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Haploid cells contain one complete set of chromosomes (n), as in gametes like sperm and egg cells, with n = 23 in humans. Free GCSE Biology notes.
Fertilisation and Development is part of Reproduction in GCSE Edexcel Higher Biology.
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Haploid cells contain one complete set of chromosomes (n), as in gametes like sperm and egg cells, with n = 23 in humans. Free GCSE Biology notes.
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Use this lesson to stay inside Reproduction while connecting the detail back to the wider Topic 3: Genetics topic.
Read the core explanations from Fertilisation and Development before testing yourself from memory.
Right now, you have about 37 trillion cells in your body. Every single one started from just two cells meeting - a sperm and an egg. But here's the puzzle: if body cells have 46 chromosomes, and sperm cells have 46 chromosomes, and egg cells have 46 chromosomes, why don't babies end up with 92 chromosomes?
The answer lies in a clever biological trick. Your body makes special cells - gametes - that have exactly half the normal number of chromosomes. When they join together at fertilisation, they restore the full set. It's like having two half-tickets that make one complete ticket.
Think of chromosomes like a recipe book. Your body cells are diploid - they have two copies of every recipe (2n = 46 chromosomes in humans). This gives you a backup copy if one gets damaged.
But gametes are different. They're haploid - they only carry one copy of each recipe (n = 23 chromosomes). This halving is crucial. When a sperm (23 chromosomes) meets an egg (23 chromosomes), the resulting zygote has the full set again: 23 + 23 = 46 chromosomes.
Meiosis is like a special factory that takes one cell with 46 chromosomes and produces four cells with 23 chromosomes each. But it's not just simple division - there's shuffling involved too.
Here's what happens: the cell goes through two rounds of division. In the first round, the chromosome pairs separate. In the second round, the individual chromosomes split. The result? Four genetically unique gametes, each with exactly half the chromosomes of the parent cell.
Trap: Many students think meiosis just cuts chromosomes in half randomly. Actually, it carefully separates whole chromosomes, ensuring each gamete gets one complete copy of each chromosome type.
Fertilisation is the moment when two half-sets of chromosomes become a complete set again. The sperm cell penetrates the egg cell, and their nuclei fuse together. This creates a zygote - the very first cell of a new organism.
Worked example: The Fertilisation Process
Trace what happens to chromosome numbers during fertilisation in humans
Answer: Zygote formed with 46 chromosomes (diploid)
The beauty of this system is mathematical precision. Every time fertilisation occurs, the chromosome number is perfectly restored. No matter how many generations pass, humans always end up with 46 chromosomes per body cell.
Once fertilisation creates the zygote, a different type of cell division takes charge: mitosis. Unlike meiosis, mitosis produces identical copies. The zygote divides into 2 cells, then 4, then 8, then 16, and so on.
Every mitotic division maintains the chromosome number. If the zygote has 46 chromosomes, every cell it produces through mitosis also has 46 chromosomes. This is crucial - you need the same genetic information in every body cell.
Here's where development gets really clever. All the cells produced by mitosis start identical, but they don't stay that way. As the embryo grows, cells begin to differentiate - they become specialised for different jobs.
Think of it like students at university. They all start with the same basic education, but then specialise into doctors, engineers, teachers. Similarly, embryo cells all have the same DNA, but different genes get switched on to create muscle cells, nerve cells, skin cells.
The amazing thing? Every specialised cell still contains all 46 chromosomes. A muscle cell has the same DNA as a brain cell - they use different parts of it.
Common exam mistakes include confusing mitosis with meiosis, forgetting that all body cells have the same chromosome number, and not explaining WHY chromosome numbers need to be halved in gametes. The key insight is that sexual reproduction requires this careful chromosome management to maintain species chromosome numbers across generations.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Haploid cells contain one complete set of chromosomes (n), as in gametes like sperm and egg cells, with n = 23 in humans. Diploid body cells have two copies of every chromosome, 2n = 46 chromosomes in humans.
A zygote is the very first cell of a new organism, formed when the nuclei of a sperm cell and egg cell fuse at fertilisation. In humans it has 46 chromosomes (diploid) and then divides by mitosis into 2 cells, then 4, then 8 and so on.
Fertilisation is the fusion of a haploid sperm cell with a haploid egg cell to form a diploid zygote. The sperm nucleus enters the egg and the two nuclei fuse, restoring the full chromosome number: 23 + 23 = 46 chromosomes.
A human body cell has 46 chromosomes, which is the diploid number (2n = 46). Gametes are haploid and have 23 chromosomes (n = 23).
An embryo develops from a zygote by mitosis, which produces genetically identical cells with the same chromosome number. As the embryo grows, cells differentiate into specialised types such as muscle, nerve and skin cells, because different genes get switched on.
The symbol n means one complete set of chromosomes (haploid) and 2n means two copies of every chromosome (diploid). In humans, n = 23 chromosomes in gametes and 2n = 46 chromosomes in body cells.
Babies do not have 92 chromosomes because gametes have exactly half the normal number of chromosomes. When a sperm with 23 chromosomes fuses with an egg with 23 chromosomes, the zygote has 23 + 23 = 46.
Fertilisation restores the chromosome number by fusing a haploid sperm nucleus with a haploid egg nucleus. The two half-sets of chromosomes combine to form a diploid zygote with 46 chromosomes, so humans always end up with 46 chromosomes per body cell.
The whole sperm does not enter the egg: only the sperm nucleus enters the egg cytoplasm. The sperm nucleus and egg nucleus then fuse to form the zygote nucleus.
Mitosis is the type of cell division that happens after fertilisation. The zygote divides into 2 cells, then 4, then 8, then 16, and every cell produced is genetically identical and diploid.
Mitosis does not change the chromosome number: every mitotic division maintains it. If the zygote has 46 chromosomes, every cell it produces through mitosis also has 46 chromosomes.
All body cells have the same DNA, and every specialised cell still contains all 46 chromosomes. A muscle cell has the same DNA as a brain cell, but different genes get switched on, so they use different parts of it.
Body cells have two copies of each chromosome because they are diploid, which gives a backup copy if one gets damaged. Diploid human body cells have 2n = 46 chromosomes.