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Chromosome
An organised structure of DNA and protein that is found in cells, a single piece of coilded DNA cointaining many genes, regulatory elements and other nucleotide sequences
Chromatid
One amoung two identical copies of DNA making up a replicated chromosome, which are jointed at their centromeres, for the process of cell division
Homologous Chromosomes
Matching chromosome pairs, the chromosomes that have the same genetic information (sister chromosomes)
Sex Cell, Gamate
Reproductive cell containing 23 unpaired chromosomes
Either egg or sperm (mature gamates)
Fertilisation
The fusion of gamates to produce a new organism --> involves sperm fusing with an ovum, which eventually leads to the development of an embryo
Sexual Reproduction
Processes that pass a combination of genetic material to offspring --> involves meiosis and fertilisation
Diploid
Full set of chromosomes (46)
Cell formed by the combination of two haploid gamates
Haploid
Half set of chromosomes (23)
Product of meiosis
Describe the main events occuring during meiosis. Differences between first and second meiotic divisions.
First division:
Diploid cell splits into 2 haploid cells
Homologous pairs are pulled apart in anaphase
Paired chromosomes align side by side wich another matching pair, forming a tetrad
Paired chromosomes sperate into two daughter cells (Haploid cells) - the sister chromatids do not split yet
Describe the main events occuring during meiosis. Differences between first and second meiotic divisions.
Second division:
Haploid cells split again leaving you with 4 haploid cells. Preserves the number of chromosomes (silimar to mitosis)
Nucleic envelope disappears, pairs line up in middle of cell, seperated to poles by spindle fibres, sister chromatids pulled apart, nuclear envelopes reform, cytokinesis divides cells
Four haploid cells are produced.
Meiosis:
Interphase
Chromosomes exist as chromotins
There are two homologous (code for the same gene) pairs (one from mum, one from dad). During S phase, DNA replicates and is connected by centromere.
Meiosis:
Prophase 1
Nuclear envelope starts disappearing
Centrosomes start facilitating the development of spindle fibres, start moving to opposite poles
Chromatin starts forming into chromatids (chromosome becomes more tightly bound)
Homologous pairs line up - can have genetic recombination (crossing over)
Meiosis:
Metaphase 1
Nuclear envelope is now gone, centrosomes have moved to opposite poles.
Spindle fibres have been generated. Microspindles attach to kinetocores on centromeres and start aligning chromosomes
Meiosis:
Anaphase 1
Homologous pairs seperate to opposite ends of cell (chromatids stay with sister chromatids)
Meiosis:
Telophase 1
Once pairs have moved to seperate ends of cell, microtubules start to disintergrate
Cytokinesis starts happening, nuclear envelope is reforming.
Completely divides by the end of telophase one leaving 2 haploid cells
Continues on to Meiosis 2 - may stay in interphase for some time
Meiosis:
Prophase 2
Nuclear envelope starts disintergrating
centrosomes start to move to opposite poles
(happening in 2 haploid cells that formed in telophase one at the same time)
Meiosis:
Metaphase 2
chromosomes line up
centrosomes are at opposite poles
Meiosis:
Anaphase 2:
Chromatid pairs seperate and get pulled to opposite sides of cell (like anaphase in mitosis) The two chromatids that split are now called sister chromosomes
Meiosis:
Telophase 2:
Cell elongates
Chromosomes are at opposite sides of cell
Nuclear envelope starts reforming
clevage is occuring (cytokinesis)

Left with 4 haploid daughter cells.
Independant assortment of chromosomes
Occurs during anaphase 1 of meiosis
Takes place when maternal and paternal chromosomes are being randomly seperated into two daughter cells.
Effect of independant assortment of chromosomes on the potential for genetic variations
The chromosomes from male and female gamates contain different genetic information which determines a persons genotype. The random segregation of chromosomes during meiotic cell division determines which genes will be inherited.
There are many different combinateions which could occur during the random assortment of gamate chromosomes - this is why genetic variation occurs.
Events that occur at crossing over of chromatids
Occurs in prophase 1 of meiosis.
Process where exchenge of genetic material between chromatids takes place.
the site of crossing over is the chiasma.
Crossing over form recombinant chromosomes, increasing genetic variation of daughter cells
Linked genes
There are so many genes that there is more than one pair on each chromosome.
If genes are found on the same chromosome for different characteristics they are called linked genes and will usually be inherited together
Explain the effect of crossing over in prodicing genetic variation
Crossing over increases the variation of genetic material in daughter cells from parent cells
Differentiate between Mitosis and Meiosis:
Mitosis
  • Occurs in the body (somatic cells)
  • DNA replicates and then divides once
  • Daughter cells are diploid (full set of chromosomes)
  • Daughter cells are genetically identical to parent cells (2 daughter cells)
  • Daughter cells grow and mature during interphase
Differentiate between Mitosis and Meiosis:
Meiosis
  • Occurs in germ cells (sex cells)
  • DNA replicates and then divides twice
  • Daughter cells are haploid (half set of chromosomes)'
  • Daughter cells have different genetic make up (4 daughter cells)
  • Daughter cells develop into gametes
  • Gametes must fuse during fertilisation to become diploid cells then embryological development promotes mitosis
Gene
Section of DNA which codes for protein production (one protein)
Genotype
The genes present in the body's cells which provide a basis for physical and functional characteristics
Phenotype
The observable physical and functional characteristics of an organism.
Can be modified by environment
Gene Pair
The two chomosomes that have intructions for the same characteristic, homologous chromosomes
Gene Locus
The specific location of a perticular gene on a chromosome
Allele
The forms of genes that affect the same characteristic byt produce different expression of that characteristic.
Usually denoted by A-dominant characteristic and a-recessive characteristic
Mutation
Permanent changes to the nucleotide sequence of the genetic material of an organism
Gene pool
The complete set of alleles in a population
Explain the role of genes in the cell's ability to produce proteins and non-protein substances
Role of genes in the cells ability to produce proteins and non-protein substances is due to hormones.
Where they send only specific receptors to target cells so that particular cell's function is altered
Describe the relationship between the appearance of a particular characteristic in an individual and the production of proteins in their genes
The genes provide a code for specific proteins to be synthesised. The sequence of nucleotides in DNA determines which proteins will be produced - these proteins influence an individual's characteristics
Relate alterations in DNA code (mutations) to the production of new alleles in a population
  • Mutation is a permament change in the DNA sequence of a gene
  • Alleles are the forms of the gene which effect characteristics which are expressed in the individual
  • If mutations are 'minor' and just create new nucleotides they generate the same gene, but with a different allele
  • Leads to new variations being produced within the population
Describe some of the causes of mutaion in terms of:
Changes in genetic material
Caused by errors in coping
  • Substitution in a base of DNA causes and incorrect amino acid in the polypeptide chain
  • bases being added or deleted can cause all the amino acids to be incorrect or result in premature termination of the polypeptide chain
  • These mutations can be passed on to future generations if individual survives to reproduce
Describe some of the causes of mutation in terms of:
The effects of mutagenic materials
Such as radiation and chemicals
Describe the chromosomal abnormaility associated with Down's Syndrome (trisomy 21)
Mutation in which the total chromosome of an organism is one more than the usual diploid number (47 chromosomes)
People with Down's syndrome have an extra chromosome 21, resulting in 47 chromosomes total
This chromosomal abnormality results in delayed mental development, heart defects, increased susceptibility to respiratory infection, 15 times greater chance of developing leukaemia and reduced life expectancy
Homozygous
Alleles of a particular gene locus are identical on both members of a pair of homologous chromosomes (aa, AA)
Hetrozygous
Alleles for a particular gene locus are not identical on both members of a pair of homologous chromosomes (Aa)
Dominant
An allele that expresses its phenotype when hetrozygous with a recessive allele (A)
Recessive
An allele that does not express its phenotype when hetrozygous with a dominant allele (a)
Autosomal
Genes that carry on non-sex chromosomes
Linkage
Gene that carries on the 23rd (sex) chromosome. aka X-linkage
Sex linked inheritance results from genes carried on either male or female chromosomes
Differentiate between complete dominance and co-dominance between alleles
Complete dominance occurs when an allele is expressed in the phenotype of the hetrozygote.
Co-dominance occurs when both alleles of a gene are fully and simultaneously expressed in the phenotype of a hetrozygote (e.g AB blood group is co-dominant)
Continuous variation
When combined effects of many genes at different loci are involved in determining trait (e.g. height)
Diversity of a characteristic in a population fur to a polygenic trait
Polygenic trait
Characteristics which result from the combined expression of many genes
Phenotype of polygenic traits is determined by two or more gene pairs, each with many alleles
Polygenic traits are usually expressed by the majority of a population
e.g. weight, height, eye colour, intelligence, skin colour
Define the role of X and Y chromosomes
  • Determine the gender
  • If a Y is present in the 23rd chromosome, the offspring will be male (XY)
  • In no Y present, the offspring will be female (XX)
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