Genes and DNA - Simple and Complex Cells, Chromosomes, DNA, and Genes
Genetics study guide by subekshas includes 14 questions covering vocabulary, What is the relationship between cells, chromosomes, DNA, genes, and alleles ? . The steps of the ladder are made of matching base pairs (A + T or C + G). The total DNA which is stored in each cell is called your genome. Half of your What is the clearly stated difference between DNA, genes, chromosomes and . It has a backbone of sugar and phosphorus, with four nucleotides joined in pairs. What percentage of the human genome is identical between individuals? What is the relationship between chromosomes, genes, and DNA? project with the goal of determining the sequence of chemical base pairs which make up DNA. 1.
When these crystals are viewed under a powerful electron microscope, the structure of molecules becomes apparent. However, electron microscopes do not produce precise visual images of what they are focused on. Instead, they generate data that have to be interpreted. Two other scientists, American geneticist James Watson and British biophysicist Francis Crick, became aware of their work and began to construct a physical model of the DNA molecule. The paper in which Watson and Crick published the results of their painstaking research has been recognized as one of the most revolutionary and influential documents in the history of science.
Watson, Crick, and Wilkins each received a Nobel Prize for their work.
- How are DNA, chromosomes, genes, and alleles related?
- What's the relationship between DNA, nucleotides, genes, and chromosomes?
Unfortunately, Franklin died before her invaluable contribution could be recognized in this way. Recently, historians of science have finally begun to recognize that without Franklin's groundbreaking work, the discovery of the structure of the DNA molecule would have been delayed by years, if not decades. Watson and Crick concluded that the DNA molecule was shaped like a double helix, two strands spiraling around each other.
A helix is the shape of a corkscrew. A double helix is the shape of two corkscrews, one intertwined with the other and curving parallel to it, like the railings of a spiral staircase. Another way to think of the double helix is to imagine a twisted rope ladder with rigid rungs, each rope forming a helix. The easiest way to think about DNA is to start by splitting the two helices [the plural form of the word helix ] apart. Think of it as sawing down through the middles of the wooden rungs of a rope ladder.
The result is two single ropes with half-rungs hanging off each rope. They serve as the four letters of the genetic alphabet. Opposite every T would be an A and vice versa and opposite every G would be a C and vice versa. The question then arises: How can just four letters create code for the thousands of proteins that the body produces and the correspondingly large number of traits they govern? The answer lies in the astonishingly large number of A-T, C-G combinations, or base pairs as they are called, that make up a gene.
If all the DNA compacted into a cell were stretched out, it would be about seven feet long. DNA and RNA The sequences of DNA that make up genes communicate instructions to the ribosomes to manufacture proteins that work together to produce an organism's traits. However, genes do not perform this function directly.
First, a process called transcription must occur. In transcription, a single gene, which could contain thousands of base pairs, unravels from the chromosome on which it is located.
The DNA that constitutes that gene then splits into its two complementary strands.
DNA, Genes and Chromosomes — University of Leicester
A special type of protein called an enzyme moves along one of the strands letter by letter and creates a corresponding strand of a substance called ribonucleic acid RNA. Three of these bases—A, C, and G—are the same in both, but in place of thymine TRNA has a base called uracil, which is abbreviated with the letter U.
As the enzyme creeps along the gene's DNA, it transcribes each base it encounters into a corresponding base on the newly emerging strand of RNA. Once the strand of RNA has been completely transcribed, it travels through the membrane that encloses the cell's nucleus into the cytoplasm. There, it attaches itself to a ribosome, providing the instructions needed to manufacture a protein. This process is called translation, and it works like this.
The thousands of proteins that any organism contains are made up of various combinations of twenty substances called amino acids. Each three-letter sequence of RNA tells the ribosome to make one of these amino acids.
The process continues in sequence, each three letters of RNA bringing into existence a new amino acid, which attaches itself to those already made. At the conclusion of the process, a complete protein has been manufactured and is ready to do its part in producing the traits that characterize the organism in which all this activity has been taking place. The three-letter sequence of RNA that codes for an amino acid is called a condon.
Together, the condons form a set of instructions—the genetic code mentioned earlier. This code is the basis for all the forms that life can take. It bridges the gap between the hereditary information that genes contain and the biochemical processes that give each individual organism the traits that define it. Mitosis The way genes encode instructions for the manufacture of proteins is similar to the mechanism they use to pass from generation to generation.
A key part of the life cycle of every cell takes place when the cell reproduces, or makes a copy of itself. Eukaryotic cells, those that make up complex organisms like plants and animals, come in two types. These types are somatic cells and sex cells, also called gametes. Somatic cells combine with each other to make up a body's tissues and organs. Sex cells combine with the sex cells of another organism to produce offspring. The two sorts of eukaryotes reproduce in different ways.
When somatic cells divide, they include a complete copy of all the genetic information contained in the original cell. That is, both corresponding sets of chromosomes are replicated in the new cell. On the other hand, when sex cells divide, only one of the two sets of chromosomes is reproduced. When the sex cell combines with the opposite sex cell of another organism—a sperm cell with an egg cell—the new cell produced by this union will then contain two matching sets of chromosomes, but one will have come from the father and the other will have come from the mother.
The reproduction of somatic cells is called mitosis; that of sex cells is termed meiosis. The purpose of mitosis is growth, so that organs and other body parts can form completely as an organism progresses from infancy to adulthood.
DNA, Genes and Chromosomes
Mitosis also creates new cells to replace those that die off at the end of their life cycles. The purpose of meiosis is to create an entirely new organism.
Compared to meiosis, mitosis is a relatively straightforward process. First, the chromosomes become thicker and double into cross-shaped forms, each limb of the cross containing a complete copy of the DNA in all of the organism's genes. Biochemists Paul Berg and Maxine Singer describe how the phenomenon continues: Two cells are formed when a membrane grows and separates the two ends of the original cell.
Each of the new cells referred to as daughter cells has a full set of chromosome pairs. If they did, the offspring resulting from the union of two sex cells would have twice the number of chromosomes—and twice the amount of genetic information—as either of the two parent cells. In humans, for example, each child would have not the required forty-six chromosomes but two times that number, or ninety-two.
Human offspring with more or less than the necessary forty-six chromosomes usually do not survive; consequently the human race would have died out after the first generation. Therefore, sex cells divide twice. The first division is like mitosis, except that during the stage when the number of chromosomes doubles, individual genes often jump from one chromosome to the other.
This is possible because similar genes occur at the same location on each of the chromosomes. The process, known as crossing over or recombination, plays a Source: Colin Tudge, The Engineer in the Garden: From the Idea of Heredity to the Creation of Life. Since one member of each pair of chromosomes has been inherited from the mother and the other from the father and since each gene has two forms allelesrecombination creates a novel arrangement of genetic information to be passed on to future generations.
Once the chromosomes have doubled and two new cells have been formed, a further division takes place to guarantee that each sex cell produced has only one set of chromosomes in humans, twenty-three rather than forty-six. Meiosis results in four cells, each.
Thus a new creature is created. It is similar to its parents because it has the same genes as they did, but it is also different because each of those genes may contain alleles different from the ones that constituted the genetic makeup of the parents. As this process continues from generation to generation, the individuals produced tend to differ to an ever-greater degree from the original parent pair. These differences enable subsequent generations to adapt to changing environment and form the genetic basis of evolution, explaining how species have changed, and new species have arisen, during the course of the history of life on Earth.
Sex Determination There is one exception to the rule that all chromosomes come in matching pairs, and that exception determines whether newly conceived organisms develop into males or females. The pair of dissimilar chromosomes have been designated X and Y. It is important to remember that although they have different names, they do form a pair in the same way their more alike counterparts do.
The Y chromosome is shorter than the X.
Genes consist of three types of nucleotide sequence: These genes are known, collectively, as the human genome. Chromosomes Eukaryotic chromosomes The label eukaryote is taken from the Greek for 'true nucleus', and eukaryotes all organisms except viruses, Eubacteria and Archaea are defined by the possession of a nucleus and other membrane-bound cell organelles. The nucleus of each cell in our bodies contains approximately 1. This DNA is tightly packed into structures called chromosomes, which consist of long chains of DNA and associated proteins.
In eukaryotes, DNA molecules are tightly wound around proteins - called histone proteins - which provide structural support and play a role in controlling the activities of the genes. A strand to nucleotides long is wrapped twice around a core of eight histone proteins to form a structure called a nucleosome. The chains of histones are coiled in turn to form a solenoid, which is stabilised by the histone H1. Further coiling of the solenoids forms the structure of the chromosome proper.
Each chromosome has a p arm and a q arm. 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.
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. 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.