Following the discovery of the structure of DNA, scientists knew that the DNA molecule carried hereditary information from one generation to the next and what the molecule looked like. But exactly how do cells use the DNA molecule?

In order for cells to use the instructions contained in the DNA molecule to make proteins, the instructions need to be transferred from the nucleus where the DNA molecule resides to the cytoplasm of the cell where proteins are made. Because DNA cannot leave the nucleus of the cell, a molecule, called messenger ribonucleic acid (mRNA), relays its message from the nucleus to structures in the cytoplasm that make proteins. Messenger RNA is created through a process called transcription. During transcription, the two strands of the DNA molecule are “unzipped,” or pulled apart, and a single-stranded molecule of mRNA matches up with the exposed base pairs. Cytosine pairs with guanine, just like it does in the DNA molecule. However, instead of adenine pairing with thymine like it does in DNA, adenine pairs with a different base called uracil (U) in the mRNA molecule.

Th e smallest part of the mRNA molecule that means something to a cell is three bases long. A group of three mRNA bases, also called a triplet, makes up a codon. In 1966, Marshall Nirenberg (b. 1927), Heinrich Matthaei (b. 1929), and Severo Ochoa (1905–1993) discovered that codons tell a cell which amino acids it needs to assemble to make a protein. Amino acids are the building blocks of proteins. This process is called translation. The mRNA codon AUG (adenine-uracil-guanine), for example, is called a start codon. Th e start codon tells the cell when to start translation. It also codes for the amino acid methionine. The codon CUA codes for the amino acid leucine. And the codons UGU and GUG code for the amino acids cysteine and valine, respectively. So, if a short string of mRNA existed in a cell that read AUGCUAUGUGUG, this string would be translated into a chain of amino acids that would read: methionine-leucinecysteine-valine.

There are 64 codon possibilities, which code for one of 20 amino acids. Because there are only 20 amino acids, and there are 64 codons, there is some overlap. In other words, several codons code for the same amino acid. Of these 64 possibilities, all but three code for a specific amino acid. The three codons UAA, UAG, and UGA signal the cell to stop translation.

For the most part, this code is universal to all types of life. Animals, plants, humans, and other organisms all have the same basic code. There are a few exceptions, but they are mainly limited to assigning one or two codons differently.

Each amino acid in a protein is called a monomer, and proteins are polymers (long strings of monomers). In other words, proteins are made up of many individual units (amino acids) linked together into a larger molecule. Proteins can also be called polypeptides. Most proteins contain 200 to 300 amino acids, but some are smaller. Smaller proteins are often called peptides. Some proteins are very large, however. The largest one found in the human body, so far, is titin, which has over 34,000 amino acids in a single chain, and is found in cardiac and skeletal muscles.

The process of cell division is necessary for life to function. In this process, one cell, the parent cell, divides into two daughter cells. This is called the cell cycle and has four distinct steps: a G1 (or gap-1) phase, an S phase, a G2 (or gap-2) phase, and mitosis. During the G1 phase, a cell grows in size and prepares to replicate (or copy) the chromosomes. The S phase is the synthesizing phase, when the DNA molecule is replicated, doubling it. Once the DNA is doubled, the cell enters the G2 phase, and prepares to divide. Collectively, the G1 phase, S phase, and G2 phase are called interphase. Mitosis is the stage in which the cell actually divides into two daughter cells, each with its own copy of the DNA molecule. In a mammalian cell, mitosis lasts only for a short time.

The process of mitosis can also be broken down into steps: prophase, metaphase, anaphase, telophase, and cytokinesis. In preparation for cell division, the DNA molecule is copied during the S phase of the cell cycle. But chromosomes are not visible in the cell at this time. The DNA molecule and all the proteins that are associated with it (the histones) are uncoiled chromatin during this phase. When prophase begins, this chromatin begins to condense into chromosomes, the DNA coils around the histones. This can be seen under a light microscope. At this point, the chromosomes consist of two sister chromatids that were formed during DNA duplication. The two sister chromatids are joined at a constricted area on the chromosome called the centromere. The chromatids contain identical genetic information. The centrioles, small organelles that will produce spindle fibers needed to allow the cell to divide, also move to the opposite ends of the cell during this phase.

During metaphase, these spindle fibers apply tension to the chromosomes, causing them to line up in the center of the cell. The spindle fibers begin to shorten, pulling the chromatids apart and toward opposite ends of the cell during anaphase. The chromatids are now two separate daughter chromosomes. The next step of mitosis is telophase, during which the daughter chromosomes arrive at the opposite ends of the cell and the spindle fibers disappear. A new nuclear membrane also forms around the two sets of daughter chromosomes and the chromosomes unravel, returning to the chromatin state. Chromosomes are no longer visible under a light microscope during telophase. Then the last phase of mitosis, cytokinesis, begins. During cytokinesis, the cytoplasm of the parent cell is divided into two daughter cells, each of which contains one set of identical chromosomes inside their new nucleus. Following cytokinesis, the two new cells return to interphase to start the cycle over again.

Mitosis is when an autosome—all chromosomes except the
sex chromosomes—divides into two daughter cells, each of which has a
full copy of the DNA molecule.

Mitosis is the process in which autosomes duplicate themselves. However, gametes are produced by a different type of cell division known as meiosis. The process of meiosis actually involves two cell divisions—meiosis I and meiosis II. During interphase, before meiosis I begins, the DNA molecule is replicated just like it is during the interphase state of the cell cycle. The first division, meiosis I, consists of four main stages: prophase I, metaphase I, anaphase I, and telophase I. During prophase I, chromatin condenses into chromosomes just like it does in mitosis prophase. As with mitosis, at this point each chromosome consists of two chromatids. The centrioles also move to opposite ends of the cell, and the spindle fibers form during prophase I, just as they do during prophase in mitosis. One difference between mitosis and meiosis occurs at the end of this phase, however. Near the end of prophase I during meiosis, sister chromatids undergo a process called crossing-over.

Crossing-over is a process in which two homologous chromosomes exchange segments. In other words, two matching chromosomes, one from the mother and the other from the father, exchange genetic material. This process is also called recombination. Genetic recombination results in genetic variation between parent and offspring, and is necessary to maintain genetic diversity. A population that has genetic diversity is better equipped to adapt and survive changes in their environment than a group with a more uniform genetic makeup.

After prophase I, metaphase I begins, during which the homologous chromosomes line up in the center of the cell. So, for example, both members of the chromosome 1 pair line up side by side. So do chromosomes 2 through 22 and the two sex chromosomes. During anaphase I, the homologous chromosomes are pulled (by spindle fibers) to the opposite sides of the cells. So each side now has chromosomes 1 through 22 and one sex chromosome. The sister chromatids are not separated at this point; only pairs of homologous chromosomes are separated. Then during telophase I, two daughter cells are formed, each one containing one chromosome from each homologous pair. Each of the chromosomes in the daughter cells has two chromatids.

Another diff erence between meiosis and mitosis is that in meiosis, the daughter cells do not return to interphase at this point. Instead, they go through another division—meiosis II. Meiosis II also has four stages: prophase II, metaphase II, anaphase II, and telophase II. Meiosis II is much like mitosis, except there is no DNA replication because each chromosome already consists of two sister chromatids. During prophase II, the centrioles move to opposite sides of the cell and spindle fibers form. During metaphase II, the chromosomes line up at the center of the cell. The sister chromatids are pulled apart and to opposite sides of the cell during anaphase II. And, four daughter cells are formed during telophase II (remember at the beginning of prophase II, there are two daughter cells, so when they divide, four daughter cells are formed.

Cell division in gametes—or sex cells—happens during the
process of meiosis, which occurs in two phases. In meiosis, a parent cell
divides twice, creating four daughter cells, each of which receives half
the amount of DNA as the parent cell.

These four daughter cells now contain only half of the chromosomes of the original, parent cell. A cell that has only half of the chromosomes that an organism contains is called a haploid cell. In humans, this means that a haploid cell would contain 23 chromosomes. Eggs and sperm are haploid cells. Haploid cells are also called gametes. When human cells undergo mitosis, all 46 chromosomes are replicated (copied) and each daughter cell contains 46 chromosomes. A cell that has an entire chromosome complement (for humans, 46 chromosomes) is called a diploid cell. Mitosis results in a diploid cell (all body cells except eggs and sperm), while meiosis results in haploid gametes (eggs and sperm). For this reason, meiosis is sometimes called reduction cell division.

In a human diploid cell, which has 46 chromosomes, 44 of them are called autosomes. Autosomes carry the bulk of the genetic material in an organism, but the remaining two chromosomes determine the organism’s sex and sex-linked traits.

In humans, there are two different sex chromosomes—an X and a Y. Normal human females have two X chromosomes, and their genotype is 46, XX. Normal human males have one X and one Y chromosome, and their genotype is 46, XY. Because females have two X chromosomes, all of their gametes (eggs) will have one X chromosome in them. Recall that gametes are made during meiosis. All of the gametes that a female makes will carry an X as the gender or sex chromosome. Males, on the other hand, can make two different kinds of gametes (sperm); some will carry an X chromosome while others will carry a Y. This is because during meiosis, half of the sperm produced will have an X and half will have a Y chromosome.

Since the male is the only one making gametes with the Y chromosome, the sperm determines the sex of the offspring. The Punnett square shows that with each pregnancy, the chance of having a boy is 50%, as is the chance of having a girl.

While humans, mammals, fruit flies, and some plants have the XY sex determination system, this is, by no means, the only type of sex determination system in existence. Birds have a similar sex determination system, but their sex chromosomes are called Z and W. The Z and W system is the reverse of the XX/XY sex determination system. In other words, in birds, males have two of the same type of sex chromosomes. Their genotypes are ZZ. Female birds, on the other hand, have two different sex chromosomes, making their genotypes ZW. Another sex determination system called the XX/XO system is used by grasshoppers and some other insects. In the XX/XO system, females have two X chromosomes just like in the XX/XY system. But males have only one sex chromosome—an X. The O stands for the absence of another chromosome.

Honeybees have an odd gender determination system. Fertilized, or diploid, eggs become females, while unfertilized, or haploid, eggs grow into males. Ants and wasps also seem to use this system. Some types of fish, alligators, and turtles do not use chromosomes to determine sex at all. Instead, the temperature of the environment determines whether the offspring will be male or female. For alligators, warmer temperatures (between 90°F and 93°F or 32°C to 34°C) result in males. While cooler temperatures (82°F to 86°F, or 28°C to 30°C) result in females. Temperatures in between these extremes result in a mix of males and females. Sea turtles are exactly the opposite—warmer nests produce almost all females, while cooler nests produce almost all males, but again, in-between temperatures result in a mix of males and females.