Understanding Chromosomes and Genes - High School Biology

During most of the cell cycle, DNA is found as chromatin. Chromatin is a mass of DNA and associated proteins. Depending on the state and activation of those proteins and on how tighly packed the DNA is around the proteins, certain genes can be turned on or off.

Chromosomes form from condensed chromatin only during mitosis (specifically during prophase), and are absent during most of the cell's cycle. Genes are units of heredity that encode the information needed to specify the amino acid sequence of proteins. The gene is the functional segment of DNA located at a specific place, or locus, on a chromosome. The cytoplasm is the material contained in the cell membrane and outside the nucleus. The nucleus is a membrane-bound organelle that contains the cell's genetic material.

An operon is a segment of DNA that is under the control of a single promoter. For example, if there are three genes required for breaking down a sugar in an operon, they will all be activated together. This makes sense, as there is no sense in activating only one or two of these genes, since all three are required to break down the sugar. For example, the control element that turns on the operator can be the sugar itself. It should make sense that the genes required to break down a sugar are only turned on if that sugar is present.

Genes are expressed when their gene products are made, to do this transcription and translation must occur to synthesize the protein which is coded for by the DNA.

The nucleotides in DNA always pair the same way; A with T and G with C. This is due to the chemical structure of each base; adenine and thymine form two hydrogen bonds when pairing, and guanine and cytosine form three. The other important thing to remember is that there is a different nitrogen base in RNA called uracil; uracil is not found in DNA, so pay attention to which molecule the question is asking about.

Synteny is the conservation of order of genes. Being able to see conserved blocks in genes when comparing two chromosomes of the same species, it indicates that at some time in evolutionary history, these blocks originated from a hypothetical common ancestor. Genes that are highly conserved among species are usually vital to the organism's viability. For example, the genes required for glycolysis to occur are required in almost all organisms.

The human genome can be described as a chain of nucleotides that is 3.2 billion base pairs long. Twenty-three different lengths that are called chromosomes segment this string of material. Genes further segment the chromosomes. The chromosomes vary in length and so do their genes. DNA of a gene on a certain part of a certain chromosome can be transcribed into mRNA, which is then translated into protein. The number of genes in our genome has been highly contested. It was originally believed that humans had over 100,000 genes. This number declined as we learned more about our genome and was more or less standardized to be around 25,000 upon completion of the Human Genome Project.

A chromosome is one molecule of DNA. It contains genetic information required for cell replication and the passing down of genetic information. Humans have 23 pairs of chromosomes, with one chromosome per pair from the mother, and the other from the father.

Humans have 23 homologous pairs of chromosomes, which adds up to 46 chromosomes total. Diploid cells have two copies of each chromosome (one from the mother, one from the father). A gamete is haploid, meaning it only contains one copy of each chromosome. The gametes (haploid) will fuse in a process known as fertilization in order to form a diploid zygote.

An allele is a variant or alternative form of a gene or gene locus. Humans are diploid—we have two alleles at each genetic locus. One allele is inherited from the mother and one allele comes from the father. Sometimes different alleles can lead to different phenotypic traits (e.g. eye color); however, most genetic variations result in little to no observable variation. An organism is considered homozygous if it possesses two of the same alleles at a particular locus. If the alleles are different, then the organism is considered heterozygous.

DNA—deoxyribonucleic acid—is found in chromosomes within a cell’s nucleus. A complete set of DNA (i.e. 46 chromosomes) is called a genome. DNA contains instructions that make humans different from other species and other individuals. DNA provides instructions for all the proteins that the body makes and is passed from adults to offspring. DNA cannot get out of the nucleus; however, RNA can. RNA is used to get the instructions from DNA out of the nucleus and into the site of protein synthesis: the ribosomes within the cytoplasm. Proteins are made of amino acids and determine the structure and function of all of the body’s cells.

Based on the definition of genome, it is the entire set of DNA found within all the chromosomes an organism contains. The human genome is contained on 23 pairs of chromosomes, which code for about 25,000 genes.

DNA nucleotides bond via hydrogen bonding to form the double helix structure. DNA is negatively charged due to the phosphates and binds to histones to form compact chromosomes in the nucleus.