What is DNA?

Chromosomes and their genes are made of a molecule called DNA.

DNA molecules carry the code that controls what your cells are made of and what they do.

DNA stands for deoxyribosenucleic

acid.

Each chromosome is a very long molecule of tightly coiled DNA.

Structure of DNA

DNA and base pairs

Base pairs hold the two strands of the DNA helix together.The rules for base pairing are…

There are millions of base pairs in a DNA molecule that always follow these rules.

A always pairs with T

C always pairs with G

Amazingly, it is the sequence of bases along a DNA molecule that forms the genetic code - it’s that simple!

What is mitosis?

Each new cell has a full set of chromosomes and is identical to the original cell.

Mitosis begins with a single cell. How many chromosomes does this cell contain?

First the cell makes a copy of each chromosome…

original cell

cell division

2 new cells

…then it divides.

What is mitosis?

Each new cell can keep on dividing by mitosis.

Mitosis makes new cells for growth and repair in all living things. That’s how you get from one cell to 50 billion!

Mitosis is also called copying division. What does this mean?

Mitosis animation

What is meiosis?

Meiosis begins with a single cell.

First the cell makes a copy of each chromosome.

Now, there are two stages of cell division…

original cell

firstdivision

seconddivision

4 new cells

How many chromosomes do the new cells have compared to the original?

What is meiosis?

Meiosis produces four new sex cells with half the original number of chromosomes.

Mitosis is also called reduction division. What does this mean?

In animals, meiosis produces ova in females and sperm cells in males.

In plants the gametes produced by meiosis are pollen grains and ovules.

Meiosis animation

How many chromosomes?

Human sex cells have a set of 23 unpaired chromosomes - that’s half the usual number.

Sex cells in animals and plants follow the same rules.

They have half the usual number of chromosomes.

They have a complete set of single chromosomes.

Why is this important for fertilization and reproduction?

Sex cells and cell division

Sex cells have half the usual number of chromosomes.

Why aren’t new sex cells produced by mitosis?

Sex cells have to be produced by a special type of cell division. This is why there are two types of cell division.

Sex cells are produced by meiosis.

In which organs does meiosis take place?

Inherited characteristics

What makes this baby human? What determines its gender?

In all living things, characteristics are passed on in the chromosomes that offspring inherit from their parents.

So all human characteristics, including gender, must be something to do with chromosomes.

Where are chromosomes found in cells?

What are chromosomes?

Chromosomes are long strands of genetic information located in the nuclei of cells.

Chromosomes are most visible during cell division when they replicate and look like this…

You will see chromosomes represented both ways.

Homologous chromosomes

In most cells chromosomes are matched in pairs based on their size and shape.

Matching pairs of chromosomes are called homologous chromosomes.

Where do homologous chromosomes come from?

Each pair of homologous chromosomes contains one chromosome that has been inherited from each parent.

chromosome from female parent

chromosome from male parent

homologouschromosomes

Human chromosomes

You inherit half your chromosomes from your mother and half from your father.

How many chromosomes do you inherit from each parent?

In human body cells there are a total of 46 chromosomes.

How many pairs of homologous chromosomes are there in human body cells?

23 pairs of chromosomes

23 unpaired chromosomes

What is meiosis?

Meiosis begins with a single cell.

First the cell makes a copy of each chromosome.

Meiosis produces four new sex cells with half the original number of chromosomes.

In animals, meiosis produces ova in females and sperm cells in males.

In plants the gametes produced by meiosis are pollen grains and ovules.

Meiosis is also called reduction division. What does this mean?

Mitosis or meiosis?

There are two types of sex chromosome with names that are very easy to remember!

X and Y chromosomes

Females have two X chromosomes, one from each parent.

X chromosome Y chromosome

XY

XXMales have one X chromosome and one Y chromosome. Which parent does a baby boy inherit its Y chromosome from?

© Boardworks Ltd 2004

Homologous chromosomes

In all living things, characteristics are passed on in the chromosomes that offspring inherit from their parents.

Chromosomes are matched in pairs that containone chromosome inherited from each parent.

So are the genes in a matching pair of chromosomes exactly the same?

chromosome from female parent

chromosome from male parent

Each chromosome may have a different version of a gene.

Different versions of a gene, that code for different versions of a characteristic, are called alleles.

The chromosomes in a matching pair contain the same type of genes that code for the same characteristics.

Different versions of genes

version for yellow petals

version for red petals

gene for petal colour

gene for petal colour

Pairs of alleles – homozygous

If the alleles in a matching pair are the same, they are called homozygous alleles.

allele for yellow petals

allele for yellow petals

allele for red petals

allele for red petals

What colour are the flowers with these homozygous pairs of alleles?

(Click twice on each bud to reveal the flower;

click again to close them.)

Pairs of alleles – heterozygous

If the alleles in a matching pair are different, they are called heterozygous alleles.

Which characteristic is expressed if alleles are different?

allele for yellow petals

allele for red petals

Some alleles are dominant to other forms of a gene and will always be expressed.

Which is the dominant allele in this heterozygous pair?

Which is the recessive allele in this heterozygous pair?

(Click twice on the bud to reveal the flower;

click again to close it.)

Representing alleles

Letters are used to represent different alleles.

The allele pair for each characteristic is called the genotype.

What colour are flowers with the genotype Rr?

A dominant allele is always a capital letter.

A recessive allele is always the corresponding small letter.

allele for red petals R

allele for yellow petals r

=

=

Genotypes and phenotypes

The allele pair for each characteristic is called the genotype.

The physical expression of an allele pair is the phenotype.

What are the phenotypes of these genotypes?

genotype:

phenotype:

RR Rrrr

(Click twice on each bud to reveal the flowers;

click again to close them.)

Genetic jargon

1. Section of DNA that codes for a particular trait or characteristic.

2. A different form of a gene that codes for a different version of a characteristic.

3. A description of the pair of alleles present for a characteristic.

4. The physical expression of the alleles.

5. Pair of alleles that produce a characteristic are the same, e.g. HH.

6. Pair of alleles that produce a characteristic are different, e.g. Hh.

8. An allele that will only be expressed when both alleles are of this type e.g. hh.

7. An allele that will always be expressed even when there is only one of these alleles present, represented by a capital letter.

Genetic jargon

Genetic jargon

gene

allele

phenotype

genotype

1. Section of DNA that codes for a particular trait or characteristic.

2. A different form of a gene that codes for a different version of a characteristic.

3. A description of the pair of alleles present for a characteristic.

4. The physical expression of the alleles.

What do these genetic terms mean?

homozygous

heterozygous

recessive

dominant

5. Pair of alleles that produce a characteristic are the same, e.g. HH.

6. Pair of alleles that produce a characteristic are different, e.g. Hh.

8. An allele that will only be expressed when both alleles are of this type e.g. hh.

7. An allele that will always be expressed even when there is only one of these alleles present, represented by a capital letter.

Genetic jargon

What do these genetic terms mean?