genetics (for mr d) Flashcards
chromosomes
- contains a single DNA molecule ( will carry a number of genes) and is associated with proteins
- constantly change appearance during life
structure of chromosome
- extremely long but still able to fit into the nucleus
- DNA is tightly packed into chromosomes
- DNA wraps around a histone, and a nucleosome is formed.
- important in packaging DNA efficiently and protecting it
structure of chromosome (eukaryotic)
- DNA coiled around small proteins known as histones.
- consist of about twice as much protein as DNA.
- before dividing, chromosomes become very condensed. (Prophase: Chromatin)
- nucleosomes fold in a way to produce supercoils
4 types of chromosomes
- metacentric: centromere position in the centre, arms are equal length
- submetacentric: centromere positioned towards one end, arms are uneven length, q arms = 2x length of p arms
- acrocentric: centromere is very close to one end, q arms and very short p arms
- telocentric: centromere at very tip of arms, no p arm visible. not present in humans
centromere
a constriction point which divides the chromosome into two sections
chromosome size
- each DNA molecule contains a number of genes
- code for 40 proteins each
- maybe present to keep genes apart so that enzymes and other molecules can easily interact with them
- chromosomes differ in size because of differences in no. of genes and amount of spacer DNA between genes
spacer DNA
genes on a DNA molecule are separated by regions called spacer DNA
- no function
locus/loci
each gene has a particular position, known as locus, on a specific chromosome
ploidy
no. of chromosome sets a cell carries
Haploid (n)
cells that contain only one set of chromosomes (e.g. gametes) (23)
Diploid (2n)
- cells that contain two sets of chromosomes; one set from each parent (e.g. somatic cells) (46)
- diploid no. of chromosomes of organisms varies widely
Homologous Chromosomes
- identical pairs of chromosomes
- carry the same genes, but not necessarily the same versions of those genes
Sex chromosomes (allosomes)
- sex determination in different organisms varies.
- in humans (and some other organisms) a special pair of chromosomes, determine the sex of the individual
heterogametic
individuals with different sex chromosomes
homogametic
individuals with two similar sex chromosomes
genes
- segments of DNA that code for a protein and can be made up of a small number of bases or a large number of bases.
- determine the inherited characteristics of organisms
- found on chromosomes
- made of unique sequences of DNA
alleles
alleles are different versions of the same gene.
karyotypes
- whole 46-chromosome set containing 23 pairs
- specifically shows an individual set of genetic information
- metaphase 1
somatic cells
all cells in the body of an organism apart from sex cells (gametes)
e.g skin cells, muscle cells
gametes
- specialised sex cells that combine in sexual reproduction
- formed by a type of cell division called meiosis
- this occurs in specialised reproductive organs (gonads)
meiosis
- only occurs in eukaryotes and in gonads
- allows for genetic variability vis genetic recombination and crossing over
- is a reduction division resulting in 4 haploid daughter cells unidentical to each other
- each daughter cell ends up with only half of the chromosomes the parent cell had
- 1 chromosome (2 chromatids) from each pair of chromosomes.
- after meiosis 2 there is still 1 chromosome but with a single chromatid
Meiotic Genetic Recombination (Crossing Over)
- occurs during meiosis as the alleles are rearranged between the four chromosomes in the starting pairs
- Prophase I
- creates daughter cells that are genetically unique
- new set of combined chromosomes are called a tetrad
- genes from non-sister chromatids are swapped to create gametes containing new and unique chromosomes that neither parent has
Genetic Variation in Sexual Reproduction
- an independent assortment of homologous chromosomes during Metaphase 1
- How homologous chromosomes line up next to each other (which one is on the left and which is on the right) is completely random
- so the combination of chromosomes that end up in a gamete is also random
meiotic cell division
Interphase- before meiosis occurs DNA is replicated, and chromosomes duplicate, result in two identical sister chromatids
P1- condense, plump and visible, crossing over occurs, nuclear envelope disappears, spindle fibres begin to form
M1- line up in the middle (equator), arranged independently (indep assortment), spindle fibres attach to each centromere
A1- spindles shorten and pull the chroms apart, reducing no. of chroms (reduction division)
T1- Cleavage furrow is formed, chromosomes decondense, 2 haploid daughter cells nuclei
P2- nuclear envelope breaks down again, SPINDLE FIBRES ARE RECREATED
M2- duplicated chroms line up in middle, spindle fibres attach to centromere
A2 spindles shorten separating sister chromatids
T2 cleavage furrow formed, chromosomes decondense, 4 haploid daughter cells
stained karyotypes
- chromosomes are stained, which shows up characteristic patterns of light and dark bands
- known as G-bands
- reflect differencesn the amounts of A & T bases and G & C bases
- banding patterns are specific and consistent
- can be used to distinguish between chromosomes
- can be used to look for subtle changes in chromosomes structure that may be associated with genetic abnormalities
duplication, inversion, translocation
- occurs when one extra but identical piece of a chromosome is copied and added to the chromosome
- occurs when one piece of a chromosome is broken up, turned upside down, and reattached, inverting the genetic material
- caused by the rearrangement of parts of non-homologous chromosomes
Genotype
- unique sequence of DNA
- two alleles a person has inherited for a particular gene
- set of alleles that an organism has inherited
Phenotype
- The individual’s observable traits
- A persons phenotype is made up of their genetics (genotype) and environmental factors
trisomy 21
down syndrome
turner syndrome
diagnosed when a female only carries one X chromosomes
klinefelter syndrome
diagnosed when a male carries two X chromosomes and a Y chromosomed
deletion/deficiency
loss of a chromosome segment
DNA
- molecules called nucleotides
- are the building blocks of DNA
- individual lines that are sequences together linearly to create a strand of DNA
nucleotide atomic structure
- a phosphate group
- a five-carbon (pentose) sugar
- a nitrogenous base
nucleotides
- DNA is a large macromolecule (large number of atoms)
- contain a phosphate molecule and a deoxyribose sugar which are bound together
- creates the sugar-phosphate backbone
- candy cane-like structure
complementary base pairs
DNA: AT GC
RNA: AU GC
2 Types of Nitrogenous Base Structures
- Purines: (Remember: Pure = Angels and Goodness) which is the Double ring structure
- Pyramidines: (Remember: Pyramids = Tools & Cements) which is the Single ring structure
DNA HELIX
- the way the nucleotides are joined results in a double-stranded DNA molecule known as a double helix
- given the base sequence of one strand, we can determine the sequence of the other because of this base-pair rule
- the two strands of DNA (held together via hydrogen bonds) are said to be antiparallel. One stand is 5’ → 3’ and the other is 3’ → 5’ (’ = Prime)
Homozygous and Heterozygous Alleles
Homozygous: the alleles in a genotype are identical
Heterozygous: the alleles in a genotype are different
Dominant and Recessive Traits
- a trait is dominant if at least one allele for the trait is present
- a trait is recessive if two copies of the allele for the trait are required for the trait to be expressed
Incomplete Dominance:
when heterozygotic organisms’ traits blend alleles to become hybrids
co-dominance
- both alleles are equally expressed
- are both dominant alleles and neither masks the other
- both genetic traits are found in these organisms
Mendel’s second law
states that the alleles of a gene controlling one trait assort independently of alleles of another gene controlling a different trait
Complete dominance
the dominant allele completely masks the effect of the recessive allele in heterozygous conditions.
environmental factors infleunce on phenotype
- organism’s environmental factors have a significant effect on their phenotype
- changes to phenotype due to environment are common amongst animal and plant organisms
epigenetics
different phenotypes that are caused by changes in gene expression rather than genetic code changes
neg. consequences of epigenetics
- E.g. Cancer suppressing genes can be switched off
- found on DNA histone proteins
- switch on/off specific genes
reprogramming
during the formation of gametes during meiosis epigenetic tags are usually erased to ensure the health of the growing embryo
Mendel’s principle of segregation
states that individuals carry pairs of alleles of each gene, which segregate into gametes during meiosis so that each gamete carries one allele of each gene
purpose of test crosses
- find out how many genes control the phenotype
- To determine if an individual showing the dominant pheno is a heterozygote or homozygote
monohybrid crosses
a cross between two individuals involving a single gene
pure-breeding organisms
homozygous in relation to the gene of interest and produce genetically identical offspring
Autosomal Incomplete Dominance
- we see incomplete dominance because the allele controls the production of an enzyme that regulates the development of the red pigment
- thus, with some functional enzyme, the colours blend
autosomal co-dominance
- alleles for coat colour are inherited
- Heterozygotes have both red and white coats, called “Roan”
carriers
heterozygous females are carriers because they are able to pass on the recessive allele, but don’t express the trait themselves
pedigrees
- can be used to follow the inheritance of traits through a family over a number of generations
dihybrid cross
- a cross of two independently assorted traits
- multiple phenotypes possible
- two alleles for each gene (trait)
- involving two gene loci
gene linkage
- the tendency for two or more genes on the same chromosomes to be inherited together
- the closer two genes are on a chromosome, the greater the likelihood that they will be inherited together
- never 100% due to crossing over (recombination), which occurs between non-sister chromatids of homologous chromosomes during meiosis
chiasma
the point where homologous chromosomes join during crossing over
