The nuclei of normal human cells carry ____ pairs of chromosomes for a total of _____. There are noticeable differences in _____ and ______ among the ____ pairs, but within each pair, the two chromosomes appear to match ______.
- shape & size
What is the exception to the rule that two chromosomes in each pair match exactly?
The male's sex chromosomes that designate X and Y
Chromosomes are cellular structures responsible for transmitting _______ _______
Chromosome theory of inheritance
the idea that chromosomes are the carriers of genes
Cell division proceeds through the precise chromosome-parceling mechanisms of _____ and ______
- mitosis (somatic/body cells)
- meiosis (gametes-egg/sperm)
In embryonic cells, the chromosomal threads split length wise in two just ______ cell division, and each of the two newly forming _______ cells receives _______ of every split thread
The kind of nuclear division followed by cell division that results in two daughter cells containing the same number and type of chromosomes as the original parent
In the cells that give rise to male and female gametes, the chromosomes composing each pair become _______, so that the resulting gametes receive ____ chromosome from each chromosome pair
The kind of nuclear division that generates egg or sperm cells containing half the number of chromosomes found in the other cells within the same organism
State 4 facts about chromosomes
- Chromosomes reside in the nucleus
- Chromosomes segregate into daughter cells during cell division
- Chromosomes are responsible for sexual dimorphism in many species including humans
- Chromosomes contain genes
Gametes and other cells that carry only a single set of chromosomes are called _______. Zygotes and other cells carrying two matching sets are called ______.
The number of chromosomes in a normal haploid cell is designated by the shorthand symbol ___; the number of chromosomes in a normal diploid cell is then ____. What would these figures be in humans?
- haploid cells in humans (n) = 23
- diploid cells in humans (2n) = 46
How do we maintain a constant 2n number of chromosomes to be maintained from generation to generation in all individual species
The halving of chromosome number during meiosis and gamete formation, followed by the union of two gametes' chromosomes at fertilization
Why must the chromosomes of every pair segregate from each other during meiosis
So that the haploid gametes will each have one complete set of chromosomes.
After fertilization forms the _____, the process of mitosis then ensures that all cells of the developing individual have ______ ______ chromosome sets
Scientists analyze the chromosomal makeup of a cell when the chromosomes are most visible. When is that?
Metaphase: a specific moment in the cell cycle of growth and division, just before the nucleus divides.
During metaphase, individual chromosomes have ______ and ______ from thin threads into compact rod-like structures. Each chromosome now consists of two identical halves known as _____ _______ attached to each other at a specific location called the _________
- duplicated and condensed
- sister chromatids
In _______ chromosomes, the centromere is more or less in the middle; in _______ chromosomes, the centromere is very close to one end.
Cells in metaphase can be fixed and stained with one of several dyes that produce banding patterns made up of ______ and ______ regions. Chromosomes that match in size , shape and banding are called ________ ________ or _______.
- lighter and darker
- homologous chromosomes or homologs
What do the two homologs of each pair contain? What is the caveat/Where do they differ?
- They contain the same set of genes
- They may carry different alleles for each gene
We use a system of notation for Metaphase chromosme classification. It uses color to indicate degrees of _______ between chromosomes. Thus, sister chromatids, which are _______ ______, appear in what manner?
- identical chromosomes
- They appear in the same shade of the same color
In the system of notation, why would homologous chromosomes appear in the same color but in different shades?
Homologous chromosomes carry the same genes but may vary in the identity of particular alleles
In the system of notation, nonhomologous chromosomes are depicted in different colors. Why?
They carry completely different unrelated sets of genetic information
There are seven distinction of interest in the picture (two are hinted) identify all
To study the chromosomes of a single organism, geneticists produce a karyotype. How?
They arrange micrographs of the stained chromosomes in homologous pairs of decreasing size
In this example of a karyotype, how many matching chromosomes are presented? How many matching vs non-matching pairs? How many chromosomes in those pairs?
- 46 chromosomes in total
- matching chromosomes: 44
- non-matching chromosomes: 2
- matching pairs: 22
- non matching pairs: 1
The 44 chromosomes in matching pairs are known as _______. The two unmatched chromosomes in the male karyotpe are called ____ ______, because they determine?
- sex chromosomes
- The sex of the individual
In many sexually reproducing organisms, two distinct chromosomes, known as the _____ chromosomes, provide the basis of sex determination.
According to our understanding of sex chromosomes, one sex carries two copies of the same chromosome (a matching pair), while the other sex has one of each type of sex chromosome (an unmatched pair). Flesh this out with a human example
For instance, the cells of normal human females contain 23 pairs of chromosomes. The two chromosomes of each pair, including the sex determining X chromosomes, appear to be identical in size and shape. In males, however, there is one unmatched pair of chromosomes, the larger of these is the X; the smaller, the Y
Apart from the difference in sex chromosomes, the two sexes are ___________ at any other pair of chromosomes. Thus, geneticists can designate women as ____ and men as ____ and represent sexual reproduction as a simple cross between ____ and ____
- XX and XY
How do we account for both the mutual of exclusion of sexes and the near 1:1 ratio of females to males, which are hallmark features of sex determination.
- The fact that sex is an inherited trait determined by a pair of sex chromosomes that separate to different cells during gamete formation that is represented by an XX * XY cross (1:1)
Draw and label a Y chromosome and an X chromosome
Several studies have shown that in humans, it is the presence or absence of the ___ that actually makes the difference; that is, any person carrying a ___ chromosome will look like a male.
State an example in which presence of a Y chromosome determines appearance of a child to be male despite the odds
Rare humans with two X and one Y chromosome (XXY) are males displaying certain abnormalities collectively called Klinefelter males. The fact they are always males shows that even two X chromosmes are insufficient for female development in the presence of a Y
4 key characteristics of Klinefelter males
- Mental retardation (sometimes)
A good contrast to Klinefelter males, these are individuals who carry an X and no second sex chromosome (XO). They are always females
4 Key characteristics of Turner females
- Lack secondary sexual characteristics (like pubic hair)
- Short of stature
- Webbed necks (folds of skin between their necks and shoulders)
Why are people who suffer from Turner syndrome invariably women
Even though they only have one X chromosome, they develop as females because they have no Y chromosome
What condition do these individuals have?
- Turner syndrome female
- Klinefelter male
What exactly is the primary determinant of maleness?
- In 1990, researchers discovered that it is not the entire Y chromosome, but rather a single Y-chromosome-specific gene called SRY (sex determining region of Y) that is the primary determinant of maleness
The evidence for SRY came from so-called ___ ______ (define)
sex reversal: the existence of XX males and XY females
In sex-reversed XX males, one of the two X chromosomes is often found to carry a portion of the ____ ________. Although in different XX males, different portions of the ____ _______ are found on the X, one particular gene, _____ is always present.
- Y chromosome
- Y chromosome
Sex reversed XY females, however, always have a Y chromosome lacking a ______ ____ _____. Explain
- functional SRY gene
- The portion of the Y chromosome containing SRY is either replaced by a portion of the X chromosome, or the Y contains a nonfunctional mutant copy of SRY
Determine the condition of these three individuals
- Sex reversed male (XX)
- Sex reversed female (XY)
- Sex reversed female (XY)
SRY is one of about _____ protein-coding genes on the Y chromosome. The two ends of the Y chromosome are called the ____________ ________, because ________ regions are present at the ends of the X chromsome
- 100 protein coding genes
- pseudoautosomal regions (PARs)
- homologous regions
How many PARs are there? Together contain about ____ genes.
- Two PARs (PAR1 and PAR2)
Where can we find copies of the 30 genes in the PARs?
On both the X and the Y chromosome
Most of the Y chromosome, however, is a _____ ______ _____, which includes SRY and also genes required for __________.
- male-specific region
The X chromosome contains about _____ genes, _____ of which have nothing to do with sex. What do those genes have something to do with?
- 1100 genes
- they encode proteins needed by both males and females
Why does having an SRY gene mean you will be male and not having SRY mean that you will be female?
- Approx. six weeks after fertilization, SRY protein activates testes development in XY (or sex-reversed XX) embryos.
- The embryonic testes secrete hormones that trigger the development of male sex organs and prevent the formation of female sex organs.
- In the absence of SRY, an ovary develops instead of a testes, and other female sex organs develop by default
Other species show variation on this XX versus XY chromosomal strategy of sex determination. In fruit flies, for example, although normal females are XX and males are XY, however, what is different?
It is ultimately the number of X chromosomes (and not the presence or absence of the Y) that determine sex.
XXY flies are _______ (why) and XO flies are ______ (why)
- female because they have two X chromosomes
- males because because they only have one X chromosome
State the mechanism of sex determination for each of the following
The sex having two different sex chromosomes is termed the __________ sex (explain). Which fits this description, human males or human females?
- heterogametic sex
- It gives rise to two different types of gametes
- human males (XY) fit the description
What happens when it is a fly vs when human:
The fertilized human egg is a single _____ cell that preserves its genetic identity unchanged through more than ____ generations of cell as it divides again and again to produce a full-term infant ready to be born.
What is the cellular mechanism that preserves genetic information through all these generations of cells?
Mitosis, the nuclear division that apportions chromosomes in equal fashion to two daughter cells
The chromosomes resemble a mass of extremely fine tangled string called ________ surrounded by the _______ ________.
- nuclear envelope
The generic term for any complex of DNA and protein found in a cell's nucleus
Envelope composed of two membranes that surround the nucleus of a eukaryotic cell
Each convoluted thread of chromatin is composed mainly of _____ and ______ (what does each do?)
- DNA: carries genetic info
- protein: serves as a scaffold for packaging and managing that information
Nucleolus (singular): large spherical organelle visible in the nucleus of interphase eukaryotic cells with a light microscope; formed by the nucleolar organizer
Nucleoli play a key role in the manufacture of ________ (function)
ribosomes (organelles that function in protein synthesis)
During the period between cell divisions, the chromatin-laden ______ houses a great deal of invisible activity necessary for the growth and survival of the cell. Name one of those activities
- The accurate duplication of all the chromosomal material
The chromatin condenses into discrete threads, and then each chromosome compacts even further into twin rods clamped together at the _______ that can be identified in karyotype analysis. Each rod in a duo is called a ______
It is an exact duplicate of the other sister chromatid to which it is connected
The repeating pattern of cell growth (as increase in size) followed by division (the splitting of one cell into two) is called the cell cycle
Only a small part of the cell cycle is spent in division or ______; the period between divisions is called ________
What is the correct order of events leading up to M phase?
- G1: interphase, gap before duplication
- S: DNA synthesis and chromsome duplication
- G2: interphase gap before mitosis
Interphase consists of three parts:
G1, Synthesis, and G2
What is the duration of G1?
From the birth of a new cell to the onset of chromosome replication
During G1, genetic material is neither _______ nor ________. During this time, how does the cell achieves most of its growth?
- duplicating nor dividing
- By using the information from its genes to make and assemble the materials it needs to function normally.
What can complicate the duration of G1? (explain)
- short answer: G0
- In rapidly dividing cells of the human embryo, for example, G1 is as short as a few hours. However, mature brain cells become arrested in a resting form of G1 known as G0 and do not normally divide again during a person's lifetime
During duplication (Synthesis), each chromosome doubles to produce _______ ______ chromatids that will become visible when the chromosomes ________ at the beginning of mitosis
- identical sister chromatids
The two sister chromatids remain joined to each other at the _________. This structure considered a single chromosome as long as?
- As long as the connection between the two sister chromatids holds
Why is replication at the S phase critical?
The genetic material must be copied exactly so that both daughter cells receive identical sets of chromosomes
Gap 2 is the interval between _________ and _______. During this phase a cell may grow (usually _____ than during G1). It also synthesizes _____ that are essential to the subsequent steps of _____ itself
- duplication and mitosis
When do microtubules become visible?
The microtubules radiate out into the ______, from a single organizing center known as the _______, usually located near the nuclear envelope.