Scientists can turn adult cells into stem cells, and in mice, they can coax those stem cells into sperm or eggs. But this final step is proving exceptionally difficult for human sex cells.
How do sex cells, known as gametes, know when to stop growing and undergo meiosis? Researchers have now discovered how a protein called STRA8 controls meiosis in germ cells.
Sperm
Sperm are long, tadpole-shaped cells that are the smallest single cells in the body. These motile cells carry half of a man’s genetic information and are haploid, meaning they don’t have paired chromosomes. They are created by a process called meiosis, which creates four sperm from a single germ cell. When sperm cell fertilizes an egg cell, the nuclei of the two cells combine to form a fertilized egg that is then ready to develop into a baby.
Each sperm cell has a head, a middle portion and a tail (flagellum) and contains 46 chromosomes. The head of the sperm cell is oblong or oval-shaped and houses the nucleus that carries the genes to be passed on. The middle portion of the sperm cell contains mitochondria, which power the cell. The tail of a sperm cell is what gives the sperm its movement, and it consists of a slender bundle of filaments that whips back and forth to propel the sperm toward an egg.
The sperm cell’s head also contains cortical granules that line its surface and help ensure only one sperm cell enters and fertilizes the female egg. The sperm’s tail also contains a chemical sensor that recognizes the egg’s unique chemical signature and activates its movement once it is within close proximity of an egg.
Eggs
The process of reproduction requires two different kinds of sex cells, called gametes. Male gametes are known as sperm and female gametes are called eggs. They are made in the special tissues of male testes and female ovaries. When a sperm cell fertilizes an egg, it produces a zygote, which is the embryo that grows into a baby.
The DNA that makes up a sex cell is very different from the DNA in body cells. This is because sex cells are made from parent cells that go through a different kind of cell division, called meiosis. During meiosis, the genetic material of each parent cell is copied and divided into two new cells. Each of these cells has a full set of 23 pairs of chromosomes.
Most of the cells in your body are called body cells. These include muscle cells, bone cells, blood cells, and nerve cells. Body cells are diploid and have two sets of chromosomes. However, sex cells are not diploid, so they have one set of chromosomes.
A sex cell that contains 23 unpaired chromosomes is called a haploid cell. It is very similar to a normal body cell except that it can only produce a zygote during sexual reproduction. Haploid sex cells also contain a specialized protein called a centromere. The centromere helps ensure that a haploid sex cell divides normally.
Ova
The cells that produce sperm and eggs are called sex cells. They are haploid and only have one set of dissimilar chromosomes, so they contain half the genetic information needed to make a complete organism (see this DNAFTB animation). When male sex cells or spermatozoa and female sex cells or ova fuse in a process called fertilization, they form a diploid cell known as a zygote that contains a mix of inherited genes from both parents.
Both sex cells are made in organs of the reproductive system known as gonads. For humans, the gonads are located in the testicles in men and in the ovaries in women.
Sex cells are different from other body cells in that they undergo a special type of cell division known as meiosis. In meiosis, the chromosomes condense into visible X-shaped structures, and each pair of sister chromatids is separated and pulled to opposite poles of the cell by microtubules associated with a structure called the centromere.
The paired sister chromatids are then randomly fused to each other in a process known as meiosis I, which results in the formation of two daughter cells with 23 chromosomes each (23 pairs of sister chromatids). Following meiosis, the cytoplasm separates around the nucleus, and the membrane that surrounds the nuclear envelope dissolves. The daughter cells then begin to differentiate into specialized cell types.
Meiosis
A sex cell’s long strands of DNA must be neatly organized. Meiosis accomplishes this by dividing a parent cell into four cells that each have half the number of chromosomes as the original cell. These cells are known as gametes and they contain only 23 chromosomes each, rather than the 46 chromosomes found in a parent’s somatic (non-reproductive) cell. Without meiosis, each new generation would inherit a full set of 46 chromosomes from the parents. Meiosis is also known as reduction division because it reduces a cell’s total number of chromosomes.
The first step in meiosis is called prophase I. It starts when sister chromatids separate from each other after the degradation of a protein complex that held them together, known as cohesin. The chromatids then move to opposite poles of the cell, where they form membranes around each set of chromosomes. This step is followed by a process called crossing over that randomly exchanges some genes between the homologous (matching) chromatid pairs. This produces a mixture of genes in each daughter cell and is responsible for producing genetic variation.
The next step is meiosis II. Unlike mitosis, meiosis II does not require DNA replication. It begins when the chromatids that remained fused in prophase I separate into two haploid secondary spermatocytes. During this stage of meiosis, each sperm cell contains a mix of 23 maternal and paternal chromosomes. This shuffled genetic diversity gives each sperm a unique combination of genes, which must then be combined with the egg of another sex cell in sexual reproduction.
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