0610 · 17.1Chromosomes, genes and proteins

Chromosomes, genes and alleles

0:00A nerve cell and a cell lining your intestine do very different jobs. Yet most body cells in the same person contain the same genes. How can the same genetic instructions produce cells that behave so differently? And how can instructions inside a nucleus control what happens elsewhere in a cell? By the end of this video, you will be able to connect chromosomes, genes and proteins, and explain how sex chromosomes are inherited.

0:22First, remember three parts of a typical animal cell. The nucleus contains genetic information, the cytoplasm is the region outside the nucleus, and ribosomes are the structures where proteins are made. Proteins are not just substances found in food. Cells make proteins themselves, and many of those proteins carry out the cell’s essential jobs. Inside the nucleus are chromosomes, which are made of deoxyribonucleic acid and contain genetic information in the form of genes.

0:49This is usually shortened to D N A. Think of a chromosome as a long package of genetic information, rather than as a single instruction. Along its length are many genes, each containing information for making a protein. Here is the precise definition you need. A gene is a length of D N A that codes for a protein. A gene is therefore part of the material making up a chromosome, not a separate structure floating beside it.

1:11Saying that it codes for a protein means that its information determines the protein’s amino acid sequence. This is the link between stored genetic information and something the cell can actually use. A useful analogy is a collection of recipe books. The chromosomes are like the books, and genes are like individual recipes within them. A recipe gives instructions for making a particular product, just as a gene contains information for making a protein.

1:35However, the cell does not read words or pictures. Its information is carried by the sequence of bases in the genetic material. Bases are the parts of the genetic material whose order carries information. You do not need the details of their chemical structure for this topic. The important idea is that changing an order can change a message, just as rearranging letters can change a word. The sequence of bases in a gene determines the sequence of amino acids used to make a specific protein.

2:00There can also be different versions of a particular gene. An allele is an alternative form of a gene. So a gene and an allele are not two unrelated kinds of genetic material. An allele is one version of that gene. Alternative forms can have different base sequences, while occupying corresponding positions on the matching chromosomes inherited from the parents. Return to the recipe analogy for a moment. Two editions of a recipe book might contain alternative versions of the same recipe at the corresponding place.

2:27Similarly, matching chromosomes can carry alternative forms of the same gene. Do not confuse this with having completely different genes at that position. The detailed inheritance of alleles belongs in the video on monohybrid inheritance, but here we need to understand what an allele is.

Protein structure and function

2:42Now let us follow the information towards a protein. Proteins are made from smaller units called amino acids, joined in a particular sequence. Imagine threading different kinds of beads onto a string, where the order of the beads represents the order of the amino acids. But a protein does not normally remain as a straight string. Its amino acid chain folds into a particular three-dimensional shape. Different sequences of amino acids give different shapes to protein molecules.

3:08This happens because the amino acids in the chain interact in ways that depend on which ones are present and where they occur. The sequence therefore influences how the chain folds. Shape matters because a protein must have a suitable shape to carry out its function, rather than simply containing the right number of amino acids. Think about an enzyme, a protein that speeds up a chemical reaction. Its active site must have a shape complementary to its substrate so that the substrate can bind.

3:34A different amino acid sequence can give a different protein shape, including a differently shaped active site. This explains why the order of genetic information matters. It helps determine the structure of a protein, and that structure affects what the protein can do. This is how genetic material controls cell function. It controls the production of proteins. Enzymes control reactions inside cells, while membrane carrier proteins help particular substances cross cell membranes.

4:00Receptor proteins for neurotransmitters allow cells to respond to these chemical messengers. You do not need the detailed processes here, as the important connection is that making particular proteins gives a cell particular capabilities. Notice that genetic material does not directly act as a membrane carrier or an enzyme. Instead, it contains the information needed to produce those proteins. It is like controlling a workshop by specifying which tools should be made, rather than doing every job itself.

4:26To explain genetic control properly, connect the base sequence to the amino acid sequence, then the protein’s shape, and finally its function.

Messenger RNA and protein synthesis

4:34But there is a location problem to solve. The gene coding for a protein remains in the nucleus, while the protein is assembled at a ribosome in the cytoplasm. The cell therefore needs a way to carry the gene’s information to the ribosome without moving the gene itself. It does this using messenger R N A. Messenger R N A is a copy of a gene. It is made in the nucleus and then moves into the cytoplasm, carrying the information needed to make the protein.

4:59This is like taking a working copy of a recipe into the kitchen while leaving the original recipe book safely in its usual place. The crucial distinction is that the information is copied, but the gene itself stays in the nucleus. In the cytoplasm, messenger R N A passes through a ribosome. The ribosome assembles amino acids into a protein molecule, using the information carried by the messenger R N A.

5:21The specific sequence of amino acids is determined by the sequence of bases in messenger R N A. So the ribosome does not choose amino acids randomly. The information specifies the order in which they are assembled. Keep the whole journey connected in your mind. A gene’s base sequence supplies information, a messenger copy carries that information into the cytoplasm, and a ribosome uses it to assemble amino acids in a specific sequence.

5:45That amino acid sequence gives the resulting protein its particular shape. For this syllabus, you do not need the detailed chemical machinery behind these steps, because understanding the information pathway is what matters.

Gene expression in specialised cells

5:57We can now answer our opening question about different kinds of cells. Most body cells in an organism contain the same genes, but many genes in a particular cell are not expressed. A gene is expressed when its information is used to make its protein. Each cell only makes the specific proteins it needs, so possessing a gene does not mean that the cell is currently using it. A nerve cell, for example, needs proteins suited to receiving and responding to signals.

6:23A cell lining the intestine needs proteins suited to its own functions, including moving substances across its membrane. Both can contain the same collection of genes while expressing different selections from that collection. Their different activities come from which instructions they use, not simply from having different sets of instructions.

Haploid, diploid and sex chromosomes

6:40Now let us look at how chromosomes are organised and inherited. A haploid nucleus is a nucleus containing a single set of chromosomes. A diploid nucleus is a nucleus containing two sets of chromosomes. A set means one of each chromosome type, so the distinction is about complete collections, not merely whether a nucleus contains a small or large number of chromosomes. In a diploid cell, there is a pair of each type of chromosome.

7:04In a human diploid cell, there are twenty-three pairs, making forty-six chromosomes altogether. One chromosome in each pair comes from the mother and the other from the father. Human gametes, the egg and sperm cells, have haploid nuclei containing twenty-three chromosomes, so each carries one set rather than two. At fertilisation, the nuclei of the egg and sperm bring their chromosome sets together. One set of twenty-three from the egg combines with one set of twenty-three from the sperm.

7:29The resulting nucleus therefore contains two sets, restoring the diploid number of forty-six chromosomes. If gametes already contained two sets, combining them would not restore the normal diploid number, which is why the single-set arrangement matters. One pair in a human diploid cell is the pair of sex chromosomes. In the usual human pattern, females have two X chromosomes, while males have one X chromosome and one Y chromosome. The X and Y chromosomes differ, so the sex chromosome pair in a male is not two identical-looking chromosomes.

7:58Nevertheless, they form the sex chromosome pair within the twenty-three pairs.

Sex inheritance and exam practice

8:03Let us work through a question. What is the probability that a child will inherit the female sex chromosome combination? Begin with the egg. Since the mother has two X chromosomes, an egg receives an X chromosome. The father has one X and one Y chromosome, so a sperm can carry either an X chromosome or a Y chromosome. Next, combine the possibilities rather than guessing from the parents’ appearance.

8:25An egg carrying an X chromosome and a sperm carrying an X chromosome produce two X chromosomes, which is female. The same egg combined with a sperm carrying a Y chromosome produces one X and one Y chromosome, which is male. These are the two possible outcomes in the usual chromosome model. The two sperm possibilities have equal probability in this model. One of the two equally likely outcomes is female, so the probability is one half, or fifty per cent.

8:50The probability of the male outcome is also fifty per cent. It is the sperm’s sex chromosome that determines which combination forms, because the egg contributes an X chromosome in either case. That does not mean every family must contain equal numbers of boys and girls. A probability describes the chance for each fertilisation, not a compulsory pattern across a small number of births. Previous children do not make the opposite outcome due next time.

9:14Also, neither parent consciously chooses which sperm fertilises the egg, so this biological explanation is not about parental control or blame. Before we practise an explanation question, check three common mistakes. A chromosome is not the same thing as a gene, because a chromosome contains many genes. A gene is not a protein, because it contains the information for making a protein. And the gene does not leave the nucleus to visit a ribosome.

9:38The messenger copy carries its information into the cytoplasm. Another mistake is to say that specialised cells must contain completely different genes. Most body cells contain the same genes, but they express different selections according to the proteins they need. Also, haploid does not mean half of every chromosome, and diploid does not mean just two chromosomes altogether. Haploid means one complete set, and diploid means two complete sets. Here is an exam-style question.

10:06Explain how a gene controls the production of a specific protein. Start by identifying the information, then follow its route from nucleus to cytoplasm, and finish with amino acid assembly. The command word explain means that you should connect the steps, rather than simply name cell structures. Try forming your answer before listening to the model answer. The sequence of bases in the gene determines the sequence of amino acids in the protein.

10:29The gene remains in the nucleus. Messenger R N A is a copy of the gene and is made in the nucleus. Messenger R N A moves into the cytoplasm and passes through a ribosome. The ribosome assembles amino acids into a protein molecule in the sequence determined by the bases in messenger R N A. If the question continues by asking why different amino acid sequences matter, extend the explanation towards shape and function.

10:53Different sequences of amino acids give different shapes to protein molecules. A protein’s shape determines whether it can carry out its particular function. For example, an enzyme needs an active site with a shape complementary to its substrate, so simply saying that a protein is different does not fully explain the consequence. Let us bring everything together. Chromosomes are made of D N A and contain genes, each a length that codes for a protein.

11:17An allele is an alternative form of a gene. Base sequence determines amino acid sequence, which determines protein shape and helps determine function. Messenger copies carry genetic information from the nucleus to ribosomes, allowing cells to make the proteins they need while leaving the genes in place. Most body cells contain the same genes, but different genes are expressed in different cells. A haploid nucleus contains one set of chromosomes, while a diploid nucleus contains two sets, with twenty-three pairs in humans.

11:45Eggs and sperm each contribute one set at fertilisation, including a sex chromosome. The usual combinations are two X chromosomes for a female and one X with one Y for a male, each with a fifty per cent chance.