3 - Genetics

Haploid

Haploid nuclei have one set of chromosomes - n. Sex cells are haploid via meiosis and possess one allele for each trait.

Diploid

Diploid nuclei have two sets of chromosomes - 2n, each. Body cells are diploid via mitosis and consist of two alleles for each trait.

Somatic cells

Any cells in the body other than sex cells

gene

A heritable DNA length found on a chromosome that codes for a specific trait

DNA

a nucleotide sequence of the genetic code of an organism contained within a chromosome

Loci

position of a gene on a chromosome

Allele

Alternative gene form that codes for variation of a characteristic, comes from gene mutations

Gene mutation

change of the nucleotide sequence in a gene affecting protein function and structure. Mutation can be caused by copying errors in DNA replicated or by external exposure.

misssense mutation

change in one amino acid of the protein sequence that creates new beneficial variations of a trait

nonsense mutation

changes a normal codon into a stop codon, thus disrupting normal functions of a trait by shortening the gene sequence

Silent mutation

A mutation that changes a single nucleotide, but does not change the amino acid created, thus no effect on traits

Sickle cell anaemia

A genetic disorder which affects the oxygen carrying haemoglobin/red blood cells due to a change in the 6th codon in the beta chain. This can lead to constant fatigue, or premature death, but a single copy of the gene is may protect against malaria.

Sickle cell anaemia cause

Non-transcribed DNA sequence changes from GAG to GUG at 6th codon position, thus the amino acid is changed from Glu to Val.

Sickle cell anemia consequences

When Glu changes to Val it alters haemoglobin structure causing it to form insoluble fibrous strands, which cannot carry oxygen well. The strands change the circular red blood cell to a sickle shape, which may form clots in the capillaries and block block supply to vital organs. Sickle cells are destroyed faster as well, leading to a low red blood cell count.

Genome

the complete genetic blueprint of an organism in their chromosomes

Human genome numbers

consists of 46 chromosomes, 3 billion base pairs, 21 thousand genes

sickle cell anemia and malaria

individuals that carry the sickle cell allele are found to be well protected against malaria.

The Human Genome Project

international project that sequenced the entire base sequence of humans. It showed that human share the majority of their sequences, and only short nucleotide polymorphisms show changes between us. Mapping - we know the number, location, size, sequences of human genes. Screening - production of gene probes to find people with gene diseases. Medicine - discovery of new proteins, led to improved treatments. Ancestry - insight of origins through comparison with other genomes, evolution and migratory patters of humans.

Oryza Sativa genome

estimated to have 38 thousand genes.

E.coli genome

estimated to have 4200 genes

Chicken genome

estimated to have 17 thousand genes

Water fleas genome

estimated to have 31 thousand genes

Prokaryotic Genetics

Prokaryotes have no nucleus but free genetic material in a nucleoid region in the cytoplasm. Only 1 chromosome from a genophore, and may contain circular plasmid DNA molecules. Containing only a few genes. No additional proteins are used for packaging, thus DNA is "Naked".

Genophore

Circular prokaryote DNA molecule that makes up the prokaryotic chromosome.

Prokaryotes and Plasmids

Sometimes prokaryotes possess plasmids, eukaryotes do not. Plasmids or genetic material can be exchanged in bacteria using their sex pili via bacterial conjugation, thus evolving new traits in generations.

Bacterial Conjugation

exchange of genetic material from one bacterial cell to another in the form of plasmids, it is a horizontal gene transfer as parents do not transfer DNA to a child through sexual reproduction

Plasmids

small circular DNA molecules that can self-replicate and autonomously synthesize proteins, perfect vectors for gene manipulation

horizontal gene transfer

The transfer of genes from one genome to another via plasmid exchange or other mechanisms that does not require transferring parental DNA to a child

Eukaryote Genetics

Eukaryotes have a nucleus that contains chromosomes consisting of linear DNA molecules that are held together by histone proteins. Histone DNA packaging allows for compact, and efficient storage.

Histone

Globular protein that holds Eukaryotic DNA together by spooling around the DNA. Histones form octamers around which DNA is wound to form a nucleosome.

Nucleosome

repeating subunits of chromatin fibers, consisting of DNA bound to 8 histones

Chromosome

a structure of DNA/nucleic acids and protein that carries genetic information

Solenoid

coiling structure of 30nm fibers that loop, compress, and fold around protein scaffolding to form chromatin

Chromatin

mixture of DNA and proteins that form chromosomes, they supercoil during cell division to form visible chromosomes

Deoxyribose Nucleic Acid structure

DNA consists of two long chains of nucleotides twisted into a double helix and joined by hydrogen bonds between the complementary bases A and T or C and G

DNA organization in eukaryotes

Eukaryotic chromosomes are linear DNA molecules that compact during mitosis or meiosis. These chromosomes are divided into 2 arms at the centromere constriction point. Naked DNA binds with 8 histones to form nucleosomes, H1 histones link nucleosomes into strings of chromatosomes, the strings then coil into solenoids that form 30nm fibers, the fibers loop, compress, and fold around protein scaffolding to form chromatin, the chromatin supercoil during cell division to form visible chromosomes

Transcription

process where a DNA sequence in a gene is copied into mRNA, T into A, A into U, C into G, G into C

Translation

Process where mRNA is decoded into an amino acid

Mutagen

agent that can cause or increase chance of mutations, can be physical, biological, chemical

Haemoglobin

The protein that carries oxygen in the red blood cells.

Eukaryotic chromosomes and genes

Eukaryotic chromosomes differ in size and in their centromere position, if we stain them with dye we can see unique banding patterns, specific genes and loci.

Loci position identifications

Numbers or letters denote the chromosome, letter p or q denotes which arm the locus is found on, the last number shows the G band location.

Telomere

repeating nucleotide at the ends of DNA molecules that do not form genes and help prevent the loss of genes

P arm

shorter arm of chromosome

Q arm

long arm of chromosome

Centromere

chromosome region where the two sister chromatids attach

chromatid

one of two identical "sister" parts of a duplicated chromosome, chromatids are held together by centromeres

homologous chromosome pairs

one maternal and one paternal chromosome pair, 4 chromatids in total, separated in gametes via meiosis to prevent chromosome doubling. The same structure, size, banding, centromere position, loci, genes, different alleles

Autosomes

Chromosomes that do not determine sex

karyograph

shows homologous pairs of stained chromosomes arranged in decreasing size and length. Sex and abnormal chromosome number can be concluded.

Down Syndrome (Trisomy 21)

A genetic disorder caused by an extra copy of chromosome 21. It is caused by a non-disjunction event in one parental gamete, and the extra genetic material causes mental and physical delays.

Heterosomes

Sex chromosomes which determine sex. Females possess a homologous pair of 2 copies of X large chromosomes, and Males are not homologous as they possess X large and Y short chromosomes. Thus, the father is always responsible for determining the sex of the offspring.

Y chromosome

shorter sex chromosome found in males that contain genes for male sex characteristics. If Y is absent, female sex organs will develop.

Karyotype analysis

Karyotyping organizes chromosomes via size and type, thus can be used to determine sex or chromosomal abnormalities. Cells are harvested from the fetus before chemically inducing cell division to make chromosomes visible, then they are stained and photographed.

Amniocentesis

A method of prenatal diagnosis where cells within amniotic fluid are collected from a pregnant mother. Done at 16 weeks if there is slight risk of miscarriage, obtained by needle inserted into the uterus.

chorionic villus sampling

sampling of placental tissue for microscopic and chemical examination to detect fetal abnormalities, done at 11 weeks with a higheterm-59r miscarriage risk

Cairns' technique DEFINITION

autoradiography or x-ray imagining technique for visualizing and measuring DNA lengths via radioactive traces.

Cairns' technique PROCESS

Cells are grown in radioactive thymidine 3H-Tsolution. Chromosomes are isolated and fixed to photographic surface which is treated with silver bromide. Radiation converts silver ions into insoluble grains which are visible via electron microscope when film is developed.

Chromosome number

feature of a particular species. Organisms with different diploid numbers usually cannot interbreed, as they cannot form homologous pairs in zygotes. When different species do interbreed, offspring's are infertile as they cannot form functional gametes.

Zygote

a diploid cell that is produced via fusion of two haploid gametes, zygotes carry two alleles of each gene, the alleles can be different or the same

Gamete

haploid sex cells, thus contain only one allele of each gene, they unite with another of the opposite sex in sexual reproduction to form a zygote.

Diploid chromosome numbers in Humans vs Others

Chromosome number does not explain genetic complexity. Homo sapiens have 46, Parascaris equorum/Roundworm have 4, Oryza sativa/rice have 24, Pan troglodytes/chimpanzees have 48, Canis familiaris/dog have 78.

Genome size numbers in Humans vs Others

Genome size does not explain genetic complexity. Largest genome is Paris Japonica/Canopy - 150 billion bp, smallest genome is Carsonella Ruddi - 160 thousand bp. T2 phage/virus - 170 thousand bp, E.Coli. - 4.6 million bp, Drosophilia Melanogaster/fruit fly - 130 million bp, Homo sapiens - 3.2 billion bp.

Genome size trends

Viruses and bacteria tend to have small genomes, prokaryotes tend to have smaller genomes than eukaryotes, plant genome sizes vary due to the capacity to self-fertilize and become polyploid.

Mitosis

Mitosis divides replicated DNA from the S-Phase in interphase and creates 2 genetically identical diploid cells from a single parent cell.

Meiosis

Meiosis divides diploid nuclei creating 4 haploid nuclei. In Meiosis 1 - homologous pairs separate and chromosome numbers are halved. In Meiosis 2 sister chromatids that were produced via DNA replication in interphase are separated.

Sister chromatids before meiosis

Before meiosis DNA was replicated during the S-phase in interphase, thus chromatids are identical. Sister chromatids are separated during Meiosis 2. Chromatids are held together by centromeres.

Bivalent

homologous chromosomes undergo synapsis to form a bivalent 4 chromatids in total

Crossing over

In P1 homologous chromosomes form bivalents which are held together at chiasmata points where crossing over of genetic material between non-sister chromatids can occur to form new gene combinations. If crossing over occurs then the 4 haploid daughter cells will be genetically different.

Meiosis 1

In Meiosis 1 homologous pairs are separated and chromosome number is halved. P1 - Chromatins rearrange into chromosomes and pair up, the nuclear membrane dissolves, non-sister chromatids trade genes at chiasmatas forming bivalents. M1 - Spindle fibers from opposing centrosomes attach centromeres on the bivalents and align them. A1 - Spindle fibers contract, shorten and split the bivalent, sister chromatids move to opposite poles. T1 - Chromosomes decondense back into chromatins, membrane may reform, cleavage furrow forms, and two haploid cells produced in cytokinesis

Centrosomes

organizing centers that contract the spindle fibers during cell division

Random assortment

Before homologous pairs are separated they are randomly positioned. Thus, in M1 - homologous pairs line up in the middle as bivalents in one of two arrangements, maternal copyleft and paternal right, or vice versa.

Sexual life cycle

Halving the chromosome numbers allows for a sexual life cycle with fusion of gametes. Sexually reproducing organisms are diploid, having one maternal and paternal chromosome. Thus, for reproduction they make haploid gametes. Fertilizing the two haploid gametes an egg and sperm forms a diploid zygote that grows via mitosis. If the chromosomes were not halved in the gametes chromosomes would double in generations.

genetic variation

crossing over, random orientation, gamete fusion from different parents, and random fertilization promote genetic variation.

Genetic variation and crossing over

Crossing over exchanges DNA segments between homologous pairs in P1, occuring between non-sister chromatids at chiasmatas. In recombination, all chromatids that make up the bivalent will be genetically different.

Genetic variation and random fertilization

A diploid zygote forms via fusion of two haploid gametes. Then it divides via mitosis and differentiates into an embryo. As meiosis produces genetically different gametes, random fertilization by egg and sperm will produce different zygotes. Identical twins are formed after fertilization, by fission of the zygote into two separate cells.

Recombinants

Chromatids that consist of DNA combinations from both homologous chromosomes, thus the offspring will have unique gene combinations.

Genetic variation and random orientation

In M1 homologous chromosomes align in the middle in random orientation to opposite poles. Thus, different maternal or paternal chromosomes can be inherited when bivalents separate in A1.

Total chromosome number combinations

In gametes the total number combination is 2^n, N - haploid number of chromosomes. In humans 2^23, by random orientation and if crossing over occurs, combinations are immeasurable.

Non-disjunction

Non-disjunction is an Error in meiosis where homologous chromosomes fail to separate thus gametes end up missing or having an extra chromosome, causing chromosomal abnormalities like down syndrome. Chromosomes fail to separate either in A1 as homologous pairs do not separate affecting 4 daughter cells, or sister chromatids fail to separate in A2, affecting 2 daughter cells.

spindle fibers

Protein structures which move the chromosomes during cell division.

Meiosis 2

In Meiosis 2 sister chromatids that could be genetically different are separated. P2 - Chromatin rearrange into chromosomes, nuclear membranes dissolve, spindle forms and centrosomes move to opposite poles, perpendicular than before. M2 - Spindle fibers line chromosomes up. A2 - Spindle fibers contract, shorten and separate sister chromatids moving them to opposite poles. T2 - Chromosomes decondense back into chromatin, nuclear membrane reforms, four haploid cells were created via meiosis. May genetically distinct if crossing over occurred in P1.

Mendel's Experiment

Mendel crossed purebred pea plants that had different traits, collected and grew the seeds and noted the traits. Then he did that again with the offspring.

Mendel Findings

Discrete factors - genes determine traits in organisms, organisms hold two types of discrete factors, each gamete contains one version of each factor, one factor is dominant over another

Law of segregation

during gamete formation via meiosis, the two alleles at a gene locus separate from each other, thus a gamete carries only one allele

law of independent assortment

alleles separate independently to one another the results are random

principle of dominance

Recessive alleles are masked by dominant alleles, co-dominance can occur

Dominant Alleles

an allele that is always expressed and masks recessive alleles

Co-dominant alleles

two dominant but different alleles that show joint-expression

Recessive alleles

alleles that only show their effects when both alleles are the same

Inheritance of ABO blood groups

A and B alleles are

dominant, and O is recessive. A and B alleles can be co-dominant, thus AB.

Red blood cell alleles

AB alleles modify the structure of antigens; O alleles do not; Incompatible blood transfusion leads to surface antigens and opposing antibodies to clump and result in hemolysis.

Genetic diseases cause

Genetic diseases are caused by recessive alleles of autosomal genes, but dominant or codominant alleles can too, and gene mutations where normal cell functions are disrupted

autosomal recessive disease

two alleles or homozygosity must be present to develop trait

autosomal dominant disease

one allele or heterozygosity must be present to develop trait, homozygous and heterozygous people develop full range of symptoms

Co-dominant genetic disease

one faulty allele and one normal allele in a heterozygote, where the normal makes the symptoms milder

cystic fibrosis

two copies of a recessive allele caused by CFTR gene mutation on chromosome 7. Causes excessive and thick mucus, clogs airways, and can lead to respiratory failure.

Huntington's disease

one dominant allele caused by HTT gene mutation on chromosome 4. The HTT gene amplifies the amount of repeats and eventually causes the HTT protein misfolds. Causes uncontrollable movements and degeneration of the nervous system, dementia.

Genetic disease rarity

Many gene diseases have been found, but they are rare as alleles that do not help in survival are not passed on. Recessive conditions are more common.

Sex-Linked genes

a gene carried on the X or Y chromosome. Sex linked inheritance differs from autosomal as males do not have XX chromosomes.