Chapter 17: Introduction to genetics Flashcards
Chromosomes
- A threadlike structure of nucleic acids and protein found in the nucleus of most living cells, carrying genetic information in the form of genes
- Each pair of chromosomes is numbered, the largest pair being no. 1. The first 22 pairs are collectively known as autosomes, and the chromosomes of each pair contain the same amount of genetic material. The chromosomes of pair 23 are called the sex chromosomes
- Each end of the chromosome is capped with a length of DNA called a telomere, which seals the chromosome and is structurally essential. During replication, the telomere is shortened, which would damage the chromosome, and so it is repaired with an enzyme called telomerase.
Genes
- Along the length of the chromosomes are the genes. Each gene contains information in code that allows the cell to make (almost always) a specific protein, the so-called gene product. Each gene codes for one specific protein, and research puts the number of genes in the human genome at between 25 000 and 30 000.
- Genes normally exist in pairs, because the gene on one chromosome is matched at the equivalent site (locus) on the other chromosome of the pair.
DNA
• Genes are composed mainly of very long strands of DNA; the total length of DNA in each cell is about a meter. Because this is packaged into chromosomes, which are micrometers (10−6 m) long, this means that the DNA must be tightly wrapped up to condense it into such a small space.
•DNA is a double-stranded molecule, made up of two chains of nucleotides. Nucleotides consist of three subunits:
-a sugar
-a phosphate group
-a base
•The DNA molecule is sometimes likened to a twisted ladder, with the uprights formed by alternating chains of sugar and phosphate units.
•The bases are linked to the sugars, and each base binds to another based on the other sugar/phosphate chain, forming the rungs of the ladder. The two chains are twisted around one another, giving a double helix (twisted ladder) arrangement. The double helix itself is further twisted and wrapped in a highly organized way around structural proteins called histones, which are important in maintaining the heavily coiled three-dimensional shape of the DNA. The term the DNA–histone material is chromatin. The chromatin is supercoiled and packaged into the chromosomes shortly before the cell divides.
The genetic code
•DNA carries a huge amount of information that determines all biological activities of an organism, and which is transmitted from one generation to the next. The key to how this information is kept is found in the bases within DNA. There are four bases:
-adenine (A)
-guanine (G)
-thymine (T)
-cytosine (C)
•They are arranged in a precise order along the DNA molecule, making a base code that can be read when protein synthesis is required. Each base along one strand of DNA pairs with a base on the other strand in a precise and predictable way. This is known as complementary base pairing. Adenine always pairs with thymine (and vice versa), and cytosine and guanine always go together. The bases on opposite strands run down the middle of the helix and bind to one another with hydrogen bonds.
Mitochondrial DNA
•Each body cell has on average, 5000 mitochondria that hold a quantity of DNA (mitochondrial DNA), which codes, for example, for enzymes important in energy production. This DNA is passed from one generation to another via the ovum, so the offspring’s complement of mitochondrial DNA is inherited from the mother. Certain rare inherited disorders that arise from faulty mitochondrial DNA are therefore passed through generations via the maternal line.
Mutation
- Mutation means an inheritable alteration in the normal genetic make-up of a cell. Most mutations occur spontaneously, because of the countless millions of DNA replications and cell divisions that occur normally throughout life. Others may be caused by external factors, such as X-rays, ultraviolet rays, or exposure to certain chemicals. Any factor capable of mutating DNA is called a mutagen. Most mutations are immediately repaired by an army of enzymes present in the cell nucleus, and therefore cause no permanent problems.
- Sometimes the mutation is lethal, because it disrupts some essential cellular function, causing cell death and the mutation is destroyed along with the cell. Often, the mutated cell is detected by immune cells and destroyed because it is abnormal. Other mutations do not kill the cell but alter its function in some way that may cause disease, e.g., in cancer. A persistent mutation in the genome that has not led to cell death can be passed from parent to child and may cause inherited disease, e.g., phenylketonuria or cystic fibrosis.
Protein synthesis
- DNA holds the cell’s essential biological information, written within the base code in the center of the double helix. The products of this information are almost always proteins. Proteins are essential to all aspects of body function, forming the major structural elements of the body as well as the enzymes (p. 28) essential for all biochemical processes within it.
- The building blocks of human proteins are about 20 different amino acids. As the cell’s DNA is too big to leave the nucleus, an intermediary molecule is needed to carry the genetic instructions from the nucleus to the cytoplasm, where proteins are made. This is called messenger (m)RNA.
Messenger ribonucleic acid (mRNA)
•mRNA is a single-stranded chain of nucleotides synthesized in the nucleus from the appropriate gene, whenever the cell requires to make the protein for which that gene codes. There are three main differences between the structures of RNA and DNA:
-it is single instead of double-stranded
-it contains the sugar ribose instead of deoxyribose
-it uses the base uracil instead of thymine.
•Using the DNA as a template, a piece of mRNA is made from the gene to be used. This process is called transcription. The mRNA then leaves the nucleus through the nuclear pores and carries its information to the ribosomes in the cytoplasm.
Transcription
- Because the code is buried within the DNA molecule, the first step is to open the helix to expose the bases. Only the gene to be transcribed is opened; the remainder of the chromosome remains coiled. Opening the helix exposes both base strands, but the enzyme that makes the mRNA uses only one of them, so the mRNA molecule is single, not double-stranded. As the enzyme moves along the opened DNA strand, reading its code, it adds the complementary base to the mRNA.
- Therefore, if the DNA base is cytosine, guanine is added to the mRNA molecule (and vice versa); if it is thymine, adenine is added; if it is adenine, uracil is added (remember there is no thymine in RNA, but uracil instead) (Fig. 17.6A). When the enzyme gets to a ‘stop’ signal, it terminates the synthesis of the mRNA molecule, and the mRNA is released. The DNA is zipped up again by other enzymes, and the mRNA then leaves the nucleus.
Translation
- Translation is a synthesis of the final protein using the information carried on mRNA. It takes place on free ribosomes in the cytoplasm and those attached to the rough endoplasmic reticulum. First, the mRNA attaches to the ribosome. The ribosome then ‘reads’ the base sequence of the mRNA.
- Because proteins are built from up to 20 different amino acids, it is not possible to use the four bases individually in a simple one-to-one code. To give enough options, the base code in RNA is read in triplets, giving a possible 64 base combinations, which allows a coded instruction for each amino acid as well as other codes, e.g., stop and start instructions. Each of these specific triplet sequences is called a codon; for example, the base sequence ACA (adenine, cytosine, adenine) codes for the amino acid cysteine.
- The first codon is a start codon, which initiates protein synthesis. The ribosome slides along the mRNA, reading the codons and adding the appropriate amino acids to the growing protein molecule as it goes. The ribosome continues assembling the new protein molecule until it arrives at a stop codon, at which point it terminates synthesis and releases the new protein. Some new proteins are used within the cell itself, and others are exported, e.g., insulin synthesized by pancreatic β-islet cells is released into the bloodstream.
Gene expression
Although all nucleated cells (except gametes) have an identical set of genes, each cell type uses only those genes related directly to its own function. For example, the only cell type containing hemoglobin is the red blood cell, although all body cells carry the hemoglobin gene. This selective gene expression is controlled by various regulatory substances, and the genes not needed by the cell are kept switched off.
Cell division
•Most body cells are capable of division, even in adulthood. Cell division usually leads to the production of two identical diploid daughter cells, mitosis, and is important in body growth and repair. Production of gametes is different in that the daughter cells have only half the normal chromosome number – 23 instead of 46, i.e., they are haploid. Gametes are produced by a form of cell division called meiosis. DNA replication takes place before mitosis and meiosis.
DNA replication
- DNA is the only biological molecule capable of self-replication. Mistakes in copying may lead to the production of non-functioning or poorly functional cells, or cells that do not respond to normal cell controls (this could lead to the development of a tumor). Accurate copying of DNA is therefore essential.
- The initial step in DNA replication is the unfolding of the double helix and the unzipping of the two strands to expose the bases, as happens in transcription. Both strands of the parent DNA molecule are copied. The enzyme responsible for DNA replication moves along the base sequence on each strand, reading the genetic code and adding the complementary base to the newly forming chain. This means that each strand of opened bases becomes a double strand, and the result is two identical DNA molecules. As each new double strand is formed, other enzymes cause it to twist and coil back into its normal highly folded form.
Mitosis
A type of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus, typical of ordinary tissue growth.
Meiosis
- Meiosis is the final step in gamete production. On fertilization, when the male gamete (sperm cell) and the female gamete (ovum) unite, the resulting zygote is diploid because each gamete was haploid.
- Unlike mitosis, meiosis involves two distinct cell divisions rather than one. Additionally, meiosis produces four daughter cells, not two, all different from the parent cells and from each other. This is the basis of genetic diversity and the uniqueness of each human individual.
