Part 2

Human chromosomes If you were to spread out all of your chromosomes from one nucleus in your body, they would look something like this. Humans have 23 pairs of chromosomes, 46 in total. One chromosome in each of the pairs comes from each of your biological parents. What are chromosomes? Chromosomes contain DNA, and they keep all the genetic information safe. Different organisms have different numbers of chromosomes. We have 46 chromosomes, guinea pigs have 64, and chimps have 48—the same number as potatoes! Chromosome pair 23 consists of the sex chromosomes. There are two types, called X and Y. These are special because they help to determine whether you are male or female. Usually, girls have two X chromosomes and boys have one X and one Y. CRAZY CHROMOSOMES The number of chromosomes in living things varies hugely and can be surprising. An elephant has 56 chromosomes, but a chicken has 78! Jumper ants These poisonous Australian ants only have one pair of chromosomes. Common garden ants have 15 pairs. Adder's tongue fern Perhaps surprisingly, plants often have lots of chromosomes. The adder's tongue fern has 1,262 chromosomes—that's the most found so far in any living thing! Atlas blue butterfly This butterfly from North Africa has 452 chromosomes. That's the largest-known number of chromosomes in an animal.

Transcription When a protein needs to be made, the gene that makes it must be activated. This process is called transcription. DNA can't leave the nucleus of the cell, so instead, a temporary copy of the DNA code is transcribed into a molecule of messenger RNA (mRNA), which can leave the nucleus. Genes make proteins Each gene usually contains the instructions for making one protein. Proteins come in different shapes and sizes and do all the work in your cells. Some of them, such as insulin, are important hormones that travel around your body in the blood. Using genes Your DNA code is like an instruction manual made up of lots of different sentences, called genes. When you need to use a gene, its code is read. Each gene is marked with a start and a stop code, like a capital letter and a period in a sentence. 1 Genes The DNA code is made up of many genes. Genes are only read once they get a "go" signal from the cell. 2 Gene unwinds Once your gene gets the "go" signal, its DNA unwinds. This separates the base pairs so that a copy can be made. 3 mRNA is made A single-stranded copy of the gene sequence is made using RNA bases. These make an mRNA molecule that can be destroyed once the required protein has been made. 4 DNA winds back up After the mRNA copy of the code has been made, the DNA is wound back up again to keep the information safe. Any damage could result in a mutation. 5 Leaving the nucleus Now we have mRNA, but we still need to read it. For this to happen, the mRNA leaves the nucleus through a pore (hole).

6 Codons Each group of three letters makes up a single word in the gene, called a codon. 7 Amino acids and tRNA Each codon codes for an amino acid and each amino acid is attached to a tRNA molecule. These pair with the codons on the mRNA. Reading the code The ribosome is incredibly fast at translating the mRNA sequence into protein. It can join up amino acids at a rate of 200 per minute. The biggest protein in the human body is called titin, and it takes almost three hours to make. Cracking the code Can you read the instructions written in the genetic code? No? Well, you'll need the help of a type of RNA called transfer RNA (tRNA) and a ribosome. They form a team that cracks the code and follows its instructions to make proteins. Ribosome The ribosome is a hard-working molecular machine. It matches the codons in the mRNA to tRNA, which brings in the amino acids needed to make proteins. This process is called translation. 8 Making proteins Amino acids make long chains, like beads on a string. The chain folds itself up into a 3-D protein and gets to work in the cell. Nearly all living things on Earth use the same genetic code to make proteins from the same animo acids. Twenty different amino acids are coded for by DNA. You can make 11 of these in your body. You must get the other nine from your food.

Copying your DNA A complete copy of your DNA is inside nearly every one of the 40 trillion cells in your body. Each time your body makes a new cell, you need to replicate, or make a copy of, all your DNA so that the new cell knows what to do. A human skin cell divides in two to form two daughter cells. Grown in a lab, these cells can help scientists learn about wound healing. DNA replication When DNA copies itself, it first unwinds into two separate strands. Then, a special protein called a DNA polymerase matches new DNA bases with the sequence on the two unwound halves of the helix. This completes the two new DNA strands. Each day, you make 330 billion feet (100 billion meters) of DNA! Why do we need new cells? Whenever you grow or repair any damage, such as a cut, you need new cells. New cells are made by cell division. Before a cell can divide, it first needs to copy all of its DNA. The "parent" cell then splits in two, making two new "daughter" cells, each with a complete set of DNA. STAGE 1 Copying chromosomes Each chromosome replicates itself to form two strands of DNA, which are stuck to each other in the middle. The membrane of the nucleus then breaks down. STAGE 2 Lining up The replicated chromosomes attach to threadlike fibers that help them line up in the center of the cell. STAGE 3 Pulled apart Pulling from the fibers separates the two halves of the replicated chromosomes. The two copies move into different halves of the cell as the cell begins to divide. STAGE 4 Daughter cells The parent cell will now become two daughter cells, each with a complete copy of DNA in a newly formed nucleus.

Do all your cells have the same DNA? Scientists used to think that cells became specialized for a particular role in the body by losing part of their DNA. We now know that different cell types contain the same DNA, so why do they look so different? Heart muscle cells The muscles in your arms and legs need to rest after use. However, heart muscle is different. It never rests and keeps working every moment until you die. FACT FILE » Heart cells beat by themselves, even when they are grown in a Petri dish! » Heart cells are full of mitochondria to give them lots of energy. » Your heart beats 100,000 times a day. Neurons Neurons, or nerve cells, are very long cells that transmit signals in and out of your brain and spine. These signals travel as fast as 250 mph (400 kph)! FACT FILE » Our brains contain about 86 billion neurons. » Your longest neuron is 3 ft 4 in (1 m) long, from your spine to your toes. » Giraffes have neurons that are more than 16 ft 6 in (5 m) long! DIFFERENT CELLS, DIFFERENT GENES Your whole genome is not active in every cell. Instead, different cells just use the genes they need. In a heart muscle cell, only the genes that are needed to make and run heart muscle are active. Other specialized genes that make different cells, such as neurons, are tightly locked away and can't be read. Gene used only in heart muscle cells Locked-away gene not used by this cell type Gene used by many different cell types Bone cells The outside of your bones is made up of a hard tissue filled with long-lasting, star-shaped cells that help take care of the bones. FACT FILE » Bones also store minerals inside your body. » The smallest bone in your body is in your ear and helps you hear. » Birds have lightweight, hollow bones to help them fly. Lung cells The lining of your lungs is made up of cells that allow oxygen from the air into your blood—and carbon dioxide out— when you breathe. FACT FILE » Lung cells are covered with little brushlike structures to sweep out gunk. » The total surface area inside both of your lungs is about the size of a tennis court! » Lungs have 1,500 miles (2,400 km) of airways.

Passing on your genes Living things pass on their genes to the next generation. This means that children often look like their parents, since they share half of their genes with their mother and half with their father. This is called genetic inheritance. Two versions of every gene The cockerpoo puppies here have two copies of each of their chromosomes—one from Mom and one from Dad. This means that they, just like humans and many other animals, have two versions of every gene. Different versions of the same gene are called alleles (a-leelz). DOMINANT ALLELES If you have two alleles of the same gene, such as the gene that controls hair-shedding, which one wins? Some alleles are dominant— their instructions override other alleles—and they always win. Dominant gene None of the puppies shed their hair, which means the dad's allele is dominant. Scientists can compare the DNA of different dog breeds to find out which genes code for which characteristics.