DNA, Genes and ChromosomesChapter

9Key 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 GeneticsGenetics is the scientific study of heredity.Heredity is what makes each individual unique.Each of the 100 x 1012 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.

DNADNA (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 DNADNA is arranged into a

double helix.

Adenine= AThymine= TGuanine= GCytosine= C

A always pairs with TC always pairs with G

Remember: DNA is double stranded

NucleotideP

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-basePhosphate

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 FeaturesEach 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 10-9 m).

Each coil is 3.4 nm long

DNA and genes are packaged into chromosomesThe 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 10-6 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

ChromosomesThe 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 chromosomesDiploid (2n) = two sets of chromosomes

Chromosome NumbersSpecies Diploid no.

Haploid no.Cattle 60 30Pig 38 19Sheep 54 27Horse 64 32Human 46 23Chicken 78 39Goat 60 30Donkey 62 31

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

Humankaryotype

Chromosomal AbnormalitiesA 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 AbnormalitiesNumerical 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 AbnormalitiesIn 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 AbnormalitiesTurner 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

Klinefelter's Syndrome

Structural AbnormalitiesWhen 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 DeterminationThe 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 systemThis 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.

MeiosisMeiosis 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 copiedthe cell divides twice, forming four

gametesthe amount of DNA is halvedthe DNA is ‘mixed up’ to increase variation

Meiosis

Fertilisation