IB Biology Year 1 - Genetics

Gene

a heritable factor of DNA that influences a specific characteristic (heritable = you get from parents and give to offspring)

Locus

Location of a gene on a chromosome. The same gene is always found in a specific location. Every human has different genes, but only by a few bases

Allele

a specific/alternate form of a gene. It takes two 2 alleles to control a trait. New alleles would have arisen by a mutation.

Geneome

all of the genetic information of an organism

-46 chromosomes

-21,000 genes

-3.5 billion base pairs

-In humans ~ 1.5% of DNA codes directly for proteins

Human Genome Project

entire base sequence of humans was sequenced

-took 19 years

-Mapped the #, location, sequence, and size of gene

gene mutation

a permanent change in a nucleotide sequence

Gene mutation information

+ new alleles are formed by mutations

+ gene mutations can be beneficial, detrimental, or neutral

+ may be inherited or acquired during a person's lifetime

Types of Gene Mutations

point and frameshift (addition/deletion)

Gene Mutation: Substitution

Where a base is changed for another base

+ affects only a single codon (aka point mutation)

+ results in single nucleotide polymorphism (genomic variant at a single base position)

+ effects can be natural or negative

Effect of Substitution

Degeneracy of the genetic code is the redundancy of the code which has multiple codons coding for the same amino acid

AAG --> lysine

AAA --> lysine

so...substitution may or may not have an effect

Silent Mutation

a mutation that doesn't change the resulting amino acid

Missense Mutations

occur when the DNA change alters a single amino acid in the polypeptide chain

Nonsense Mutations

occur when the DNA change creates a premature STOP codon which shortens the polypeptide

Negative Effects of Substitution: Sickle Cell Missense Mutation

Sickle Cell Anemia: disorder caused by a missense mutation on hemoglobin a protein in red blood cells that carries oxygen

The structure of hemoglobin is altered, making it fibrous and insoluble (red blood cells end up in a crescent moon shape)

Results in blood clotting and anemia (low red blood cell count)

Causes of Mutation

+ errors in DNA replication or repair

+ Mutagen: any agent that causes mutation

- the chance of mutation can be greatly increased by external factors

Types of Mutagen

Physical: mutagenic forms of radiation (such as UV and x-ray)

Chemical mutagens (heavy metals and nitrosamines in tobacco)

Biological: viruses, bacteria

Germline Mutation

occurs in gamete or germ cells. can be passed onto offspring --> can cause inheritable genetic disease

Somatic Mutation

occurs in somatic cells (non-reproductive). Most cancers are somatic mutations which is why those mutations are localized to a particular organ or tissue

Mutations in somatic cells cannot be passed on from one generation to the next

Mutations are a source of genetic variation

+ original source of all genetic variation

+ can be essential for evolution by natural selection

Eukaryotic Chromosomes come in pairs

come in pairs in normal cells (1 complete DNA set from mom and dad)

Homologous Chromosomes

a pair of similar chromosomes that are the same size. They carry the same genes in the same locations but can be different alleles

Karyogram

stained photo of all the homologous chromosomes in order of decreasing length

Karyotype

description of your chromosomes; most human cells contain 23 pairs of chromosomes

22 pairs of chromosomes named autosomes (genes that don't determine sex)

2 sex chromosomes (1 pair) - pair 23

+ XY in males

+ XX in females

Down Syndrome

+ caused by trisomy 21

+ 21st pair is not a pair but a triplet

Diploid Cells

+ nuclei have pairs of all homologous chromosomes (2n)

+ human diploid cells have 46 chromosomes (2n=46)

+ All cells EXCEPT for reproductive cells are diploid (new diploid cells created via mitosis)

Haploid Cells

+ nuclei have one set of chromosomes instead of two (n)

+ Human sperm and egg cells have 46/2 = 23

+ Haploid cells exist for the purpose of sexual reproduction (they are sex cells or gametes)

2 types of cell division

Mitosis (somatic cells)

Meiosis (sex cells)

Cell division produces new "daughter" cells that can form multicellular tissues or entirely new organisms

3 basic stages to cell division

1. DNA Replication (genetic material is duplicated - happens in S phase)

2. Nuclear Division (DNA is separated into two nuclei)

3. Cytokinesis (cytoplasm is divided to produce two individual daughter cells)

Meiosis Overview

ME - O - SHIT (quadruplets)

1 diploid cell becomes 4 haploid cells

Gametogenesis = creation of gametes

Meiosis I

- PMAT + cytokinesis

- separates the homologous pairs (as an X) from each other "reduction division"

- cytokinesis happens and leaves 2 cells with half of the number of chromosomes

Meiosis II

- PMAT + cytokinesis

- sister chromatids get separated from each other: X's become halved

- looks a lot like mitosis

- after 2nd cytokinesis you have 4 cells with chromosomes. These 4 cells are 4 gametes

Meiosis and Genetic Diversity

Meiosis needed for sexual reproduction which increases genetic diversity

Increases Genetic Diversity by 2 processes:

1. crossing over during Prophase I where homologous chromosomes swap DNA fragments

2. random orientation during Metaphase I and II

Non-Disjunction (when meiosis goes wrong)

DNA fails to separate during anaphase like should. Non-disjunction is usually fatal.