Base pairs, DNA, amino acids, proteins, genes, chromosomes, genetics - a short introduction.
The DNA within just about every living cell contains all the information necessary to So genes are like books, chromosomes are like bookshelves, the set of an. Genes are made of a chemical called DNA or deoxyribonucleic acid and are Like a recipe book, it holds the instructions for making all the. What are chromosome abnormalities and how often do they occur? Think of it this way: DNA is in genes, genes are on chromosomes.
The p arm from the French word 'petit', meaning small is the short arm, and the q arm the next letter in the alphabet is the long arm. In their replicated form, each chromosome consists of two chromatids. Chromosome unraveling to show the base pairings of the DNA The chromosomes - and the DNA they contain - are copied as part of the cell cycle, and passed to daughter cells through the processes of mitosis and meiosis.
Read more about the cell cycle, mitosis and meiosis Human beings have 46 chromosomes, consisting of 22 pairs of autosomes and a pair of sex chromosomes: One member of each pair of chromosomes comes from the mother through the egg cell ; one member of each pair comes from the father through the sperm cell.
- The Cell: A Molecular Approach. 2nd edition.
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- Organizing DNA Information
A photograph of the chromosomes in a cell is known as a karyotype. The autosomes are numbered in decreasing size order. Karyotype of a human male Prokaryotic chromosomes The prokaryotes Greek for 'before nucleus' - including Eubacteria and Archaea lack a discrete nucleus, and the chromosomes of prokaryotic cells are not enclosed by a separate membrane. Most bacteria contain a single, circular chromosome.
The chromosome - together with ribosomes and proteins associated with gene expression - is located in a region of the cell cytoplasm known as the nucleoid. The genomes of prokaryotes are compact compared with those of eukaryotes, as they lack introns, and the genes tend to be expressed in groups known as operons.
DNA, Genes and Chromosomes
The circular chromosome of the bacterium Escherichia coli consists of a DNA molecule approximately 4. In addition to the main chromosome, bacteria are also characterised by the presence of extra-chromosomal genetic elements called plasmids. These relatively small circular DNA molecules usually contain genes that are not essential to growth or reproduction.
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How are DNA chromosomes and genes related?
The experiments that defined the role of DNA were derived from studies of the bacterium that causes pneumonia Pneumococcus. Virulent strains of Pneumococcus are surrounded by a polysaccharide capsule that protects the bacteria from attack by the immune system of the host. Because the capsule gives bacterial colonies a smooth appearance in culture, encapsulated strains are denoted S.
Mutant strains that have lost the ability to make a capsule denoted R form rough-edged colonies in culture and are no longer lethal when inoculated into mice. In it was observed that mice inoculated with nonencapsulated R bacteria plus heat-killed encapsulated S bacteria developed pneumonia and died.
Importantly, the bacteria that were then isolated from these mice were of the S type. Subsequent experiments showed that a cell-free extract of S bacteria was similarly capable of converting or transforming R bacteria to the S state. Thus, a substance in the S extract called the transforming principle was responsible for inducing the genetic transformation of R to S bacteria. In Oswald Avery, Colin MacLeod, and Maclyn McCarty established that the transforming principle was DNAboth by purifying it from bacterial extracts and by demonstrating that the activity of the transforming principle is abolished by enzymatic digestion of DNA but not by digestion of proteins Figure 3.
Although these studies did not immediately lead to the acceptance of DNA as the genetic material, they were extended within a few years by experiments with bacterial viruses. In particular, it was shown that, when a bacterial virus infects a cell, the viral DNA rather than the viral protein must enter the cell in order for the virus to replicate.
Moreover, the parental viral DNA but not the protein is transmitted to progeny virus particles. The concurrence of these results with continuing studies of the activity of DNA in bacterial transformation led to acceptance of the idea that DNA is the genetic material. DNA is extracted from a pathogenic strain of Pneumococcus, which is surrounded by a capsule and forms smooth colonies S. At the time of Watson and Crick's work, DNA was known to be a polymer composed of four nucleic acid bases—two purines adenine [A] and guanine [G] and two pyrimidines cytosine [C] and thymine [T] —linked to phosphorylated sugars.
Given the central role of DNA as the genetic material, elucidation of its three-dimensional structure appeared critical to understanding its function. Analysis of these data revealed that the DNA molecule is a helix that turns every 3.
In addition, the data showed that the distance between adjacent bases is 0. Comparative studies of this kind have revealed not only that mice and humans share most of the same genes, but also that large blocks of the mouse and human genomes contain these genes in the same order, a feature called conserved synteny Figure Conserved synteny can also be revealed by chromosome painting, and this technique has been used to reconstruct the evolutionary history of our own chromosomes by comparing them with those from other mammals Figure Figure Conserved synteny between the human and mouse genomes.
Regions from different mouse chromosomes indicated by the colors of each mouse in B show conserved synteny gene order with the indicated regions of the human genome A.
For example the genes more Figure A proposed evolutionary history of human chromosome 3 and its relatives in other mammals. A At the lower left is the order of chromosome 3 segments hypothesized to be present on a chromosome of a mammalian ancestor.
Along the top are the patterns of more Chromosomes Exist in Different States Throughout the Life of a Cell We have seen how genes are arranged in chromosomes, but to form a functional chromosomea DNA molecule must be able to do more than simply carry genes: This process occurs through an ordered series of stages, collectively known as the cell cycle. The cell cycle is briefly summarized in Figureand discussed in detail in Chapter Only two of the stages of the cycle concern us in this chapter.
During interphase chromosomes are replicated, and during mitosis they become highly condensed and then are separated and distributed to the two daughter nuclei.
How Is a Cell's DNA Like the Books in a Library? | Education - Seattle PI
The highly condensed chromosomes in a dividing cell are known as mitotic chromosomes. This is the form in which chromosomes are most easily visualized; in fact, all the images of chromosomes shown so far in the chapter are of chromosomes in mitosis. This condensed state is important in allowing the duplicated chromosomes to be separated by the mitotic spindle during cell division, as discussed in Chapter Figure A simplified view of the eucaryotic cell cycle.
During interphase, the cell is actively expressing its genes and is therefore synthesizing proteins. Also, during interphase and before cell division, the DNA is replicated and the chromosomes are duplicated.
During the portions of the cell cycle when the cell is not dividing, the chromosomes are extended and much of their chromatin exists as long, thin tangled threads in the nucleus so that individual chromosomes cannot be easily distinguished Figure We refer to chromosomes in this extended state as interphase chromosomes.
A comparison of extended interphase chromatin with the chromatin in a mitotic chromosome. A An electron micrograph showing an enormous tangle of chromatin spilling out of a lysed interphase nucleus. B A scanning electron micrograph of a mitotic chromosome: These basic functions are controlled by three types of specialized nucleotide sequence in the DNAeach of which binds specific proteins that guide the machinery that replicates and segregates chromosomes Figure Figure The three DNA sequences required to produce a eucaryotic chromosome that can be replicated and then segregated at mitosis.
Each chromosome has multiple origins of replication, one centromere, and two telomeres. Shown here is the sequence of events a typical more Experiments in yeasts, whose chromosomes are relatively small and easy to manipulate, have identified the minimal DNA sequence elements responsible for each of these functions. One type of nucleotide sequence acts as a DNA replication originthe location at which duplication of the DNA begins.
Eucaryotic chromosomes contain many origins of replication to ensure that the entire chromosome can be replicated rapidly, as discussed in detail in Chapter 5. After replication, the two daughter chromosomes remain attached to one another and, as the cell cycle proceeds, are condensed further to produce mitotic chromosomes. The presence of a second specialized DNA sequence, called a centromereallows one copy of each duplicated and condensed chromosome to be pulled into each daughter cell when a cell divides.
A protein complex called a kinetochore forms at the centromere and attaches the duplicated chromosomes to the mitotic spindleallowing them to be pulled apart discussed in Chapter The third specialized DNA sequence forms telomeresthe ends of a chromosome.
Telomeres contain repeated nucleotide sequences that enable the ends of chromosomes to be efficiently replicated. Telomeres also perform another function: We discuss this type of repair and the other features of telomeres in Chapter 5.
In yeast cells, the three types of sequences required to propagate a chromosome are relatively short typically less than base pairs each and therefore use only a tiny fraction of the information-carrying capacity of a chromosome.
Although telomere sequences are fairly simple and short in all eucaryotes, the DNA sequences that specify centromeres and replication origins in more complex organisms are much longer than their yeast counterparts. For example, experiments suggest that human centromeres may contain up tonucleotide pairs. It has been proposed that human centromeres may not even require a stretch of DNA with a defined nucleotide sequence; instead, they may simply create a large, regularly repeating protein - nucleic acid structure.
We return to this issue at the end of the chapter when we discuss in more general terms the proteins that, along with DNA, make up chromosomes.
DNA, genes and chromosomes — University of Leicester
Recall from earlier in this chapter that human chromosome 22 contains about 48 million nucleotide pairs. Stretched out end to end, its DNA would extend about 1. This remarkable feat of compression is performed by proteins that successively coil and fold the DNA into higher and higher levels of organization. Although less condensed than mitotic chromosomes, the DNA of interphase chromosomes is still tightly packed, with an overall compaction ratio of approximately fold.
In the next sections we discuss the specialized proteins that make the compression possible.