B12 Reproduction Flashcards
(25 cards)
Difference between mitosis and meiosis
-mitosis is a type of nuclear division that gives rise to cells that are genetically identical
-it is used for growth,repair of damaged tissues,replacement of cells and asexual reproduction
-meiosis is a type of nuclear division that gives rise to cells that are genetically different
-it is used to produce gametes (sex cells)
Sexual production
-process involving fusion of the nuclei of two gametes to form a zygote and the production of offspring that are genetically different from each other:
-the gametes of animals are sperm cells and eggs cells
-the gametes of flowering plants are pollen cells and eggs cells
Gametes
-Gametes are formed by meiosis as they are non identical.
-a normal cells has 46 chromosomes. There ate sets of chromosomes (23 pairs)
-in each pair, one chromosome is from the father and the second set is from mother
-each gamete has 23 chromosomes and they fuse in fertilisation
-the genetic information from each parent is mixed, producing variation in offspring
Asexual production
-asexual production does not involve sex cells or ferilisation
-only one parent is required so there is no fusion of gametes and no mixing of genetic information
-As a result, the offspring are genetically identical to the parent and to each other
-asexual production is defined as a process resulting in genetically identical offspring from one parent
-only mitosis is involved in asexual production
Meiosis
-cells in reproductive organs divide by meiosis to form gametes
-the number of chromosomes must be halved when gametes are formed
-otherwise, there would be double the number of chromosomes after they join at fertilisation in the zygote
-this halving occurs in meiosis and so it is described as a reduction division in which the chromosome number is halved from diploid to haploid,resulting in genetically different cells
-it starts with chromosomes doubling themselves as in mitosis and lining themselves up in centre of cell
-after this happened the cells divide twice so that only one copy of each chromosome passes to each gamete
-We describe gametes as being haploid-having half the normal number of chromosomes
-meiosis produces 4 haploid cells
Process of meiosis
-each chromosome is duplicated (makes identical copies of itself), forming X-shaped chromosomes
-First division: the maternal+paternal chromosome pairs line up along the centre of the cells and are pulled apart so that each new cell only has copy of each chromosome-the sections of DNA are swapped between maternal +paternal chromosome pairs,creating recombinant chromosomes
-second division: the chromosomes line up along the centre of the cell and the arms of chromosomes are pulled apart
-a total of 4 haploid daughter cells will be produced
Importance of meiosis
-produces gametes
-increases genetic variation offspring
-meiosis produces variation by forming new combinations of maternal and paternal chromosomes every time a gamete is made, meaning that when gametes fuse randomly at fertilisation,each offspring will be different from any others
Fertilisation
-Gametes join at fertilisation to restore the normal number of chromosomes
-when the male and female gametes fuse,they become a zygote
-This contains the full number of chromosomes,half of which came from male gamete and half from female gamete
-the zygote divides by mitosis to form 2 new cells,which then continue to divide and after a few days from an embryo
-cell division continues and eventually many of the new cells produced become specialised to perform particular functions and form all the body tissues of the offspring
-the process of cells becoming specialised is known as cell differentiation
The Genome
-The entire set lf the genetic material of an organism is known as a genome.
-Biologists now know the entire human genome
The structure of DNA
-The genetic material in nucleus of a cell is composed of a chemical called DNA
-DNA, or deoxyribonucleic acid, is the molecule that contains the instructions of growth and development of all organisms
-DNA is a polymer made up of two strands forming a double helix
-DNA is contained in structure called chromosomes
-Chromosomes are located in the nucleus of cells
Genes
-A gene is a short length of DNA found on a chromosome
-Each gene codes for a particular sequence of amino acids
-These sequences of amino acids form different types of proteins
-structural proteins,enzymes and hormones
-Genes control our characteristics as they code for proteins that play important roles in what our cells do
The human genome project
-The human genome project was the name of the international, collaborative research effort to determine the DNA sequence of the entire human genome and record every gene in human beings
Important because:
-from medical perspective, as it has already and will continue to improve our understanding of the genes linked with different types lf disease and inherited genetic disorders, as well as helping us find treatments
-also made it possible to study human migration patterns from past, as different populations of humans living in different parts of world have developed very small differences in their genomes
Key terms
Gamete-an organism’s reproductive cell (egg in female and sperm in males) which has half the number of chromosomes (23)
Chromosome-a structure found in the nucleus which is made up of a long strand of DNA
Gene-A short section of DNA that codes for a protein,and therefore contribute to a characteristic. Some characteristics are controlled by a single gene, however most characteristics are the result of many different genes interacting
Alleles-The different forms of the gene-humans have two alleles for each gene as they inherit one from each parent
Dominant allele-only one is needed for it to be expressed and for corresponding phenotype is observed
Recessive allele-two copies are needed for it to be expressed and for the corresponding the phenotype to be observed
Homozygous-when both inherited alleles are the same
Heterozygous-when one of the inherited alleles is dominant and other is recessive
Genotype-the combination of alleles an individual has
-the physical characteristics that are observed in the inidvidual
Genetic inheritance
-Alleles can be dominant or recessive
-A dominant allele only needs to be inherited from one parent in order for the characteristic to show up in the phenotype
-a recessive allele needs to be inherited from both parents in order for characteristic to show up in the phenotype
-If there is only one recessive allele, it will remain hidden and the dominant characteristic will show
-If the two alleles of a gene are the same, they are homozygous
-An individual could be homozygous dominant or homozygous recessive
-if two alleles are different they are heterozygous
-dominant allele is a capital letter and recessive is lowercase
Multiple gene inheritance
-Most characteristics are a result lf multiple genes interacting,rather than a single gene
-Characteristics that are co trolled by more than one gene are described as polygenic
-polygenic characteristics have phenotypes that can show a wide range kf combinations in feature
-polygenic inheritance is difficult to show using genetic diagrams because of the wide range of combinations
Predicting inheritance
-Monohybrid inheritance is the inheritance of characteristics controlled by a single gene
-this can be determined using a genetic diagram known as a Punnet square
-A punnet square shows the possible combinations of alleles that could be produced in offspring
-From this, the ratio lf these combinations can be worked out
How to construct punnet square
-Determine the parental genotypes
-Select a letter that has clearly different lower case for example Aa, Bb,Dd
-Split the alleles for each parent and ad them to the Punnet square around the outside
-Fill in the middle four squared of the Punnet square to work out the possible genetic combinations in the offspring
-You may be asked to comment on the ratio of different allele combinations in the offspring,calculate percentage chances of offspring showing a specific characteristic or just determine the phenotypes pf the offspring
-Completing a Punnet square allows you to predict the probability of different outcomes of monohybrid crosses
Family trees
-A family tree diagrams are usually used to trace the pattern of inheritance of a specific characteristic through generations of family
-This can be used to work out the probability that someone in the family will inherit the genetic disorder
Inherited diseases
-some disorders are inherited
-these disorders are caused by the inheritance of certain slleles
-e.g systic fibrosis and polydactyly are two genetic disorders that can be i herited
Cystic fibrosis
-a genetic disorder of cell membranes
-results in the body producing large amounts of thick,sticky mucus in the air passages
-over time, this may damage the lungs and stop them from working properly
-cystic fibrosis is cause by a recessive allele (f)
This means:
-people are heterozygous won’t be affected by the disorders but are “carriers”
-people must be homozygous recessive in order to have the disorder
-if both parents are carriers, the chance of them producing a child with cystic fibrosis is 25% (1in4)
-if only one parent is carrier there is no chance of producing a child with cystic fubrosis
Polydactyly
-polydactyly is a genetic disorder that causes someone to be born with extra fingers or toes
-Polydactyly is caused by dominant allele (D)
- this means even if only one parent is a carrier, the disorder can be inherited by offspring
Impact of inherited disease-embryo screening
-IVF is the process by which embryos are fertilised in a laboratory and then implanted into mother’s womb
-a cell can be taken from embryo before being implanted and its genes can be analysed
-it is also possible to get DNA from the cell of an embryo that’s already in the womb and analyse its genes in the same way
-Genetic disorders can be inherited during this analysis
Has led to many economic,social and ethical concerns:
-in IVF embryo might be destroyed if alleles causing a genetic disorder are found in its genes
-pregnancy might be prematurely terminated if embryo already in womb is found to have alleles causing a genetic disorder within its genes
Positives and negatives of embryo screening
Arguments for:
-reduces the number of people suffering (ethical)
-treating disorders is very expensive (economic)
-there are many regulations in place to stop it getting out of hand (social)
Against:
-could encourage people to pick characteristics-creating designer babies (ethical)
-it is expensive to carry out screening (economic)
-may promote prejudice as it suggests that those with genetic disorders will not live a full life or are unwanted (social)
-decisions about terminating a pregnancy has to be made (social)
-the procedure can lewd to miscarriage (social)
Screening techniques conducted
Pre-implantation:
Screening during IVF:
Embryos produced by IVF to be tested before they are implanted in the mother,so only babies without disorder are born
Post-implantation:
Amniocentesis:
Carried out at around 15-16 weeks of pregnancy. It involves taking some of the fluid from around the developjng fetus. This fluid contains fetal cells,which can the be used ofr genetic screening
Chorionic villus sampling:
Chorionic villus sampljng of embryonic cells is done at an earlier stage of pregnancy-between 10 and 12 weeks- by taking a small sample of tissue from the developjng placenta. This again provides fetal cells to screen
