Ib Biology Topic 3

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Heritable

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passed on from parent to offspring

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Characteristic

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genetic trait, e.g. hair colour or blood type

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Gene locus

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the specific position of a gene on a chromosome

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Allele

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one of the possible alternatives of a gene, occupying a specific position on a chromosome, that controls the same trait

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Alleles (basic)

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different forms of the same gene that differ by one or only a few bases

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Mutation

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a random, rare change in genetic material

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What does base substitution mutation result in?

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A single base being changed

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When do base substitution mutations occur?

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When DNA replication doesn't work correctly

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What does a base substitution mutation do?

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It causes a different amino acid to be placed in the growing polypeptide chain

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Are base substitution mutations harmful, neutral or beneficial?

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They can be all three

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Genome

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the whole genetic information of an organism

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Beginning and completion years for the Human Genome Project

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Began 1990, complete 2003

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Shape and number of chromosomes in prokaryotes

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Circular and one. Nucleoid region of a bacterial cell contains a single, long, continuous circular thread of DNA

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Plasmid

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An extra ring of DNA, not connected to the main chromosome. It replicates independent of the chromosomal DNA.

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Purpose of plasmids

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Not required in normal circumstances, but may help the cell adapt to unusual circumstances

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What are eukaryote chromosomes composed of?

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Linear DNA molecules associated with histone proteins

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Homologous chromosomes

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a chromosome pair with genes for the same characteristics at corresponding loci

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Diploid

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having two homologous copies of each chromosome (2n), usually one from the mother and one from the father

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Are most cells in the human body diploid or haploid?

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Diploid

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Haploid

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One chromosome from each pair

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Which cells are haploid?

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Sex cells/gametes, aka eggs and sperm

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What does "n" stand for?

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The number of chromosomes in a haploid cell, i.e. 23

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Is the number of chromosomes a species has indicative of the species complexity?

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No

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Diploid number of chromosomes for Homo sapiens

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46

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Diploid number of chromosomes for Pan troglodytes (chimpanzees)

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48

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Diploid number of chromosomes for Canis familiaris (domestic dog)

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78

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Diploid number of chromosomes for Oryza sativa (rice)

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24

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Diploid number of chromosomes for Parascaris equorum (roundworm)

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2

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Karyogram

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an image which shows the chromosome number of an organism in homologous pairs of decreasing length

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Process of creating a karyogram

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Chromosomes are stained during mitosis and a micrograph is taken of them

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Sex chromosomes

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the pair of chromosomes responsible for determining the sex of an individual

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What is different about the 23rd pair of chromosomes in humans?

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They're the sex chromosomes, and are the only pair where the two chromosomes can be very different in size and shape

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Autosome

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a non-sex chromosome

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Who was Gregor Mendel?

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An Austrian monk who is known as the father of genetics

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What did Gregor Mendel do?

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Basically he crossed a tonne of pea plants, had a lot of reliability because of doing lots of repeats, discovered the 3:1 dominant:recessive ratio, and the Mendelian Laws of Inheritance

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What happens when gametes fuse?

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The gametes (n) fuse to form a zygote (2n), so two copies of each gene exist in the diploid zygote. Copies of the gene can have the same alleles (homozygous) or different alleles (heterozygous)

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Dominant

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expresses its trait irrespective of other alleles (is expressed in both homozygous and heterozygous)

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Recessive

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only expresses its trait in the absence of the dominant allele (only expressed in homozygous)

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Co-dominant

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Pairs of alleles that both affect the phenotype (expressed in heterozygous)

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Examples of autosomal recessive diseases

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Albinism, Cystic Fibrosis, Phenylketonuria (PKU), Sickle Cell Disease/Sickle Cell Trait, Thalassemia

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Sex-linked gene

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a gene located on an X or Y chromosome

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Are genetic diseases common or rare?

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Genetic diseases are very rare. Even the most frequently occurring autosomal recessive diseases only affect about 1 in 2000 people

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How many gametes produced by meiosis?

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Four

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Does DNA replication occur before meiosis?

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Yes, in the S-phase of interphase.

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What happens in Prophase I?

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1. Chromosomes become more visible as the DNA becomes more compact

2. Homologous chromosomes are attracted to each other and pair up

3. Crossing over occurs

4. Spindle fibres made from microtubules form

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What happens in Metaphase I?

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1. The homologous chromosomes line up across the cell's equator by random orientation

2. The nuclear membrane disintegrates

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What happens in Anaphase I?

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Spindle fibres from the poles attach to chromosomes and pull them to opposite poles of the cell

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What happens in Telophase I?

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1. Spindles and spindle fibres disintegrate

2.The chromosomes uncoil and new nuclear membranes form

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What happens at the end of meiosis I?

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Cytokinesis (splitting into two separate cells)

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What happens in Prophase II?

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1. DNA condenses into visible chromosomes again

2. New meiotic spindle fibres are produced

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What happens in Metaphase II?

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1. Nuclear membranes disintegrate

2. The individual chromosomes line up along the equator of each cell in random orientation

3. Spindle fibres from opposite poles attach to each of the sister chromatids at the centromeres

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What happens in Anaphase II?

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1. Centromeres of each chromosome split, releasing each sister chromatid as an individual chromosome

2. The spindle fibres pull individuals chromatids to opposite ends of the cell.

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What happens in Telophase II?

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1. Chromosomes unwind their strands of DNA

2. Nuclear envelopes form around each of the four haploid cells, preparing them for cytokinesis