Sources of Variation

Concept 1: DNA & Chromosomes

Success Criteria

  • I can describe the general structure of DNA.

  • I can describe chromosome structure, in terms of sister chromatids and homologous pairs.

  • I can describe chromosome number, in terms of haploid and diploid cells.

Vocabulary

Energy

Joule

Transformation

DNA Structure

Success Criteria:
I can describe the general structure of DNA.

Hei mahi

Do now in your book/online notes:

Write everything you know about:

1) What DNA is.

2) Why all organisms have DNA.

DNA is the molecule involved in passing on genetic information to offspring.

Structurally, DNA consists of two strands of alternating sugar and phosphate units around each other in a double helix. Connecting the two strands, like the rungs of a ladder, are hydrogen bonds between the pairs of bases.

There are 4 bases, and they pair up according to the complementary base-pairing rule:

  • A pairs with T

  • G pairs with C

The combination of a base, a deoxyribose sugar, and a phosphate unit is called a deoxyribonucleotide, or nucleotide for short.

Since there are four types of bases, there are four types of DNA nucleotides.

Therefore, DNA is a polymer, because it is made up of many monomers (nucleotides).

Hei mahi

Hei mahi in your books/online:

Draw a homologous pair of chromosomes. On your diagram, label the following:

gene maternal paternal sister chromatids homologous pair centromere

Chromosome Structure & Number

Success Criteria:
I can describe chromosome structure, in terms of sister chromatids and homologous pairs.
I can describe chromosome number, in terms of haploid and diploid cells.

DNA is packaged into chromosomes

Loosely packed DNA is in the form of chromatin. DNA is in this state during the interphase stage of the cell cycle.

Tightly packed DNA is in the form of chromosomes. DNA is in this state during the cell division stage of the cell cycle.

Sister chromatids

Sister chromatids are the identical copies of DNA created after DNA replication.

One sister chromatid was the original copy, the other sister chromatid was the product of DNA replication.

Sister chromatids are joined by a centromere.

Homologous pairs

Chromosomes come in pairs called homologous pairs. Homologous pairs look similar (same length, centromere position) and carry the same genes, but they are NOT identical.

This is because one homologue (chromosome from this pairing) comes from mum, the other homologue comes from dad. Mum and dad’s DNA are NOT the same.

Chromosome number

The number of chromosomes a cell has depends on:

  1. Species (different species have different chromosome numbers). E.g. humans have 46 chromosomes.

  2. Gamete vs somatic cells (gametes have half the number of chromosomes). E.g. human body cells have 46 chromosomes, compared to human sex cells which have 23 chromosomes.

Any cell with two chromosome sets is called a diploid cell, and has a diploid number of chromosomes, abbreviated 2n.

E.g. Human somatic cells have a diploid number of 46 (2n = 46).

Any cell with a single set of chromosomes is called a haploid cell, and has a haploid number of chromosomes, abbreviated n.

E.g. human sperm or eggs cells have a haploid number of 23 (n = 23).

C1 Task 1: Watch this video.

C1 Task 2: Complete some of these worksheet activities.

SciPAD

Page 102 - Chromosomes and Chromatids.

Education Perfect

Task called 'Concept 1: DNA & Chromosomes'.

M2.5 C1. DNA + Chromosomes Worksheet.pdf

Concept 2: Genes & Genetic Variation

Success Criteria

  • I can distinguish between genes and alleles, dominant and recessive, homozygous and heterozygous, and genotype and phenotype.

  • I can describe genetic variation.

  • I can list the sources of genetic variation.

Vocabulary

Energy

Joule

Transformation

Hei mahi

Hei mahi in your books/online:

Draw one diploid cell and one haploid cell.

Genes & Alleles

Success Criteria:
I can distinguish between genes and alleles.

What is a gene?

A gene is a small section of DNA on a chromosome that codes for a particular protein. Each type of chromosome carries a unique set of genes.

All individuals inherit 2 copies of each gene - 1 from mum on the maternal homolog, 1 from dad on the paternal homolog.

The homologs in each homologous pair carries the same gene in the same locus (exact location of a gene on a chromosome). The two copies of any given gene could be the same, or different versions of the same gene.

What is an allele?

Alleles are alternative forms/variations of a gene. E.g. blue and brown are alternative forms of the eye colour gene.

Dominant vs recessive

Alleles may either be dominant or recessive.

A dominant allele is always expressed, even if only one copy is present. Dominant alleles are represented by a capital letter. E.g. Dominant brown eye allele = B

A recessive allele is only expressed if a dominant allele is NOT present. Recessive alleles are represented by a lowercase letter. E.g. Recessive blue eye allele = b

Homozygous vs heterozygous

For any gene, a person may have the same two alleles, known as homozygous (E.g. curly/curly alleles), or two different alleles, known as heterozygous (E.g. curly/straight alleles).

Genotype vs phenotype

The genotype is the combination of alleles, and can be expressed as a phenotype (what we actually observe).

Genetic Variation

Success Criteria:
I can distinguish between genes and alleles.

Genetic variation is the difference in DNA sequences between individuals within a population. These differences in DNA sequence lead to different genotypes, and this can be observed as differences in physical traits (phenotype).

These differences in phenotypes can range from size and colour, to resistance to disease and environmental conditions.

These are the sources of genetic variation:

  • Mutations

      • Gene mutations

      • Chromosomal mutations

(Notice that the three to the right (meiosis, mate selection, and random fertilisation) are all to do with sexual reproduction).

  • Meiosis

      • Independent assortment

      • Crossing over/recombination

      • Segregation

  • Mate selection

  • Random fertilisation

C2 Task 1: Watch this video.

C2 Task 2: Complete some of these worksheet activities.

SciPAD

Page 103 - Genes, Loci, and Alleles.

Page 104 - Variation.

Education Perfect

Task called Task called 'Concept 2: Genes & Genetic Variation'.

M2.5 C2. Genes + Genetic Variation.pdf

Concept 3: Mutagens & Mutations

Success Criteria

  • I can list some mutagens that can cause DNA mutations.

  • I can describe the consequences of gametic and somatic mutations.

  • I can describe how mutations cause genetic variation.

Vocabulary

Energy

Joule

Transformation

Mutations & Inheritance

Success Criteria:
I can describe the consequences of gametic and somatic mutations.
I can describe how mutations cause genetic variation.

A mutation is a permanent change in the base sequence of DNA. This change could be an addition, deletion, or swapping of DNA bases.

Mutations may be:

  • Harmful mutations can negatively change protein function and decrease the chance of survival.

  • Neutral mutations have no effect on protein function.

  • Beneficial mutations can positively change protein function and increase the chance of survival.

Note that only beneficial and harmful mutations change the phenotype, because they alter protein function. Neutral mutations do not alter the phenotype.

Not all mutations are inherited.

Gametic mutations form a new allele in sperm/eggs/ovum/pollen. Gametic mutations can be passed onto the offspring/next generation via sexual reproduction/ fertilisation.

Somatic mutation forms a new allele in a body cell which may affect the individual, but cannot be passed onto the offspring.

Mutations are the ONLY source of NEW alleles and NEW genetic information.

Therefore, gametic mutations increase genetic variation in a species.

Mutagens

Success Criteria:
I can list some mutagens that can cause DNA mutations.

Mutations can suddenly happen as a result of errors during DNA replication, or they can be induced by mutagens such as:

  1. Ionising radiation (e.g. UV, X-rays)

  2. Viruses and microorganisms

  3. Alcohol, fatty and processed foods

  4. Environmental poisons and irritants.

C3 Task 1: Watch this video.

C3 Task 2: Complete some of these worksheet activities.

SciPAD

Page 106 - Mutation as a Cause of Genetic Variation

Education Perfect

Task called 'Mutagens & Mutations'.

M2.5 C3. Mutations + Mutagens Worksheet.docx.pdf

Concept 4: Meiosis & Sexual Reproduction

Success Criteria

  • I can compare the pros and cons of sexual reproduction.

  • I can describe the purpose of meiosis.

  • I can describe the mechanism for generating 4 haploid gametes (meiosis).

Vocabulary

Energy

Joule

Transformation

Sexual Reproduction

Success Criteria:
I can compare the pros and cons of sexual reproduction.

Sexual reproduction is the type of reproduction that involves a life cycle where a haploid gamete (n) combines with another haploid gamete (n) to produce a diploid zygote (2n).

The diploid zygote (2n) goes through mitosis to create all of the diploid somatic cells in the body (2n).

Meiosis

Success Criteria:
I can describe the purpose of meiosis.
I can describe the mechanism for generating 4 haploid gametes (meiosis).

What's the point of meiosis?

The purpose of meiosis is to make haploid gametes.

DNA replication has to happen before Meiosis.

Before meiosis, DNA is replicated during the interphase part of the cell cycle to create sister chromatids.

M2.5 C4. Meiosis Video.mp4

Meiosis involves TWO cell division events

After the chromosomes get replicated during interphase producing sister chromatids, the diploid cell divides TWICE, producing 4 haploid gametes (n).

The 4 sister chromatids in a homologous PAIR, are sorted into 4 haploid daughter cells. This is how meiosis reduces the number of sets of chromosomes from 2 (diploid) to 1 (haploid).

C4 Task 1: Watch this video.

C4 Task 2: Complete some of these worksheet activities.

SciPAD

Page 108 - Meiosis as a Cause of Variation.

Education Perfect

Task called 'Meiosis & Sexual Reproduction'.

M2.5 C4. Meiosis & Sexual Reproduction.pdf

Concept 5: Genetic Variation from Meiosis

Success Criteria

  • I can discuss the mechanisms for generating 4 genetically unique gametes (i.e. crossing over, independent assortment, and segregation).

Vocabulary

Energy

Joule

Transformation

Do now:

Form groups of 2-3 students.

Pick TWO number. (E.g. 5 and Jack)

Take 4 cards of the same number/letter, TWO in black, and TWO in red.

Meiosis has two results:

  1. Reduction of the chromosome number from diploid to haploid.

  2. Production of genetically different gametes (mixing existing alleles into new combinations).

The number of chromosomes is reduced to haploid because the cell divides twice to produce four daughter cells.

Genetically unique gametes are produced due to: independent assortment, crossing over, and segregation.

Crossing Over

Success Criteria:
I can discuss the mechanisms for generating 4 genetically unique gametes (i.e. crossing over, independent assortment, and segregation).

During meiosis I, corresponding segments of non-sister, inward facing homologous chromosomes/chromatids can exchange, producing:

  1. Recombinant chromosomes/chromatids (mix of both parents)

  2. Non-recombinant chromosomes/chromatids (unaltered by crossing over)


New combinations of existing alleles arise from crossing over, resulting in what is called recombination.

The gametes which have chromosome segments from both parents are called recombinants.

Segregation

Success Criteria:
I can discuss the mechanisms for generating 4 genetically unique gametes (i.e. crossing over, independent assortment, and segregation).

The law of segregation states that:
The two alleles for a gene segregate (separate) during gamete formation, and end up in different gametes.

During meiosis II, the two alleles of every gene are segregated (separated) when chromatids separate. This means each gamete only receives one allele for each gene.

Segregation results in all daughter cells being unique because each gamete has:

  • A different combination of alleles from each other.

  • Only has half the chromosomes of the parent cell.

Independent Assortment

Success Criteria:
I can discuss the mechanisms for generating 4 genetically unique gametes (i.e. crossing over, independent assortment, and segregation).

The law of independent assortment states that:
Each pair of alleles segregates independently of all other pairs of alleles on non-homologous chromosomes.

With independent assortment, alleles from non-homologous genes are shuffled, creating more genetic variation in gametes.

Homologous pairs of chromosomes line up at random along the cell equator, independently of other homologous pairs.

Which homologue ends up in which daughter cell is random. This results in gametes with a unique combination of chromosomes and therefore alleles.

Every homologous pair can line up in two ways, so a cell with 3 pairs of chromosomes could produce 2 x 2 x 2 = 8 kinds of gametes.

Humans have 23 homologous pairs of chromosomes, so a human cell could produce 2^23 (over 9 million!) different kinds of gametes by independent assortment.

C5 Task 1: Watch this video.

C5 Task 2: Complete some of these worksheet activities.

SciPAD

Page 109 - Meiosis as a Cause of Variation - Independent Assortment.

Page 110-111 - Meiosis as a Cause of Variation - Crossing Over

Page 112 - Meiosis as a Cause of Variation - Segregation

Education Perfect

Task called 'Genetic Variation from Meiosis'.

M2.5 C5. Genetic Variation from Meiosis.pdf