Intro to Genetics Flashcards
What is Genetics?
Genetics & genomics is the understanding of genes and chromosomes, how they are inherited and what happens when genes and/or inheritance go wrong.
Genetics and genomes cross many boundaries including legal, ethical, religious, social, familial and medical.
Genetics play a part in essentially every medical condition except possibly trauma.
What is a genome?
Genome: complete genetic content of an organism.
Explain DNA and Chromosomes?
The human genome contains 3x10^9 bp of DNA, this DNA is divided up into discrete packages (chromosomes). A chromosome contains a single molecule of DNA. Chromosomes consist of chromatin: DNA and protein (histones) packages to form a coiled structure.
DNA is a double helical structure where each strand has a sugar back bone with nucleic acids that each have a 5 prime and 3 prime ends. (We always read 5’ to 3’)
The human genome is not only existent in the nucleus of cells but also in the mitochondria. It is thought that 80% of our genome is transcribed to produce RNA to regulate gene expression.
What are Genes?
The gene is an evolving concept:
Traditionally the DNA sequence of bases that contains all the information necessary to make a specific peptide or RNA molecule.
It is Proposed that “a union of genomic sequences encoding a coherent set of potentially overlapping functional products”
- This includes structural (coding) as well as regulatory elements.
-Traditionally each gene considered to be localised at a specific point (locus) on the chromosome BUT in reality regulatory (and even coding) elements can be elsewhere along a chromosome.
What are Introns and Exons?
Exons: contain sequences (mostly) coding for specific polypeptide (expressing regions)
Introns: found between exons (intervening regions), may be involved in regulation of gene expression
The structural part of a gene includes both exons and introns (usually transcribed). Typically introns occupy greater regions that the exons.
What are Regulatory sequences?
Regulatory sequences control when, where and how much gene is expressed. Include:
- promoter regions (5', typically upstream of the first exon, often contain specific sequences such as TATA box-5' TATAA3') - enhancer/silencer regions which further modify expression (may be upstream, downstream, in introns or even quite far away from the coding regions)
How does DNA replication occur?
It occurs during the S phase of cell division so that there are two copies during cell division. It is beneficial that the two copies are as close as possible, so semi-conservative replication is used.
There are many enzymes involved- builds only in one direction (5’ to 3’). One complimentary strand made in fragments that are later joined together. Errors can occur in replication and are able to be repaired.
The ends of the strands (telomeres) need to be conserved and so there are enzymes that monitor that process. Telomeres shorten as we get older and may be associated with aging.
Explain the concept of gene expression.
Gene expression can be constitutive (happening all the time). These are products that are made by all cells all of the time and are often called ‘housekeeping genes’; the expression is constant.
The expression can also be regulated and may depend on timing (developmental), place (cell type), amount, or in response to signals and it can be under very tight control.
What is Transcription?
After binding to the promoter, RNA polymerase synthesises an RNA copy of the coding (sense) DNA strand, using non-coding (anti-sense strand) as a template (RNA is complementary to template)
What are some sources and causes of genetic variation?
Sources of Variation:
- genetic factors - environmental factors
Causes of Variation:
- mutations (large and small) which are now referred to as 'variants' - sexual reproduction (meiosis/independent essortment/crossing over & fertilisation (which is random) leads to mixing & matching of different forms of a gene- recombination)
Most natural variation is normal and determines the way we look, behave etc.
Some variation can cause disease/genetic conditions.
How can Variation occur?
Gross chromosomal changes (number of chromosomes, structural rearrangements) are often pathogenic
Copy number variations (CNVs) are a variation in the amount of specific sequences of DNA (ranging from Kbs to Mbs) - majority are benign but some are pathogenic.
Changes to DNA sequences can be benign or pathogenic and can effect coding regions, promoter and other regulatory sequences, slice sites and non coding regions.
- Base pair substitutions: can be silent (synonymous- no change in amino acid, or in non-coding regions with no function), missense (change to amino acid - effect variable), nonsense mutations (can create stop codons - these tend to be pathogenic: truncated proteins or mRNA degradation)
- Deletion and insertions: usually frameshift (typically pathogenic when out of frame)
What are genotypes and phenotypes?
Genotype: genetic constitution of an organism
Phenotype: physical characteristics (morphological, biochemical, molecular) of an organism, or cell
What is an allele?
Allele: an alternative sequence variant of a gene
What is a polymorphism?
Polymorphism: where there are at least two, or more, relatively common alleles of a gene in the population (more than 1%)
- different alleles of genes responsible for ABO blood groups - polymorphisms in medical genetics often used to mean benign variation.
Explain the concept of Dominance in inheritance?
Dominant: a phenotype is dominant if the trait can be seen in individuals who are heterozygous for an allele (only one copy is needed)
Recessive: a phenotype is recessive if the trait is seen only in the individuals who are homozygous for the allele (need both copies)
Co-dominant: a phenotype in which the effects of both alleles are seen in a heterozygote (ie. a heterozygote’s phenotype is distinct from and not intermediate between those two homozygotes such as ABO groups)
Incomplete Dominance: a heterozygote’s phenotype is intermediate between those of two homozygotes (achondroplasia)
