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Sexual Reproduction

We'll tell you about meiosis in a minute, but first let's review a few basic facts about cells, genes, chromosomes and DNA. 

Cells, Genes, DNA and Chromosomes

Cells are the building blocks of life -- and DNA is their instruction book. DNA and the cell's environment together control how the cell will grow, divide and function. DNA codes for the proteins that are made by the cell. It works like this:

Inside the nucleus of every cell, there are long thin strands of chromosomes. These chromosomes are composed of proteins and twisted chains of DNA -- chains that wind around and around into a shape called a double helix – see the diagram above. A helix has a shape like a spring, so you can imagine a double helix as two springs that are wound together. Each gene is one tiny part of this double helix -- Genes are arranged in a linear way along the chromosome, a bit like beads on a string. Chromosomes come in pairs, so in humans there are 23 pairs, making 46 chromosomes altogether. The sequence of genes is the same for both members of a chromosome pair. Each gene -- which, as we said, is a tiny part one of the chromosomes -- contains a specific sequence of DNA.

As we have learned before, each chromosome comes in a pair, so each gene is represented twice, once on each chromosome of the pair. Even though these pairs of genes code for the same trait, they are often not identical. We call differences in genes for the same trait variations. The various genes that code for a particular trait are called alleles. In a diagram below, you will see genes for beetle color; both alleles code for the same trait – code – but each codes for a different color Alleles are important when it comes to passing on physical characteristics (traits), because they can be either dominant or recessive. Alleles (genes) that are expressed (seen) in an organism are called dominant, while alleles that are not seen are called recessive. We'll tell you more about this in our next Instruction.