Chromosomes, Genes and Cell Division

Chromosomes, DNA and Genes

Chromosomes are made of DNA. The DNA contains genetic information in the form of genes. A gene is a length of DNA that codes for a protein. An allele is an alternative form of a gene.

Worked example (gene and allele): Distinguish between a gene and an allele. Step 1. A gene is a length of DNA that codes for a protein. Step 2. An allele is an alternative form of a gene. Answer: gene: codes for a protein; allele: alternative form.

Inheritance of Sex

Sex in humans is inherited using the X and Y chromosomes. Females have two X chromosomes (XX). Males have one X and one Y chromosome (XY).

All egg cells carry an X chromosome. Half of the sperm carry an X chromosome and half carry a Y chromosome. At fertilisation, if an X sperm fuses with an egg the child is a girl (XX). If a Y sperm fuses with an egg the child is a boy (XY).

Each fertilisation has a 1 in 2 chance of giving a girl (XX) and a 1 in 2 chance of giving a boy (XY). The expected ratio is 1
1.

Worked example (sex determination): State the chance that a baby will be a boy, and explain your answer. Step 1. A boy is XY. This needs a Y sperm. Step 2. Half of the sperm carry a Y chromosome and each is equally likely to fertilise the egg, so the chance is 1 in 2 (50%). Answer: 1 in 2 (50%).

Worked example (independent chances): A couple already has three sons. State the chance that their fourth child is a girl. Step 1. Each fertilisation is a separate event. Step 2. Half of the sperm carry an X chromosome, so the chance is still 1 in 2 (50%). Answer: 1 in 2 (50%).

Genes, Proteins and Cell Function

ExtendedFrom bases to proteins

The sequence of bases in a gene determines the sequence of amino acids used to make a specific protein. Different sequences of amino acids give different shapes to protein molecules.

ExtendedHow DNA controls cell function

DNA controls cell function by controlling the production of proteins. These include:

Worked example (genes and protein shape): Explain why a gene with a different sequence of bases may produce a protein that works differently. Step 1. A different sequence of bases can give a different sequence of amino acids. Step 2. This can give the protein a different shape, so it may not work in the same way. Answer: different bases; different shape.

Making a Protein

ExtendedMaking a protein, step by step

1. The gene coding for the protein remains in the nucleus.

2. mRNA is a copy of the gene.

3. mRNA molecules are made in the nucleus and move to the cytoplasm.

4. The mRNA passes through ribosomes.

5. The ribosome assembles amino acids into protein molecules.

6. The specific sequence of amino acids is determined by the sequence of bases in the mRNA.

Worked example (making a protein): State where mRNA is made, and where it goes to be used. Step 1. mRNA is made in the nucleus, as a copy of a gene. Step 2. It moves to the cytoplasm and passes through ribosomes. Answer: made in the nucleus; used at ribosomes.

Worked example (the ribosome): State what the ribosome does, and what determines the sequence of amino acids. Step 1. The ribosome assembles amino acids into protein molecules. Step 2. The sequence of amino acids is determined by the sequence of bases in the mRNA. Answer: assembles amino acids; mRNA bases decide order.

Gene Expression, Haploid and Diploid

ExtendedNot every gene is used

Most body cells in an organism contain the same genes. However, many genes in a particular cell are not expressed, because the cell only makes the specific proteins it needs.

ExtendedHaploid and diploid nuclei

A haploid nucleus contains a single set of chromosomes. A diploid nucleus contains two sets of chromosomes. In a diploid cell there is a pair of each type of chromosome. In a human diploid cell there are 23 pairs.

Worked example (same genes, different cells): A nerve cell and a muscle cell from the same person have the same genes. Explain why the cells are different. Step 1. Each cell only expresses some of its genes. Step 2. So each cell makes different proteins, and has different features. Answer: different genes expressed.

Mitosis

ExtendedWhat happens in mitosis

Mitosis is nuclear division giving rise to genetically identical cells. The exact replication of chromosomes occurs before mitosis. During mitosis the copies of the chromosomes separate, so the chromosome number is maintained in each daughter cell.

ExtendedThe roles of mitosis

Worked example (chromosome number): A human body cell has 46 chromosomes and divides by mitosis. State the number of chromosomes in each daughter cell. Step 1. The chromosomes are copied, and the copies separate. Step 2. The chromosome number is maintained, so each daughter cell has 46. Answer: 46 chromosomes.

Stem Cells

ExtendedUnspecialised cells that divide

Stem cells are unspecialised cells that divide by mitosis to produce daughter cells. These daughter cells can become specialised for specific functions.

ExtendedLinking ideas

Worked example (explaining stem cells): Describe what a stem cell is and how it can lead to specialised cells. Step 1. A stem cell is an unspecialised cell that divides by mitosis. Step 2. Its daughter cells can become specialised for specific functions. Answer: unspecialised; divide by mitosis; become specialised.

Worked example (same genes in daughter cells): Explain why the daughter cells formed when a stem cell divides have the same genes as each other. Step 1. Stem cells divide by mitosis. Step 2. Mitosis gives genetically identical cells. Answer: mitosis; genetically identical.

Meiosis

ExtendedA reduction division

Meiosis is involved in the production of gametes. It is a reduction division in which the chromosome number is halved from diploid to haploid, resulting in genetically different cells.

ExtendedMitosis and meiosis compared

Worked example (meiosis and gametes): A human diploid cell has 46 chromosomes. State the number of chromosomes in a gamete, and name the type of division. Step 1. Meiosis halves the chromosome number. Step 2. Each gamete is haploid, with 23 chromosomes. Answer: 23; meiosis.

Worked example (restoring the diploid number): Explain why gametes must be haploid, and what happens to the chromosome number at fertilisation. Step 1. Fertilisation is the fusion of the nuclei of two gametes. Step 2. If gametes were diploid the number would double each generation; fusing two haploid nuclei gives a diploid zygote. Answer: two haploid nuclei fuse; zygote diploid.