Section 3.0DNA is the Inherited Material Responsible for Variation

Goals for this Section:

1. Describe the relationship among chromosomes, genes, and DNA

2. Explain how DNA stores genetic information

3. Distinguish between cell division during sexual and asexual cell reproduction

4. Investigate the transmission of characteristics from parents to offspring

Goals – Continued…

5. Identify examples of different types of inheritance

6. Identify examples of dominant and recessive characteristics

3.1 – DNA – The Transmitter of Genetic Code

• Variation within a species is necessary for the survival of the species

• The source of this variation is the genetic material that is present in all cells

DNA and Variation

• As you have seen, sexual reproduction involves recombining the genes of two parents in one offspring

• However, variation also exists in those species that reproduce asexually

• This is because of slight changes, or mutations in their DNA

What Produces Variation?

• Essentially, all variation results from slight differences in the genetic code of each individual

• This genetic code is contained in the molecule known as DNA

• Canadian scientist Oswald Avery was one of the first people to suggest that DNA was a molecule that stored information that coded for inherited characteristics

DNA

• DNA is an abbreviation for deoxyribonucleic acid

• All living organisms carry DNA in their cells

• In some cases, the DNA is found in the nucleus, but it also may be found elsewhere in the cell

• DNA was first identified in 1869, but its role was not understood until 1944, when Avery confirmed its role in passing on heritable traits

• However, at this time, the actual structure of DNA was not understood

Watson & Crick

• In 1953, James Watson and Francis Crick determined that DNA has a structure similar to a spiral staircase

http://www.chem.ucsb.edu

The Structure of DNA

• DNA consists of a sugar-phosphate backbone with nitrogen base pairs in-between

http://www.biologycorner.com

The Genetic Code

• The instructions for the characteristics that an organism displays are contained in the genetic code

• The genetic code consists of the four nitrogen base pairs

• The sequence of these nitrogen bases makes up genes that code for proteins made by the cell

Chromosomes

• A chromosome is a single strand of DNA

• A single chromosome can hold numerous genes

• Human cells contain 46 chromosomes, arranged into 23 pairs

• However, other species have different numbers of chromosomes

Genes

• All genes come in pairs• Each pair of genes carries DNA for

the same trait (for example, leg length in a fly)

• These pairs of genes are always found at the same position on a chromosome

• However, the code for each gene in the pair may be different

Alleles• Offspring will inherit genes from both

of their parents

http://www.csulb.edu

Multiple Genes & Alleles

• Often a trait depends not only on a single gene

• As well, there is often more than one or two possible alleles for a single trait

3.2 – Cell Division

• Cell division can be associated with sexual or asexual reproduction

• The type of cellular division involved will affect the number of chromosomes present at the end of the process of cellular division

Mitosis

• Mitosis is the form of cellular division used during asexual reproduction

http://www.bbc.co.uk

Meiosis

• Meiosis produces gametes (sex cells)

http://www.bbc.co.uk

A Comparison of Mitosis and Meiosis

Characteristic Mitosis Meiosis

Original number of chromosomes per cell

Final number of chromosomes per cell

Number of cell divisions

3.3 – Patterns of Inheritance

• For centuries humans have been breeding animals to enhance desired traits

• Over time, purebred strains of various types of animals and crops were developed

Genetics Terms

• Purebred

• Hybrid

More Genetics Terms

• Dominant Trait

• Recessive Trait

Example – Crossing Purebred White & Black Cats

• Assume the black coat colour is dominant to white in cats

• What would be the result of breeding a purebred white cat and a purebred black cat?

Example – Breeding of Hybrids

• Now assume that two of the offspring from the previous example are bred together

• What would be the results of this breeding?

Other Patterns of Inheritance

• Occasionally, some traits show incomplete dominance, where a trait is a mixture of the traits of the parents

• For instance, if a white snapdragon is crossed with a red snapdragon, the offspring will be pink

• In some cases, because multiple alleles may be present, an offspring may not have traits similar to either parent

• Many traits are in fact far too complex to explain using simple patterns of dominant and recessive genes

Environmental Factors

• Although genes for a particular trait may be present, it may not be expressed because of environmental factors

• To be expressed, genes must be turned “on” or “off”, and this often depends on factors in the environment

Examples of Traits with Environmental Factors