Organisation of genetic material Each chromosome in the nucleus contains thousands of genes.

Mitosis & Meiosis

How many chromosomes in gametes? If gametes had the same number of chromosomes as body cells, what problem would this cause at fertilization? The embryo would have double the number of chromosomes – 92 instead of 46 in humans. Only one chromosome from each homologous pair in the parent cell is copied to the gametes during cell division. This means that human gametes only have 23 chromosomes. Gametes are said to be diploid cells.

Fertilisation Fertilization is the stage of sexual reproduction when gametes fuse. This is the first step in the creation of a new life. When an egg cell is fertilized, it becomes a zygote. The zygote has a full set of chromosomes. This zygote divides by mitosis many times and becomes an embryo.

Mitosis & Meiosis

FeatureMitosisMeiosis Used for growth & repair Used for gamete production Use for asexual reproduction Cell divides once Cell divides twice 4 new cells formed Two new cells formed New cells contain one set of chromosome New contain two sets of chromosomes New cells show genetic variation New cells genetically identical Copy and complete

Embryonic stem cells Have the potential to become any type of cell! Can differentiate into all cell types

Stem cells Could be used to treat illnesses or replace damaged cells

Becoming specialized Tissue cells continue to divide and differentiate, each time becoming more and more specialized. nerve cells red blood cells stem cell tissue cells cardiac muscle cells

Chromosomes and their genes are made of a molecule called DNA. The backbone is made of a sugar called deoxyribose and phosphates, the cross links are made of bases. It has a double helix structure. DNA stands for deoxyribonucleic acid. Each chromosome is a very long molecule of tightly coiled DNA. What is DNA?

The genetic code Each gene codes for a particular combination of amino acids This makes a specific protein Alleles have different DNA sequences and code for different proteins

Homozygous alleles If the alleles for a characteristic are the same, the organism is said to be homozygous for that characteristic. So the organism has a homozygous genotype. allele for brown eyes allele for blue eyes

Heterozygous alleles The phenotype expressed by heterozygous alleles will depend on which allele is dominant and which allele is recessive. If the alleles for a characteristic are different, the organism is said to be heterozygous for that characteristic. So the organism has a heterozygous genotype. allele for brown eyes allele for blue eyes ?

lung alveoli mucus Cystic Fibrosis A disorder of cell membranes It is caused by a recessive allele Individuals with the condition are homozygous for the faulty allele Those who are heterozygous are carrier and have no symptoms

Polydactyl People with the disorder have extra finger and toes It is caused by a dominant allele They can be heterozygous or homozygous for the faulty allele