# Biology, Answering the Big Questions of Life/cell division3

* INTERMEDIATE

## What is sex?

Sexual reproduction is the mixing of chromosomes to make organisms with new combinations of traits.

## Is sex the only way that organisms reproduce?

No. There are two forms of reproduction: Sexual reproduction, and Asexual reproduction.
When an organism makes an exact copy of itself, this is called asexual reproduction. The offspring (children) are called clones.
Sexual reproduction is when two organisms combine their chromosomes to make offspring that are different from their parents.

## What is the advantage of Sex?

Sexual reproduction makes offspring that can survive in conditions that the parents could not. It creates many varied offspring, and it increases the chance that some of an organism's offspring will survive.

## When is sexual reproduction better than asexual reproduction?

When conditions change.
If the environment is stable and good for an organism, then asexual reproduction is favored. If an organism can grow well now, then exact copies of that organism should be able to grow well too.
If conditions change radically, or if they constantly change, there is a probability that the organism will die and if all of his offspring are identical, there is a good chance that all of his offspring will die with him.
Sex produces organisms different from the parent. There is a good chance that some of those offspring will thrive no matter what new conditions occur.
Many organisms can reproduce both sexually and asexually. Humans for example can only reproduce sexually.

## Why do brothers and sisters look different from each other if they have the same mother and father?

Because humans reproduce by a process called sexual reproduction that mixes up the traits of the parents to make new , different offspring.
Humans have 46 chromosome in each normal cell. These chromosomes store all the information needed to make a person. In order to make children, however, each parent must give only half of the information that they need to make a person to each child.
Humans each have two copies of 23 types of chromosome in each cell. organisms that have two copies of their chromosomes are called diploid. When sex cells are made, one of each of the twenty three chromosomes makes it into each sex cell.
In addition to this, chromosomes will shuffle their sequences before reproduction in a process called crossing over. This shuffling of information usually makes each chromosome after meiosis look different than they did before meiosis.
Therefore each sex cell has a virtually unique combination of one parent's chromosomes. These cells are then combined to form an even more unique combination of traits.
So although brothers and sisters have the same parents, they are very unlikely to have gotten the same combination of chromosomes from them, and these differences make each person look unique.
It is this very uniqueness which is the strength of sexual reproduction. When people create different offspring, they each have different strengths and weaknesses that makes it possible for them to do well under different conditions. If everyone was the same then they would all be equally sensitive to the same diseases for example, but for sexually reproducing species, some will survive change better than others. This makes it more likely that no matter what comes, at least one of the offspring will survive to reproduce.

## What is the human life cycle?

When we talk of how a cell grows and reproduces, we call it the cell cycle. When we talk of how an organism grows and reproduces, we call it a life cycle.
Humans are born as babies. There are two sexes: male and female. They grow to adulthood. They make sex cells (called gametes). Sperm for males. Eggs for females. The sperm and the egg mix and make a single cell called a zygote. The zygote grows into a baby.

## Other life cycles

Other organisms have different life cycles. Many protists, for example reproduce both sexually and asexually.

## How are gametes made?

Gametes are made by a process called Meiosis.

## What is Meiosis?

Meiosis is a form of cell division that makes four cells which each have half of the chromosomes on the parent.

## Why do we need Meiosis?

Meiosis is needed to make gametes for sexual reproduction.
In sexual reproduction, Eukaryotic cells make organisms with new combinations of chromosomes by mixing the DNA of two parents.
In order to make the new individual have the same number of chromosomes as the parents, the parents must make sex cells with half of the number of chromosomes as the parent so that when they fuse, the new organism has the same number of chromosomes as the parent.

## What are the steps of meiosis?

Meiosis is the process in which one mother cell splits into four daughter cells, each with half the chromosomes of the mother cell.
Meiosis is similar to mitosis, but the original cell splits twice making four cells instead of splitting once to make two cells. These two divisions are called Meiosis I and Meiosis II.
The stages have the same names as in mitosis, only with a number to distinguish which division they occur in. The stages in order are Prophase I, Metaphase I, Anaphase I, Telophase I, interkinesis (a type of interphase), Prophase II, Metaphase II, Anaphase II, Telophase II.
Meiosis is the process in which one mother cell splits into four daughter cells, each with half the chromosomes of the mother cell.