Chapter 9 Key Knowledge: molecular genetics: genome, gene expression, genes as units of inheritance transmission of heritable characteristics: genes as units of inheritance, eukaryote chromosomes, alleles, meiosis

What is Genetics Genetics is the scientific study of heredity. Heredity is what makes each individual unique. Each of the 100 x cells in our body (except the red blood cells) contains the entire human genome. In the nucleus of every cell is the genetic information “blueprint” to construct the entire individual. The blueprint is contained in the Deoxyribonucleic acid (DNA). Function of DNA: Genetic code for almost every organism. Provide template for protein synthesis.

The cell’s nucleus contains chromosomes made from long DNA molecules. The diagram below shows the relationship between the cell, its nucleus, chromosomes in the nucleus, genes and DNA.

DNA DNA (deoxyribonucleic acid) molecules are large and complex. They carry the genetic code that determines the characteristics of a living thing. Except for identical twins, each person’s DNA is unique. These variations in DNA are responsible for each person’s varying traits.

Nucleotide Bases of DNA DNA is arranged into a double helix. Adenine= A Thymine= T Guanine= G Cytosine= C A always pairs with T C always pairs with G Remember: DNA is double stranded

Nucleotide P A Nucleotide sub-units in DNA are assembled head-to-tail forming a chain. Each nucleotide has a pentose sugar (deoxyribose in DNA and ribose in RNA), a phosphate and an nitrogen- containing base. C N-base Phosphate Sugar

When nucleotides join to form a chain, a bond forms between the sugar of one of the nucleotides and the phosphate group of the next and so on. The chains runs from ‘head-to-tail’. With a phosphate group at the head carbon end (also known as the 5’ “five prime” end) and a carbon in the pentose sugar molecule at the tail end (also known as the 3’ “three prime” end).

3’ and 5’ refer to the position of the carbon in the sugar which bonds to the next nucleotide

P A P A 5’ end 3’ end Weak Hydrogen bonds form between base pairs. T T 1’ 2’ 3’ C C 4’ 5’ 1’ 2’3’ 4’ 5’

5’ end 3’ end 5’ end 3’ end

DNA Molecule Key Features Each DNA molecule consists of two nucleotide chains. The chain runs in opposite directions and are said to be anti-parallel. The sugar-phosphate backbones are on the outside and they coil around each other to form a molecule with constant diameter of 2 nm (2 x m). Each coil is 3.4 nm long

DNA and genes are packaged into chromosomes The length of DNA in one cell reaches about 1.8 m but it fits into a nucleus, which in a human cell is only 6μm wide (6 x m). During mitosis and meiosis DNA is tightly packaged into chromosomes. Chromosomes consist of DNA and therefore carry genes. Chromosomes come in various shapes and sizes

DNA is coiled around small proteins called histones. Where DNA is wrapped around a core of histone proteins it forms a particle called a nucleosome. This gives the DNA strand the appearance of a strand of beads. This arrangement of DNA wrapped around histones serves to package the DNA efficiently and to protect it from enzymatic degradation. When eukaryotes prepare to divide, the DNA condenses into chromosomes. The nucleosomes fold themselves in a regular manner, producing supercoils. When they are highly condensed they can be seen under a light microscope. Packaging DNA into Chromosomes

You must know the different parts of a chromosome: Centromere Chromatids Histones Nucleosome

Packaging of DNA Histone proteins

Chromosomes The DNA in every cell is located in rod like segments called chromosomes. Chromosomes occur in pairs in every cell of our body (somatic cells) except in the sperm and ova (gametes). This is called the diploid chromosome number and is denoted 2n. Sperm and ova have half the number of chromosomes. This is called the haploid number of chromosomes and is denoted n.

How many chromosomes? All nucleated cells of an organism contain a fixed number of chromosomes. The number of chromosomes in somatic cells is a characteristic of a species. The number of sets of chromosomes in a cell is called the ploidy level. Haploid (n) = one set of chromosomes Diploid (2n) = two sets of chromosomes

Chromosome Numbers SpeciesDiploid no.Haploid no. Cattle6030 Pig3819 Sheep5427 Horse6432 Human4623 Chicken7839 Goat6030 Donkey6231

Human Chromosomes Humans have 22 pairs of autosomes and 1 pair of sex chromosomes (either XX or XY). Human karyotype

Chromosomal Abnormalities A karyotype, as you saw in the previous slide, refers to a full set of chromosomes from an individual. This can be compared to a "normal" karyotype for the species via genetic testing. A chromosomal abnormality may be detected or confirmed in this manner. Chromosome abnormalities usually occur when there is an error in cell division following meiosis or mitosis. These abnormalities can be organised into two basic groups: numerical and structural.

Numerical Abnormalities Numerical abnormalities are called Aneuploidy (an abnormal number of chromosomes). These occur when an individual is missing either a chromosome from a pair (Monosomy) or has more than two chromosomes of a pair (Trisomy, Tetrasomy, etc.).

Numerical Abnormalities In humans, an example of a numerical abnormality is Trisomy 21 (Downs Syndrome) (an individual with Down Syndrome has three copies of chromosome 21, rather than two).

Numerical Abnormalities Turner Syndrome is an example of a Monosomy where the individual has only one sex chromosome; an X.

Klinefelter's Syndrome Three sex chromosomes are associated with Klinefelter’s syndrome. XXY These individuals are males with some breast development, little body hair is present, and such people are typically tall, with or without evidence of mental retardation. Males with XXXY, XXXXY, and XXXXXY karyotypes have a more severe form of the syndrome, and mental retardation is expected.

Klinefelter's Syndrome

Structural Abnormalities When the chromosome's structure is altered. This can take several forms: Deletions: A portion of the chromosome is missing or deleted. Duplications: A portion of the chromosome is duplicated, resulting in extra genetic material. Translocations: When a portion of one chromosome is transferred to another chromosome. There are two main types of translocations. In a reciprocal translocation, segments from two different chromosomes have been exchanged. In a Robertsonian translocation, an entire chromosome has attached to another at the centromere - in humans these only occur with chromosomes 13, 14, 15, 21 and 22. Inversions: A portion of the chromosome has broken off, turned upside down and re-attached, therefore part of the genetic material is inverted.

Structural Abnormalities

Chromosomes and Sex Determination The XX/XY system This is the most familiar sex-determination systems, as it is found in human beings, most other mammals, as well as some insects. In the XY sex-determination system, females have two of the same sex chromosomes (XX), while males have two distinct sex chromosomes (XY) The ZW/ZZ system This is found in birds, some insects and other organisms. In this system the females have two different kinds of chromosomes (ZW), and males have two of the same chromosomes (ZZ).

Other Means of Sex Determination Many other sex-determination systems exist. In some species of reptiles, including alligators, crocodiles and some turtles, sex is determined by the temperature at which the egg is incubated.

Meiosis Meiosis results in cells with half the number of chromosomes, 23 instead of the normal 46. These are the gametes (ova and sperm). Human gametes contain 23 single chromosomes. The main features of meiosis are: the chromosomes are copied the cell divides twice, forming four gametes the amount of DNA is halved the DNA is ‘mixed up’ to increase variation

Meiosis

Fertilisation