Chapter 9: Chromosomes and

DNA Replication

Higher Human Biology

Unit1: Cell function and Inheritance

Learning Intentions.• Describe chromosomes as vehicles of

inheritance.• Specifically explain about....1.Genes as regions of chromosomal DNA.2.DNA replication, its importance, and its

relationship to nuclear division.3.The normal chromosome complement,

homologous pairs, autosomes and sex chromosomes.

DNADNA• DNADNA is often called

the blueprint of life..• In simple terms,

DNA contains the instructions for making proteins within the cell.

Why do we study DNADNA?

We study DNA for many reasons, e.g.•Its central importance to all life on Earth.•Medical benefits such as cures for diseases.•Better food crops.

Maurice Wilkins (1916-2004) and

Rosalind Franklin (1920-1958) Used X ray

crystallography to determine chemical structure of DNA

Research formed bases of Watson and Crick’s DNA double helix theory

X-Ray diffraction of DNA

James Watson(1928-) and Francis Crick (1916-2004)

Determined double-helix structure of DNA in 1953.

DNA Structure

Sugar-phosphate backbone

Base

DNA Structure – the instructions to make you is

in your genesy

• A gene is a unit that controls inherited characteristics.

• Genes are held on chromosomes.

• There are 46 chromosomes in the each human body cell. These are arranged as 23 pairs.

• In sex cells (Egg and sperm) there are only 23 chromosomes.

Genes and Chromosomes DNA Structure

Chromosome made of

hundreds of genes

Centromere not made of

genes

Site (locus) of a gene

Chromosome Complement • Each DNA strand is split into chromosomes• All sexually reproducing animals have pairs of

chromosomes. • One set of each pair comes from each parent species.• Each species of plant or animal has a characteristic

number of chromosomes (chromosome complement).– Humans have 23 pairs– Lions have 19 pairs– African wild dogs have 39 pairs– Mosquito have 3 pairs– Some ferns have 500-600 pairs

Autosomes and Sex Chromosomes

• In the human chromosome complement there are 22 pairs of homologous chromosomes called autosomes and play no part in sex determination.

• The last pair- pair number 23- determine the sex of the individual and are known as the Sex Chromosomes.

autosomes Sex

chromosomes

Human Karyotype• 22 pairs of homologous

chromosomes – autosomes.

• 1 pair – sex chromosomes.

• All cells have double set of chromosomes (diploid) except gametes which have 1 set (haploid).

Human Male Karyotype

Remember the karyotype looks different for males and females

Diploid and Haploid• A haploid cell (e.g. a sex cell)

has a single set of chromosomes (one of each type).

• A diploid cell: has a double set of chromosomes (two of each type which form pairs).

• The chromosome complement of a haploid cell is represented by the letter n.

• The diploid by 2n.

Diploid/Haploid numbers in various species

WHAT’s THE POINT CALLER!!!!!! The diploid

number is always double the haploid number!

The Human Life Cycle

• From the time of conception our cells carry out mitosis.This allows us to make more and more cells and become bigger. Our cells also carry out mitosis if we need to repair damaged cells.

Human sperm fertilising an egg. The fertilised eg will now be known as a zygote.

Fertilisation

A baby starts life as a zygote (a fertilised egg). The zygote is one cell containing 46 chromosomes - 23 from the sperm and 23 from the egg. The zygote undergoes mitosis to eventually become a fully grown baby made of many cells. It continues to carry out mitosis as it grows and repairs throughout life.

There are two types of Nuclear Division

•Mitosis – Nucleus of normal body cell divides into 2 daughter nuclei

•Meiosis – Sex cell production. Nucleus of gamete mother cell divides into 4 daughter nuclei with 23 chromosomes.

Mitosis – We will expand on mitosis and meiosis in chapter 10

This is how the body makes new cells when growing or repairing damaged cells

• Mitosis is the process by which the nucleus of a normal body cell (or zygote) divides into 2 daughter nuclei.

• Each of these receives exactly the same number of chromosomes as were present in the original nucleus.

• Mitosis is followed by division of the cytoplasm to form two daughter cells and sometimes the entire process of cell division is loosely referred to as MITOSIS.

In the first stage of mitosis the chromosomes copy all their information and become shorter and thicker. They become known as two chromatids joined in the centre by a centromere.

Chromatids

Centromere

Spindle fibres attach.

Chromatids get pulled apart and become chromosomes.

Two new daughter cells formed each identical to the parent cell.

Stages of Mitosis (again we will learn this properly in Ch10).

Gamete Mother cells

• These are cells found in the testes in men and ovaries in women that are responsible for producing gametes (sex cells).

Meiosis (more in Chapter 10)

• This is the process by which a cell called the gamete mother cell found in the testes in men and ovaries in women undergo division to form 4 new sex cells ( sperm or egg) each containing 23 chromosomes.

46

23

23

23

23

92

The gamete mother cell copies all its information so for a while it appears to have 92 chromosomes. It undergoes two meiotic divisions to produce 4 gametes( sex cells) each with 23 chromosomes in them.

Task: Torrance pg 70 Qu’s 1-4

Why do we need DNA replication?

• When cells divide the daughter cell must receive an exact copy of the genetic material from the parent.

• In order for this to happen the DNA must be replicated or copied.

The importance of DNA replication

• DNA replication ensures that an exact copy of the species’ genetic information is passed from cell to cell during growth and from generation to generation.

• If DNA failed to replicate itself, the process of mitosis and meiosis would come to a halt. DNA replication is therefore essential for the continuation of life.

DNA UNWINDS AND UNZIPS

DNA helicase unzips the 2 strands

1. Replication starts at a specific sequence on the DNA molecule. DNA helicase is an enzyme which unwinds and unzips DNA, breaking the weak hydrogen bonds that join the base pairs, and forming two separate strands each with exposed bases.

Free Nucleotides Bind

Complementary base pairing• DNA helicase completes

the splitting of the strand.• Meanwhile, free nucleotides

that have been activated are attracted to their complementary bases. Weak hydrogen bonds are formed between the bases.

• Each chain acts as a template.

DNA Polymerase – Makes the backbone

• Once in place the activated nucleotides are joined together by DNA polymerase.

• DNA polymerase joins between the sugar of one nucleotide and the phosphate of the next by strong covalent bonds, forming the phosphate-sugar backbone.

Replication FinishedThe result is that there are two DNA molecules, each

with one new synthesised strand of DNA and one strand from the original.

The daughter molecules have a base sequence identical to one another and the original DNA molecule.

The DNA is then rewound by another enzyme.

Summary: see next diagram!The 7 Steps of Replication

1. Original DNA requires replication.2. Replication starts at a specific sequence on the DNA molecule.

DNA helicase unwinds and unzips DNA, breaking the hydrogen bonds that join the base pairs, and forming two separate strands.

3. The new DNA is built up from the four nucleotides (A, C, G and T) that are abundant (free nucleotides) in the nucleoplasm.

4. These nucleotides attach themselves to the bases on the old strands by complementary base pairing. Where there is a T base, only an A nucleotide will bind, and so on.

5. DNA polymerase joins the new nucleotides to each other by strong covalent bonds, forming the phosphate-sugar backbone.

6. A winding enzyme winds the new strands up to form double helices.

7. The two new molecules are identical to the old molecule.

DNA Replication

Extension: Not required, FYI: Semi-conservative

Replication• The model of DNA synthesis (we have

just learned) is also called Semi-conservative replication.

• Discovered by Meselsohn and Stahl (1958).

• 2 new strands each containing 1 of the original strands

• Ensures exact copy of genetic information is passed from cell to cell and from one generation to the next enabling continuity of life.

Task: Torrance pg 71 Qu’s 1-4

Essay – 2002

• Give an account of DNA replication under the following headings:– DNA structure; (7)– DNA replication (3)

(10)