英文医学精品课件：Nucleus and Chromatin
细胞生物学课件英文版-基因和染色体分析
PCR is used in forensic analysis
VNTRs: Variable Number Tandem Repeats
STRs: short tandem repeats •4 to 40 repeats in various positions (loci)
“DNA fingerprint”
– Conserves energy
• Don’t make what you don’t need!
• Multicellular
– Allows specialization of cell types
– Allows growth and development
• How do we know that
Fig 10-23, 10-24
How can we change DNA information?
Make site specific changes
Transform into a strain that has no
DNA repair
Fig 10-34
Reporter Genes
Fig 10-29
organism • Mechanisms to determine the sequence of DNA • Ways to alter and change DNA sequences • Ways to put DNA back into organisms
• And if we can do all that: use stem cells to alter/repair damage
• What are some of the specialized proteins produced in these cell types?
细胞生物学课件英文版-DNA和染色体
Fig. 5-22
Final levels of chromatin packing: heterochromatin & chromatids
Euchromatin (less condense)
Chromatin organization within a nucleus
Fig. 5-19
Nucleolus: rRNA transcription!
Special parts of chromosomes:
Fig. 5-16
How is the intervening DNA packed & packaged?
strain.
What part of the cell contains genetic material?
Fig. 5-4
What part of the cell contains genetic material?
T2 virus (bacteriophage): contains only DNA and protein Injects the genetic material into the host cell (E. coli)
Fig. 5-7
inside
outside
Fig. 5-6
Genome: complete set of DNA/genetic information
Human cells: ~two meters, ~3.2 x 109 nucleotides, ~25,000 genes
mRNA
医学英语词汇ppt课件
Nursing Process Health Assessment Community Health Nursing Nursing Care of the Oldest Old Promoting Coping in Hospitalized Children Midwifery and Perinatal Care Principles and Methods of Traditional Chinese Medicine in Nursing
2、部分医学词汇中英之间音或形相近 音近： penicillin:盘尼西林 Gua Sha: 刮痧 形近： milk tooth:乳齿 chicken breast:鸡胸
3、大量医学英语词汇包含词根、词缀 Electrocardiogram：心电图
Autotransplantation:自体移植
The respiratory system 6、肺lung： 词根：pneumon(o)-,pulmo(n)(o)例词：pneumonia肺炎；pulmonary肺的 7、胸chest： 词根：thorac(o)例词：thoracentesis胸腔穿刺术；
Practice
Respiratory distress syndrome Pneumonia Lung ventilation Metabolic compensation dyspnea 呼吸窘迫综合征 肺炎 肺通气 代谢代偿 呼吸困难
The circulatory system
The heart ,blood vessels, lymph vessels and lymph nodes all make up the system whereby blood is pumped to all the body tissues, bringing with it food ,oxygen and other substances ,and carrying away waste materials.
医学专业英语课件_15
Biography---James Watson
Born in Chicago, April 6, 1928 His father‘s ancestors were originally of English descent His mother‘s father was Scottish-born taylor married to a daughter of Irish immigrants who arrived in the US about 1840 Spent entire boyhood in Chicago
Phenotype: The observable characters of an organism; the result of the way the genes are expressed
Genetic defect: Pathological changes that occur by duplication, deletion or rearrangement of DNA
1962, The Nobel Prize in Physiology or Medicine Died at 2004
Biography---Paul Berg
Born on June 30, 1926, New York
Gained early recognition/influence when he delineated the key steps in which DNA produces proteins Best known for his development of a technique for splicing together DNA from different types of organisms.
【医学PPT课件大全】第二章 相关的生物学基础知识
1、核小体的组成
146bpDNA ＋
组蛋白八聚体→ 核小体的核心颗 粒
146bp的核心 DNA
直径约10nm
组蛋白八聚体
146bp的核心DNA在组蛋白八聚体上盘绕1.8圈
核小体的结构
* 组蛋白H1把核小体“封锁”起来 Histone H1
核小体的结构
Histone H1
Chromatosome 166 bp, 2 superhelical turn
具有裸露的质粒DNA
细胞骨架
无
细胞增殖方式 无性分裂
真核细胞
有 有 DNA与组蛋白及非组蛋白 结合，存在于染色体中， 染色体位于细胞核中 有 有
线粒体DNA、叶绿体DNA
有 以有丝分裂为主
2、原核细胞和真核细胞在 分子生物学上的区别
A、同时连续 阶段性 区域性
B、膜分化
C、信息量
动物细胞
植 物 细胞
三、生物膜系统
1、生物膜系统（biomembrane system） 2、质膜（plasma membrane） 3、内膜系统（internal membrane）
第二节 染色体（Chromosome ）
一、染色体的基本特征
A. 稳定性 B. 连续性 C. 控制性 D. 变异性
二、原核细胞染色体
200 bp
Histone H1
Nucleosome
连接 DNA
<10 to > 100 bp 平均 55 bp
The 10 nm fiber is a continuous strong of nucleosomes.
每个核小体单位 包括：200bp左 右的DNA、一 个组蛋白八聚体、 一分子H1
Chapter1英文版生化
CHAPTER 1 DNA and Chromosome1.1 DNA is genetic material1.2 DNA Structure1.3 Chromosome1.4 Genome and Gene1.1 DNA is Genetic Material*1.1.1 DNA is the genetic material of bacteria 1928, Fred Griffith ~16 years1944,Oswald Avery Colin MacLeod Maclyn McCarty~1/104•转化（Transformation ）•转化因子（Transforming principle )1/100001952Alfred Hershey Martha Chase1.1.2 The genetic material of phage T2 is DNA1.2 DNA Structure1.2.1 A nucleic acid (DNA/RNA) is a polynucleotideDNA: dA, dG, dC, dT2′-H (deoxyribose sugar)RNA: A, G, C, U2′-OH (ribose sugar)Direction: 5′-p 3′-OH1.2.2 DNA is a double helixKey term: Complementary Base Pair A-T, C-GDNA double helix ——a research legacyYesterday’s research –today’s legacyB-form DNA DNA double helix1.2.3 Supercoiling structure of DNAPositive supercoilingThe DNA is twisted around itself in thesame sense as the two strands within thedouble helix,causing the DNA strandsto wound around one another moretightly, so that there are more base pairsper turn.Negative supercoilingThe DNA is twisted around itself in theopposite sense as the two strands withinthe double helix, causing the DNAstrands to be twisted around one anotherless tightly, so there are fewer base pairsper turn.Type I topoisomerasesType II topoisomerases链接反应Summary of topoisomerasesA type I topoisomerase is an enzyme that changes the topology of DNA by nicking and resealing one strand of DNA without requiring any input of energy.A type II topoisomerase is an enzyme that changes the topology of DNA by nicking and resealing both strands of DNA. The hydrolysis of ATP may be used to drive the reactions.E. coli DNA Gyrase is a type II topoisomerase that can introduce negative supercoils.1.3 Chromosome1.3.1 Viral genomes are packaged into their coatsNote: Viruses havegenomes of double-stranded or single-stranded DNA orRNA.1.3.2 Prokaryotic chromosome, a supercoiled nucleoidThe nucleoid is the region in a prokaryotic cell that contains the genome. The DNA is bound to proteins and is not enclosed by a membrane.1.3.3 Eukaryotic chromatin and chromosomesChromatin describes thestate of nuclear DNA andits associated proteinsduring the interphase(between mitoses) of theeukaryotic cell cycle.Nucleosome is the subunit of chromatinreleased by digestion ofchromatin with micrococcalnuclease.Nucleosome and HistonesHistones are extremely richin the positive charged aminoacids lysine and arginine.Nucleosome:Histones: H1×1, H2A ×2,H2B ×2, H3 ×2,H4 ×2DNA: ~200bp DNACore Particle:Histones: H2A ×2, H2B ×2,H3 ×2, H4 ×2DNA: 146bp DNACore particle of nucleosomeThe N-terminal tails of histones are modifiedmethylation, acetylation,and phosphorylation.The sites that are modified areconcentrated in the N-terminaltails. These modificationshave important effects on thestructure of chromatin and incontrolling gene expression.Chromatin RemodelingChromatin structure is dynamicInduced change in chromatin structureReplication, gene expressionHistone modificationAcetylation by histone acetyltransferase(HAT)Methylation by methyl transferasesPhosphorylation by kinasesDNA modificationsMethylation of cytosine (5-methyl C) in CpG islandsHeterochromatin1928, staining differences in nuclei lead to terms euchromatin and heterochromatinHeterochromatinDark stainingGenetically inactiveFew genes, those present repressedReplicates late in S phaseCentromeres and telomeres are heterochromaticPortion of Y and inactivated X chromosomesPosition effect when genes are translocated to location adjacent toheterochromatinDNA CondensationThe path of nucleosomes in the chromatin fiberChromatin proteins are subdivided into histones and nonhistones. Nonhistone proteins:e.g. high mobility group proteins (HMG protein);and other DNA binding proteins.Human ChromosomesA chromosome is a discrete unit ofthe genome carrying many genes.Each chromosome consists of avery long molecule of duplex DNAand an approximately equal mass ofproteins. It is visible as amorphological entity only duringcell division.ChromosomeHuman Chromosomes1.4 Genome and GeneThe genome is the complete set of sequences in the genetic material of an organism. It includes the sequence of each chromosome plus any DNA in organelles.One gene : one enzyme hypothesis is the basis of modern genetics.One gene : one polypeptide chain: a gene is a stretch of DNA coding for a single polypeptide chain.A gene is a stretch of DNA that functions as a unit to give rise to an RNA or protein product; it may include exons and introns .An exon is any segment of a gene that is represented in the mature RNA product.An intron is any segment of a gene that is transcribed, but removed from the transcript in RNA splicing.1.4.1 GeneSome DNA sequences code for more than one protein1.4.2 Genome Size and Gene NumberRepetitive DNAUnique (single copy):Most of the genes, a small percentage of the DNASlightly repetitive (1-10 copies): Some genesMiddle repetitive (10 to several hundred copies):e.g. rRNA genesHighly repetitive (several hundred to several million copies):e.g. satellite DNABuoyant Density AnalysisSatellite DNAsSome DNAs, when subjected to buoyant density analysis, give a density profile that is not a single peak as expected for random variations in the AT/CG content from one region of DNA to anotherSome densities are over representedSatellite DNAsShort sequence tandem repeatsHeterochromatic regions, mostly associated with centromeresThe human genome has fewer genes than expected•Only 1% of the human genomeconsists of coding regions.•The exons comprise ~5% of eachgene, so genes (exons plus introns)comprise ~25% of the genome.•The human genome has 30,000-40,000 genes.•~60% of human genes arealternatively spliced.•Up to 80% of the alternativesplices change protein sequence, sothe proteome has ~50,000-60,000members.1.4.3 Individual genomes showextensive variation•Single nucleotide polymorphism(SNP)describes a polymorphism (variation in sequence between individuals) caused by a change in a single nucleotide. This is responsible for most of the genetic variation between individuals.•Restriction fragment length polymorphism(RFLP) refers to inherited differences in sites for restriction enzymes (for example, caused by base changes in the target site) that result in differences in the lengths of the fragments produced by cleavage with the relevant restriction enzyme.Restriction fragment length polymorphism(RFLP)Short Tandem Repeat (STR) MarkersMultiplex STR AnalysisT w o d i f f e r e n t i n d i v i d u a l sAmpFlSTR ®SGM Plus™kitgenes1.4.4 ExtranuclearExtranuclear genes reside outside the nucleus in organellessuch as mitochondria and chloroplasts.Human GenomeCharacteristics of Prokaryotic GenomeThe prokaryotic chromosome exits as a nucleoid without the membrane around.The structure of prokaryotic chromosome is much more simple than eukaryotic chromosome.Many prokaryotic gene are polycistronic(多顺反子），and some genes are overlapping (重叠）.Prokaryotic chromosomal DNA do not contain highly repeated sequences.Prokaryotic genes do not contain the noncoding introns.Characteristics of Eukaryotic GenomeThe eukaryotic cell contains a membranous nucleus that includes the chromosome as carriers of genetic information.The chromosomal DNA of eukaryotes is packaged into a highly regular nucleoprotein complex called chromatin.A typical eukaryotic gene is monocistronic(单顺反子）. Many DNA segments contain highly repeated sequences. Eukaryotic genes are split with the noncoding introns.断裂基因（interrupted gene）内含子(intron)、外显子(exon)Summary1. 中心法则(Central dogma)；2. 复制(Replication)，转录(Transcription)，翻译(Translation) ，逆转录(Reverse transcription)；3. 拟核(Nucleoid)，染色体(Chromosome)，染色质(Chromatin)；4. 核小体(Nucleosome)及其核心颗粒，组蛋白(Histones)及其修饰；5. 基因(Gene)，外显子(Exon)，内含子(Intron)；6. 单核苷酸多样性(SNP)，限制性片段长度多样性(RFLP)；STR DNA与RNA的区别；DNA是遗传物质的证据；DNA作为遗传物质的特征；Watson&Crick双螺旋模型；拓扑异构酶I和II的作用机制；原核和真核基因组的主要差异。
最新(医学专业英语)医学英语术语学ppt课件
2 .理解词义，记忆单词
Masto-
乳房固定术
Gastro-
胃固定术
-pexy ColpohysteroHepato-
阴道子宫固定术 肝固定术
Cysto
一、学习医学英语构词法的重要性 二、英语发展简史 三、医学英语术语的基本特征 四、医学英语术语构词规则 五、医学英语术语的读音规则 六、医学英语构词法的常用词缀
一、学习医学英语构词法的重要性
pneumonoultramicroscopicsilicovolcanoconiosis
Pneumono-ultra-microscopic-silico-volcano-coniosis
✓ 几乎在所有希腊语词根后加-o-； ✓ 拉丁语词素后面可以加o或i; ✓ 当结合型与另一个以元音字母开头的词根或后缀 结合时，
结合型中的元音字母常省略；
✓ 并列关系的词素之间； o或i为词素所不可缺少的部分时， 可以保留。
① saved
Gastro -pexy gastropexy Meno -pause menopause
cocci ganglia
apex cervix
Apices Cervices
（四）字母的互通
• ae→e，oe→e，ou→u，ph→f，y→i • haemorrhage→hemorrhage oedema→edema • oulitis [u’laitis]→ulitis [ju’laitis] • sulphonamide→sulfonamide
细胞结构英文版27张幻灯片
NAD+
cytoplasm
fermentation
Electron carrier produced in glycolysis
fermentation lactate
when oxygen is available
Contains digestive enzymes to digest food in protists or destroy aging organelles
Endomembrane System
Rough Endoplasmic
Reticulum
Smooth Endoplasmic
Reticulum
Structure
Cilia and Flagella
Function(s)
• Move substances across cell surface in lungs, fallopian tubes
• Cell movement in sperm
Connections Between Cells
Citric acid cycle
Electron Transport chain
oxygen Outside cell
mitochondrion
2 ATP
2 ATP
32 ATP
Energy Yield from Glucose
Without Oxygen, Fermentation Occurs
Structure
Mitochondrion
Function
Captures energy from organic molecules, producing ATP
医学细胞生物学 细胞核(nucleus)精品PPT课件
核 2. 核基质结合蛋白：与核基质结合的酶、细胞调控蛋
骨
白等 。
架
3. RNA：维持核骨架的三维结构。
DNA： 不是骨架结构成分，只是功能的结合。
核
骨
mRNA聚合酶
架 mRNA
的
结
构
核骨架通道
（二）功 能
1.参与DNA的复制； 2.参与基因的表达 ； 3.参与前体RNA的加工修饰 及定向运输；
4.参与细胞分裂中DNA的包 装和染色体的构建；
5.与细胞分化相关。
分化程度越高，细胞的RNA合成能力越旺盛， 核骨架越发达。
第三节 染色质和染色体
❖ 染色质(chromatin)： 间期细胞核中，能被碱性染料染色的物质。呈细
丝状、形态不规则，弥散于细胞核内。
❖ 染色体(chromosome)： 有丝分裂时，染色质组装、凝集成光镜下可见的
棒状或点状结构。
二、染色质的种类
（一）常染色质
指在细胞间期呈松散状、染色较浅，且有转 录和复制活性的染色质。
（二）异染色质
指在细胞间期呈凝缩状 态，染色较深，缺乏转录活性 的染色质。
1、结构异染色质
在所有细胞核内都呈异固缩的染色质。多 位于着丝粒区、端粒、次缢痕区及染色体短
臂的某些节段。
2、兼性异染色质
在不同细胞类型或不 同发育时期出现的异染色 质区。
核孔复合体-“捕鱼笼” 模式 图
胞质纤维
四、核膜的主要功能
1.核膜的区域化作用，使转录和翻译在时空上 分离;
2.控制着细胞核与细胞质的物质交换;
（1）自由扩散
无机离子，如K+、 Ca2+ 、 Mg2+、 Cl-离子，水分子等； 分子量小于5KD的单糖、氨基酸和核苷酸。
医学细胞生物学细胞核nucleus-医学精品
3、非组蛋白(nonhistone) ：
是染色体上与特异DNA序列结合的蛋白质，所以又 称序列特异性DNA结合蛋白。为一类带负电的酸性蛋白。
有种属和组织特异性，在整个细胞周期都能进行 合成。
功能：
（1）与特异性的DNA序列相识别、结合，启动并促进基 因的转录和复制，调控基因的表达；
（2）促进核小体结构中DNA分子的进一步的盘曲、折叠 参与染色体结构的构建。
两种核转运受体：
1）核转运受体（入核素）
2）能识别RNA或RNA结合蛋白的受体（出核素）
3.在细胞分裂中参与染色质 的定位与分离
4.核膜参与合成生物大分子
外层核膜与粗面内质网相似， 因此可以参与细胞蛋白质的合成。 如，核周间隙分布的多种结构蛋 白及酶。
第二节 核纤层与核骨架
一、核纤层
真核细胞中，紧贴内核膜下的一层高电子密度 的纤维蛋白网。
第七章 细胞核(nucleus)
细胞核的特点：
位置：一般位于中央，有的位
于细胞的一侧或边缘。
形状：多种多样，与细胞形状
相适应。
数量：一般为单核，有的为
双核或多核。
大小： 5-30um
核体积
核质比＝= 细胞质体积
细胞核的主要结构：
核膜 核纤层 染色质 核基质 核仁
第一节 核膜
核膜(nuclear envelope)：
成分：核纤层蛋白A、B、C
功 能：
1.为染色质提供附着的位点 2.为核膜提供支架，稳定核膜 3.与细胞核构建有关
外核膜 内核膜
核膜镶嵌蛋白
B AC
核周间隙
二、核骨架
核骨架：又称核基质，是间期核内，除去核膜、核纤
层染色质和核仁之外，以非组蛋白为主组成 的纤维网架结构。
医学专业英语课件12
• The word achondroplasia is derived from Greek and means "without cartilage formation," although individuals with achondroplasia do have cartilage. During fetal development and childhood, cartilage normally develops into bone, except in a few places, such as the nose and ears. In individuals with achondroplasia, something goes wrong during this process, especially in the long bones (such as those of the upper arms and thighs). The rate at which cartilage cells in the growth plates of the long bones turn into bone is slow, leading to short bones and reduced height.
医学遗传学-09人类染色体英语 PPT课件
The bulk of constitutive heterochromatin is composed of noncoding DNA sequences that are repeated hundreds of thousands or even millions of times.
Medical Genetics
Medical Genetics
Humans have 23 pairs of chromosomes, which makes the diploid number 46. The diploid number is the number of chromosomes of a normal cell.
Medical Genetics
09 人类染色体
human chromosome
Medical Genetics
A chromosome is the visible state of genetic material during a phase of the division of the cell (metaphase).
Medical Genetics
The highest degree of condensation can be found in the chromosomes (mitosis, meiosis) during cell division.
Medical Genetics
The haploid number is the number of chromosomes in a gamete. Usually, the diploid number is twice the haploid number.
第十章细胞核nucleus与染色体
亲核蛋白与核定位信号
亲核蛋白（karyophilic protein） 在细胞质内合成后，需要或能够进入细胞核内发挥功能的 一类蛋白质 核质蛋白（nucleoplasmin）的入核转运 核定位信号 (nuclear localization signal，NLS)
NLS是存在于亲核蛋白内的一些短的氨基酸序列片段， 富含碱性氨基酸残基，如Lys、Arg，此外还常含有Pro。 NLS的氨基酸残基片段可以是一段连续的序列（T抗原），也可 以分成两段，两段之间间隔约10个氨基酸残基（核质蛋白）。 NLS序列可存在于亲核蛋白的不同部位，在指导完成核输入后并 不被切除。 NLS只是亲核蛋白入核的一个必要条件而非充分条件
三种构型DNA的主要特征
DNA构型的生物学意义
A型
Z型
B型
DNA构型的生物学意义
沟（特别是大沟）的特征在遗传信息表达过程中起关键作用 沟的宽窄及深浅影响调控蛋白对DNA信息的识别 三种构型的DNA处于动态转变之中 DNA二级结构的变化与高级结构的变化是相互关联的， 这种变化在DNA复制与转录中具有重要的生物学意义。
第十章 细胞核与染色体
nucleus and chromosome
细胞核是真核细胞内最大、最重要的细胞器,是细胞
遗传与代谢的调控信息中心.
细胞核的结构组成： ◆ 核被膜 (内膜, 外膜, 核孔复合体, 核纤层)
◆ 染色质
◆ 核仁 ◆ 核骨架 (核基质)
第一节 核被膜与核孔复合体
1 核被膜
●结构组成 ●核被膜的功能 ●核被膜有规律地解体与重建
核被膜的功能
构成核、质之间的天然选择性屏障 避免生命活动的彼此干扰 保护DNA不受细胞骨架运动 所产生的机械力的损伤 核质之间的物质交换与信息交流
医学专业英语课件_15
Biography---Francis Crick
Born on June 8, 1916 at Northampton, England In 1937, obtained a B.Sc. In Physics at London Started to learn biology in 1937, interrupted by World War Two During the war, he worked as a scientist for the British Admiralty Left the Admiralty in 1947 to study biology
Genome: Genetic endowment of a species Gene mapping: Determining location of genes on chromosome
Gene sequencing: Determining identity of genes from the distinctive order (sequence) of base pairs, such as A-T and G-C Chromosome: Threadlike structure in the nuclei of plant and animal cells; it carries the linearly arranged genetic units (genes)
The Human Genome Project
Began at 1990
Founded by US department of Energy (DOE), US National Institute of Health (NIH) in collaboration with Britains Wellcome trust Draft sequence was published in 2001
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The Philadelphia chromosome
A chromosomal abnormality in CML (Chronic Myelogenous Leukemia) Chromosome 22 gets shorter Chromosome 9 gets longer
The Philadelphia Chromosome: t(9;22) Translocation
The beads are histones DNA wraps around histones
Histones are an octamer (2 each of H2A, H2B, H3, and H4 proteins)
is used to construct proteins. 3. Chromosome: A linear sequence of genes and non-coding
sequences arranged on a single strand of DNA helix. 4. Genome: A collection of chromosomes that is the sum
9
9+22ຫໍສະໝຸດ Phbcr abl
bcr-abl
FUSION PROTEIN WITH TYROSINE KINASE ACTIVITY
Chromosome Translocation
Part of one chromosome, or an entire chromosome has been Attached to another chromosome.
Two types of nucleic acid (DNA and RNA)
DNA is the genetic material RNA is primarily used for protein synthesis
Organizational Hierarchy
1. DNA: Elemental building block of life. 2. Gene: A specific sequence of DNA whose information
A translocation between chromosomes 9 and 22 is written as: t(9,22)
Chromosomes exist in condensed (metaphase) or open states (interphase)
During interphase, DNA looks like beads on a string
G-banding Idiogram
telomere
centromere
Numerical position eg 4q26.2
telomere
Short arm (p) Long arm (q)
Karyotype abnormalities can be used to diagnose human disease
total of genetic information necessary to code for an organism.
The nucleus is an organelle for packaging DNA
Chromosomes exist in condensed (metaphase) or open states (interphase)
Karyotype: A visual display of human metaphase chromosomes G-banding: Geimsa stain of karyotype
Dark bands: A-T rich regions Light bands: G-C rich regions
Chromatin: DNA plus the proteins that package it within the cell nucleus. Chromosome: A DNA molecule with its attendant proteins that moves as an independent unit during mitosis and meiosis. Chromatid: Before DNA replication, each chromosome consists of a single DNA molecule plus proteins and is called a chromatid. Sister Chromatids. After replication, each chromosome consists of two identical DNA molecules plus proteins. These are called sister chromatids. Centromere: The centromere regulates the movements of the chromosomes during mitosis and meiosis and can be visualized as a constricted region where sister chromatids are held together most closely. Kinetochore: The substructure within the centormere that binds microtubules and directs the movements of chromosomes in mitosis. Telomere: The specialized structure at either end of the chromosomal DNA molecule. P arm: The short arm of a chromosome Q arm: The long arm of a chromosome