Genes, alleles and genomes and chromosomes

What are genes and how are they named?

A section of DNA that codes for a protein

• Named according to loci (location) and function

How are genes different in prokaryotes and eukaryotes?

Eukaryotes: A DNA sequence within linear chromosomes in the nucleus

Prokaryotes: A DNA sequence within a single circular chromosome (and sometimes smaller rings called plasmids) located freely in the cytoplasm in a region called the nucleoid

What are some examples of proteins produced by the code of gene?

• Enzymes: Speed up chemical reactions

• Channel/carrier proteins: Transport across membranes

• Antibodies: Assist in immune response

• Hormones: Chemical messengers in the bloodstream

What is an allele?

A different version of a gene 

• Eukaryotes have 2 copies of each gene (one materal and one paternal)

How is the proteome different to the genome?

Genome: All the genetic information (all the base pairs) found in one complete set of an organisms chromosomes

• Includes genes in the mitochondria and chloroplasts (which have their own DNA)

Proteome: The complete set of proteins expressed by an organism

Describe prokaryotic chromosomes, how are they structured?

• One circular chromosome found in the cytoplasm

• Additional genetic material in plasmids (small rings of DNA)

• Plasmids can be replicated during the cells' life and shared between cells

• This increases genetic diversity (slightly different plasmids in each cell) which increases chances of survival

What is the structure of eukaryotic chromosomes? How are they formed?

• Linear chromosomes found in the nucleus (only visible during cell division)

• DNA segments are wound around histone proteins to form nucleosomes

• Nucleosomes combine with linking DNA and fold to become chromatin fibres

What are nucleosomes?

The basic structural unit of DNA packaging in eukaryotic cells that consists of a segment of DNA wound around a core of histone proteins

How does studying the human genome and understanding gene functon improve detection/diagnosis of human diseases and disorders?

• Studying the human genome helps identify genes linked to diseases

• Understanding gene function helps to determine how mutations to these genes cause disease

• Genetic mutations that cause disease then can be identified

• Genetic tests to detect these variants can be made

• Leading to earlier and more accurate detection and diagnosis

How is the centromere different to the kinetochore? How does the centromere tell you which arm is which?

Centromere: The constricted region of a chromosome that seperates it into a short arm (p) and long arm (q)

• The kinetochore forms on the centromere duirng cell division, being a protein structure for microtubules to attach to

Why is the telomere important for chromosome stability?

Telomere: The region of non-coding DNA located at the end of chromosomes

• Prevent chromosomes from fraying or sticking to each other

• Contain non-coding DNA that provides room for error

• Becomes shorter with each division, causing risk of errors to increase with age

What are the components of a bivalent chromosome?

• 4 chromatids that form 2 pairs of homologous sister chromatids which are connected to form bivalent chromatids

Bivalent chromosomes: A paired, connected set of homologous chromosomes formed during Meiosis I

Sister chromatids: Two identical copies of a single replicated chromosomes joined by a centromere

Why is gene loci important?

Gene loci: The physical location of a gene or DNA sequence on a chromosome

Knowing gene loci allows scientists to determine the exact gene sequence, gene function and variation

What ultimately causes chromosomal abnormalities?

Chromosomal variability: The differences in the structure or number of chromosomes within individuals, populations, or species

• Chromosomal variability results in abnormalities where there are missing, extra, or structurally altered chromosomes

How is a chromatid different to an unduplicated chromosome?

Unduplicated chromosome: A single, linear chromosome consisting of one chromatid that has not yet been replicated

Chromatid: One of the two identical halves of a replicated chromosome

• A chromatid is specificially one half of a replicated chromosome

How are the 4 types of chromosomes classified? What are these types?

• Based on centromere position and length of short (p) and long (q arms

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Metacentric: Central centromere (p + q roughly equal)

Submetacentric: Centromere slightly off centre (p arm shorter)

Acrocentric: Centromere severely offset (p arm very short)

Telocentric: Terminally located centromere (lacking p arm)

What information helps scientists classify chromsomes?

1. Size

2. Banding pattern

3. Centromere positon (4 types)

How are homologous chromosomes identified?

By a matching pair of chromosomes consisting of one maternal and paternal chromosome which contain the same gene sequence and loci

• Centromere positon and length is the same however alleles for each gene can differ

What is the relationship between organism and gene/chromosome number?

• No relationship between chromosome and gene number

EG More chromosomes doesn’t = more genes

• No relationship between organism complexity and chromosome/gene number

Why are autosomes important for survival?

Autosomes: Chromosomes not involved in sex determination

• Humans contain 22 pairs of autosomes which have autosomal genes

Function: Carry genes for body development, physical characteristics and cellular functions

Inheritance: One of each autosome from each parent

How do sex chromosomes trigger sexual development?

Sex chromosomes: Chromosomes that determine sex

• Humans have 1 pair which contain sex-linked genes however they also have genes for other traits

Function: Contain sex-linked genes that trigger female/male sexual development

How does the X chromosome differ from the Y chromosome? Why does everyone have an X chromosome?

Feature	X chromosome	Y chromosome
Size & shape	Large	Small
Gene count	900	55
Role	Sex + essential non-sex traits (vision, blood clotting)	Male sex determination + reproductive development
Genetics	Both genders (XX, XY)	Males only (XY)
Inheritance	Mothers to all children, fathers to daughters	Father to son only

What can be identified from karyotypes?

• Individual's sex

• Different species by chromosome number

• Structural genetic variability such as duplication, inversion or deletion

• Numerical genetic variability such as aneuploidy (trisomy, monosomy)

How do karyotypes help detect chromosomal abnormalities?

Karyotype: A visual representation of chromosomes from the cell nuclei of an individual

• A person's chromosomes are isolated from their cells and stained before a picture is taken of them through a microscope

• Chromosomes are rearranged according to centromere location and size (largest to smallest) to then be compared/identified

How do chromosomal abnormalities occur? What kinds of abnormalities/variation are there?

• When there are missing, extra, or structurally altered chromosomes

Numerical: Abnormal number

EG Aneuploidy (extra or missing), trisomy (3 instead of 2), Monosomy (1 instead of pair)

Structural: Altered structure from breakage or incorrect rejoining

EG Deletion, duplication, translocation (portion broken off and attached to another chromosome)

Why do mitochondria and chloroplasts have their own chromosomes? How do these chromosomes comapre to nuclear chromosomes?

• As these organelles evolved from free-living bacteria and retained some of their original circular chromosomes

Mitochondrial & Chloroplast Chromosomes	Nuclear Chromosomes
Circular DNA	Linear DNA
Located in the organelles	Located in the nucleus
Contain relatively few genes	Contain many genes
Replicate independently via binary fission	Replicate during the S phase of the cell cycle
Code for some organelle proteins	Code for most cell proteins

What is Trisomy 21 and what is it’s effect on development and health?

• Down Syndrome, a chromosomal disorder due to a partial or complete extra copy of Chromosome 21

• Occurs in 3 forms

Health + developmental effects:

• Short stature and distinct facial features

• Mild to moderate physical and cognitive impairment

• Increased risks of heart, respiratory, digestive, hearing, vision and thyroid gland problems

How can Down Syndrome occur?

• In 3 forms: Trisomy 21, mosaicism, tanslocation

1. Trisomy 21 (95% of cases): 3 complete copies in all cells

2. Mosaicism (1-2% of cases): 3 copies in some but not all cells

3. Translocation (3-4% of cases): Partial copy of chromosome 21 attached to another chromosome

What does an extra X chromosome result in and what are the characteristics of a person who has this?

• Klinefleter syndrome (XXY)

• Genetically male as they possess a Y chromosome

• Sterile (undeveloped testes), taller than average height

• Feminisation of secondary sexual characteristics

EG Breast development, reduced facial and body hair

What chromosomal abnormaltiy results in Turner Syndrome? How does this affect the individual?

• A numerical abnormality as they possess only one X chromosome (XO)

• Genetically female as they possess an X chromosome

• Sterile (undeveloped ovaries)

• Symptoms include short stature, wide chest, webbing of neck and heart defects