologous pair, one chromosome comes from the mother, and one chromosome comes from the father. Homologous chromosomes are very similar, but they are not identical. They carry the same genes eg, hair or eye color , but they may not code for the same trait eg, blonde hair or brown eyes. Occurs before cell division.

Consists of three stages: Gap 1 growth , S phase DNA replication and Gap 2 continues growth, prepares for cell division. Tell us what you think about Healio. What is a Genome? Whole-Exome Sequencing vs. Visit Healio. Your Module Progress. Module 1. Module Content. Genes are small sections of DNA within the genome that code for proteins.

They contain the instructions for our individual characteristics — like eye and hair colour. If you have any other comments or suggestions, please let us know at comment yourgenome.

Can you spare minutes to tell us what you think of this website? Open survey. In: Facts In the Cell. Related Content:. What is a cell? Recall that prokaryotic cells do not possess a nucleus. Prokaryotic chromosomes are less condensed than their eukaryotic counterparts and don't have easily identified features when viewed under a light microscope.

Figure 2: A the appearance of DNA during interphase versus mitosis. During interphase, the cell's DNA is not condensed and is loosely distributed.

A stain for heterochromatin which indicates the position of chromosomes shows this broad distribution of chromatin in a mouse cell upper left. The same stain also shows the organized, aligned structure of the chromosomes during mitosis.

HP1 and the dynamics of heterochromatin maintenance. Nature Reviews Molecular Cell Biology 5, All rights reserved. Figure Detail. Figure 3 Eukaryotic chromosomes consist of repeated units of chromatin called nucleosomes , which were discovered by chemically digesting cellular nuclei and stripping away as much of the outer protein packaging from the DNA as possible. The chromatin that resisted digestion had the appearance of "beads on a string" in electron micrographs — with the "beads" being nucleosomes positioned at intervals along the length of the DNA molecule Figure 3.

Nucleosomes are made up of double-stranded DNA that has complexed with small proteins called histones. The core particle of each nucleosome consists of eight histone molecules, two each of four different histone types: H2A, H2B, H3, and H4. The structure of histones has been strongly conserved across evolution, suggesting that their DNA packaging function is crucially important to all eukaryotic cells Figure 4. Histones carry positive charges and bind negatively charged DNA in a specific conformation.

In particular, a segment of the DNA double helix wraps around each histone core particle a little less than twice. The exact length of the DNA segment associated with each histone core varies from species to species, but most such segments are approximately base pairs in length. Furthermore, each histone molecule within the core particle has one end that sticks out from the particle. These ends are called N-terminal tails , and they play an important role in higher-order chromatin structure and gene expression.

Figure 4: The nucleosome structure within chromatin. Each nucleosome contains eight histone proteins blue , and DNA wraps around these histone structures to achieve a more condensed coiled form. Figure 5: To better fit within the cell, long pieces of double-stranded DNA are tightly packed into structures called chromosomes. Although nucleosomes may look like extended "beads on a string" under an electron microscope, they appear differently in living cells.

In such cells, nucleosomes stack up against one another in organized arrays with multiple levels of packing.

The first level of packing is thought to produce a fiber about 30 nanometers nm wide. These 30 nm fibers then form a series of loops, which fold back on themselves for additional compacting Figure 5. The multiple levels of packing that exist within eukaryotic chromosomes not only permit a large amount of DNA to occupy a very small space, but they also serve several functional roles.

For example, the looping of nucleosome-containing fibers brings specific regions of chromatin together, thereby influencing gene expression. In fact, the organized packing of DNA is malleable and appears to be highly regulated in cells.