Biology Chapter 5: Genetics and DNA

Genetics and Evolution - Final Revision

• Intended Learning Outcomes:

  • Explain the structure of genetic material and the cellular mechanism of heredity
  • Analyze patterns of inheritance
  • Discuss the theory and mechanisms of evolution
  • Explain how inherited and learnt traits can evolve at the population level

• Assessment Methods:

  • Assignment 1 (50%): Written exam on Genetics (17.12.24)
  • Assignment 2 (50%): Evolution Case Study (1500 words)
  • Written Exam (1 hour): Section A (short answer & multiple choice - 60%), Section B (inheritance patterns - problem solving using Punnett squares - 40%)

What is Genetics?

• Genetics is the study of genes and chromosomes.
• Genetics is the science of heredity, explaining similarities between parents and offspring.
• Genes are units of heredity transferred from parent to offspring, influencing traits.
• Humans have approximately 20,000+ genes.

DNA

• DNA (deoxyribose nucleic acid) is the genetic material inherited from parents.
• DNA is present in the nucleus of all cells, tightly packed as chromosomes.
• DNA controls all chemical changes in cells, e.g., cell type formation (muscle, blood, nerve).
• DNA determines the organism type produced (e.g., buttercup, giraffe, human).

DNA Molecule

• DNA is a large molecule with a long chain of subunits called nucleotides.
• Nucleotides consist of a sugar (deoxyribose), a phosphate group (-PO4), and an organic base (A, C, T, G).

Nucleotides

• Ribose is a sugar similar to glucose, with five carbon atoms (mRNA).
• Deoxyribose is almost identical to ribose but lacks one oxygen atom (DNA).

The Double Helix

• Paired DNA strands coil into a spiral structure called the double helix.
• The sugar-phosphate chains form the backbone of the helix.
• Bases are paired on the interior of the helix.

Replication

• Before cell division, DNA strands unwind and separate.
• Each strand creates a new partner by adding appropriate nucleotides.
• Two identical double-stranded DNA molecules are formed.
• Each nucleus contains identical DNA.

Chromosomes

• Chromosomes are found in the nucleus.
• Chromosomes are paired, with 23 pairs in humans (46 single chromosomes).
• Exceptions include sex cells (egg and sperm).
• Before cell division, chromosomes condense, becoming visible.
• Histones are proteins that help condense DNA into chromatin.
• Chromatin is the unraveled condensed structure of DNA used for packaging in the nucleus.
• Replicated chromosomes are called sister chromatids joined at the centromere.
• Short chromosome arms are 'p' arms; longer ones are 'q' arms.

Chromosomes and Genes

• Chromosomes come in pairs, and genes are also in pairs, inherited from each parent.
• Genes of a pair control the same characteristic.

Mitosis

• Mitosis occurs throughout the animal's life for growth and to repair damaged cells.
• Mitosis results in two identical cells with the same genetic material.
• The number of chromosomes remains the same.

Meiosis

• Meiosis occurs only in sperm and egg cells.
• Meiosis involves two divisions, producing four unique haploid cells from a single cell.
• Genetic information is halved, and each resulting cell is genetically unique from the original cell and other resulting cells.
• In mammals, one female gamete becomes an egg; the other three are polar bodies.

Mendel's Laws

• Mendel was the first biologist to use quantitative mathematics to explain results.
• Mendel predicted genes occur in pairs, and one gene of each pair is present in gametes.

Dihybrid Cross

• If F1 generation is self-pollinated, 4 phenotypes are observed (e.g., tall/purple, tall/white, short/purple, short/white) - 9:3:3:1 ratio

Why Dihybrid Cross Works/Doesn't Work

• Genes on different chromosomes are inherited independently - the dihybrid cross works.
• Genes very close together on the same chromosome are linked; the dihybrid cross often doesn't work.

Incomplete Dominance

• A heterozygote has a phenotype that's a blend of both alleles.
• E.g., Red flower crossed with a white flower produces a pink flower.

Codominance

• Both alleles in a heterozygote contribute fully to the phenotype, creating a mixture of the traits.
• E.g., Roan coat color in horses.

Sex Chromosomes

• Females have two X chromosomes; males have one X and one Y chromosome.
• Traits on the Y chromosome are passed only from father to son.

X-inactivation in Female Mammals

• During development, one X chromosome in each cell is inactivated (Barr body).
• This means different cells in a female will have different active X-chromosomes.
• Example: calico cats.

Sex-linked Punnett Squares

• In cats, the gene for coat color (calico) is codominant.
• Calico females have black and orange patches on a white coat; males can only be black or orange, not calico.

Polygenic Traits

• Polygenic traits are controlled by two or more genes.
• Many phenotypes are possible for polygenic traits, greatly influenced by environment.
• Growth rate in animals or human height is an example.

Epistasis

• The effect of a gene at one locus influences the expression of another gene at a different locus.
• Examples often involve coat color in animals, where one gene may control whether the animal makes pigment, and another gene may control what color the pigment is.

DNA Transcription and Translation

• DNA transcription involves creating mRNA from DNA, happening inside the nucleus.
• DNA translation involves using mRNA to build proteins in the cytoplasm involving ribosomes.

Types of Mutations

• Germline mutations are inherited; somatic mutations are not.
• Chromosome mutations change chromosome structure; point mutations change one nucleotide.
• Point mutations can be silent, missense, or nonsense mutations.

Genetic Engineering

• Inserting genes from one organism into another involves various methods, including using gold particles, viruses, or plasmids, and by using restriction enzymes.

Cloning

• Natural cloning occurs via asexual reproduction in some organisms.
• Artificially, vertebrates can be cloned by transferring the nucleus from a body cell to an egg cell, which has had its nucleus removed.

Stem Cells

• Stem cells can produce all cells in the body (embryonic stem cells).
• Specialized stem cells (e.g., skin, bone marrow) are limited in the cells they can generate.

Any Questions?

• Exam time: 1:00 PM - 2:15 PM)