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Genetic Material
Reproduction is a characteristic of all living organisms. During reproduction, the instructions for
inherited traits are passed from parents to offspring through their genetic material. The passage of
these genetic instructions from one generation to the next generation is heredity.
Organization of Genetic Material
gene → DNA molecule → chromosome → genome
It is important to know how genetic material is organized. The genome of an organism refers to its
complete genetic makeup and includes the organism's entire set of chromosomes. The total number
of chromosomes in a normal human is 46: 23 chromosomes from the mother and 23 from the father.
Chromosomes, Genes, & DNA
Genetic information is encoded in DNA. DNA is located in the chromosomes of cells. Chromosomes
appear very dark (when viewed under a microscope) and are located in the cell nucleus, if the cell
has one.
A gene is a section of DNA located
on a chromosome.
A section of a chromosome that
controls a particular trait is a gene.
Genes are a kind of blueprint for an
organism. They contain all the
information necessary to build,
repair, and keep the organism
running, including how to make all
the different proteins and other
materials the body needs.
Sexual & Asexual Reproduction
Reproduction is the process through which an organism passes its genetic information to its offspring. The
ability to reproduce is necessary for the survival of a species. The two main types of reproduction are asexual
reproduction and sexual reproduction.
Asexual Reproduction
In asexual reproduction, a single parent passes copies of its genes to each of its offspring, so all of
the offspring are genetically identical to the parent.
In general, this form of reproduction is used by simple organisms, such as bacteria, because only one
parent is required for asexual reproduction. Also, asexual reproduction is
generally quicker and simpler than sexual reproduction. However, populations
of organisms produced through asexual reproduction have less genetic diversity
and are therefore less likely to have any members survive catastrophic change
in their environment.
There are several different types of asexual reproduction:
•
Budding—Budding occurs when an offspring grows out of the body of the
parent organism. Hydra reproduce by budding.
•
Fragmentation —Fragmentation occurs when
parts of an organism break off. The organism fragments
regrow their missing parts through regeneration and
develop into new individuals. Sea stars (starfish)
reproduce through fragmentation and regeneration.
•
Binary Fission—Binary fission occurs
when a prokaryotic organism undergoes
cellular division and divides into two
new organisms. Amoeba reproduce by
binary fission.
•
Mitosis—Mitosis is a special type of asexual reproduction that occurs in eukaryotes. During mitosis,
genetic material is replicated within the nucleus of a cell. Then, the nucleus divides into two new
nuclei, and in most cases, the entire cell divides during cytokinesis into two new daughter cells. The
daughter cells are genetically identical to each other and to the parent cell. They have the same
number of chromosomes as the parent cell.
Sexual Reproduction
In sexual reproduction, two parents each contribute genetic material to their offspring. Because both
parents contribute genetic material, the offspring have traits of both parents, but they are not exactly
like either parent. This creates more diversity in a population of organisms. For sexual reproduction to
occur, each parent must form a sex cell, also known as a gamete. Gametes are formed by meiosis.
Meiosis is a two-step process of cell division which produces four cells that each contain half of the
number of chromosomes present in the parent cell. A female gamete is called an egg, and a male
gamete is called a sperm. When an egg's nucleus and a sperm's nucleus fuse, fertilization occurs,
and a fertilized egg, known as a zygote, forms. The zygote retains the chromosomes from the egg
cell and the sperm cell, so it has the correct number of chromosomes for the species. The zygote
divides over and over again through mitosis and develops into a mature organism. All the new cells
produced from the fertilized egg have the same genetic information as the fertilized egg, although the
cells may specialize or differentiate to express different genes and become different kinds of cells.